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Signaling through GPR109a, the putative receptor for the endogenous ligand β-OH butyrate, inhibits adipose tissue lipolysis. Niacin, an anti-atherosclerotic drug that can induce insulin resistance, activates GPR109a at nM concentrations. GPR109a is not essential for niacin to improve serum lipid profiles. To better understand the involvement of GPR109a signaling in regulating glucose and lipid metabolism, we treated GPR109a wild-type (+/+) and knockout (-/-) mice with repeated overnight injections of saline or niacin in physiological states characterized by low (ad libitum fed) or high (16 h fasted) concentrations of the endogenous ligand, β-OH butyrate. In the fed state, niacin increased expression of apolipoprotein-A1 mRNA and decreased sterol regulatory element-binding protein 1 mRNA independent of genotype, suggesting a possible GPR109a independent mechanism by which niacin increases high-density lipoprotein (HDL) production and limits transcriptional upregulation of lipogenic genes. Niacin decreased fasting serum non-esterified fatty acid concentrations in both GPR109a +/+ and -/- mice. Independent of GPR109a expression, niacin blunted fast-induced hepatic triglyceride accumulation and peroxisome proliferator-activated receptor α mRNA expression. Although unaffected by niacin treatment, fasting serum HDL concentrations were lower in GPR109a knockout mice. Surprisingly, GPR109a knockout did not affect glucose or lipid homeostasis or hepatic gene expression in either fed or fasted mice. In turn, GPR109a does not appear to be essential for the metabolic response to the fasting ketogenic state or the acute effects of niacin.

This study was aimed to observe the effects of vitamin B 1 , vitamin B 2 , and niacin supplementation on hepatic gene expression profiles in mice exposed to acute hypoxia. Thirty mice were randomly divided into normal, acute hypoxia, and acute hypoxia plus vitamin B 1 , vitamin B 2 , and niacin supplementation groups and fed corresponding diets for 2 weeks and then exposed to a simulated altitude of 6000 m for 8 h. Hepatic gene expression profiles were analyzed using a microarray technique. Several biochemical markers were also assayed. The results showed that a total of 2476 genes were expressed differentially after acute hypoxia exposure (1508 upregulated genes and 968 downregulated genes). Compared with the acute hypoxia group, there were 1382 genes differentially expressed (626 upregulated genes and 756 downregulated genes) in the acute hypoxia plus vitamin B 1 , vitamin B 2 , and niacin supplementation group. Pathway analysis indicated that carbohydrate, lipid, and amino acid metabolism, as well as electron transfer chain, were improved to some extent after vitamin B 1 , vitamin B 2 , and niacin supplementation. Supportive results were obtained from biochemical assays. Our findings suggest that the supplementation of vitamin B 1 , vitamin B 2 , and niacin is beneficial in improving nutritional metabolism partly via gene expression under acute hypoxia condition.

Background: Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disorder in industrialized countries, yet its pathophysiology is incompletely understood. Small-molecule metabolite screens may offer new insights into disease mechanisms and reveal new treatment targets., Methods: Discovery (N = 33) and replication (N = 66) of liver biopsies spanning the range from normal liver histology to non-alcoholic steatohepatitis (NASH) were ascertained ensuring rapid freezing under 30 s in patients. 252 metabolites were assessed using GC/MS. Replicated metabolites were evaluated in a murine high-fat diet model of NAFLD., Results: In a two-stage metabolic screening, hydroquinone (HQ, p(combined) = 3.0 × 10(-4)) and nicotinic acid (NA, p(combined) = 3.9 × 10(-9)) were inversely correlated with histological NAFLD severity. A murine high-fat diet model of NAFLD demonstrated a protective effect of these two substances against NAFLD: Supplementation with 1% HQ reduced only liver steatosis, whereas 0.6% NA reduced both liver fat content and serum transaminase levels and induced a complex regulatory network of genes linked to NALFD pathogenesis in a global expression pathway analysis. Human nutritional intake of NA equivalent was also consistent with a protective effect of NA against NASH progression., Conclusion: This first small-molecular screen of human liver tissue identified two replicated protective metabolites. Either the use of NA or targeting its regulatory pathways might be explored to treat or prevent human NAFLD., (© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Objective: The hepatic metabolism of six compounds newly synthesized as retinoid X receptor agonists was characterized in rat and human liver microsomes to examine the relationship between their hepatic metabolism profiles and side chain structures, considering the interspecies difference., Materials and Methods: The compounds used have a 6-[N-ethyl-N-(3-alkoxy-4-isopropylphenyl)amino]nicotinic or 6-[N-ethyl-N-(4-alkoxy-3-isopropylphenyl)amino]nicotinic acid skeleton, in which the isopropoxy, isobutoxy or cyclopropylmethoxy group is employed for the alkoxy group. These compounds were incubated with the microsomes, and their Michaelis--Menten parameters were determined. The incubation study was also performed with various cytochrome P450 (CYP) inhibitors to examine their susceptibilities to the inhibitors. In addition, a molecular docking simulation was conducted to assess the compound's spatial configuration with the CYP isoform when necessary., Results: The Michaelis--Menten parameters determined are comparable between rats and humans for the compounds having 3-isobutoxy, 4-isobutoxy, 4-isopropoxy and 4-cyclopropylmethoxy groups. However, it was indicated that all compounds except that having the 3-isobutoxy group are metabolized in a different manner between rats and humans. That is, the extent of the contribution of each CYP isozyme is different between those two species. A molecular docking simulation showed that the spatial configuration of the compound to be associated with CYP2D6 markedly changes depending on whether the isobutoxy group is situated at the 3- or 4-position., Conclusion: A slight difference in the side chain structures markedly alters the compound's metabolic profile, which amplifies the interspecies difference regarding the profile, increasing the difficulty in characterizing the profile in humans with the structural-property relationship and interspecies extrapolation.

Our aim was to characterize the effects and the underlying mechanisms of the lipid-regulating agent Niaspan(®) on both insulin action and triglyceride decrease in 20 nondiabetic, dyslipidemic men with metabolic syndrome receiving Niaspan(®) (2 g/day) or placebo for 8 weeks in a randomized, cross-over study. The effects on plasma lipid profile were characterized at the beginning and the end of each treatment period; insulin sensitivity was assessed using the 2-step euglycemic hyperinsulinemic clamp and VLDL-triglyceride turnover by measuring plasma glycerol enrichment, both at the end of each treatment period. The mechanism of action of nicotinic acid was studied in HuH7 and mouse primary hepatocytes. Lipid profile was improved after Niaspan(®) treatment with a significant-28% decrease in triglyceride levels, a+17% increase in HDL-C concentration and unchanged levels of fasting nonesterified fatty acid. VLDL-tri-glyceride production rate was markedly reduced after Niaspan(®) (-68%). However, the treatment induced hepatic insulin resistance, as assessed by reduced inhibition of endogenous glucose production by insulin (0.7±0.4 vs. 1.0±0.5 mg/kg · min, p<0.05) and decrease in fasting hepatic insulin sensitivity index (4.8±1.8 vs. 3.2±1.6, p<0.05) in the Niaspan(®) condition. Nicotinic acid also reduced insulin action in HuH7 and primary hepatocytes, independently of the activation of hepatic PKCε. This effect was associated with an increase in diacylglycerol and a decrease in tri-glyceride contents that occurred in the absence of modification of DGAT2 expression and activity. Eight weeks of Niaspan(®) treatment in dyslipidemic patients with metabolic syndrome induce hepatic insulin resistance. The mechanism could involve an accumulation of diacylglycerol and an alteration of insulin signaling in hepatocytes., (© Georg Thieme Verlag KG Stuttgart · New York.)

Nonalcoholic fatty liver disease (NAFLD), a leading cause of liver damage, comprises a spectrum of liver abnormalities including the early fat deposition in the liver (hepatic steatosis) and advanced nonalcoholic steatohepatitis. Niacin decreases plasma triglycerides, but its effect on hepatic steatosis is elusive. To examine the effect of niacin on steatosis, rats were fed either a rodent normal chow, chow containing high fat (HF), or HF containing 0.5% or 1.0% niacin in the diet for 4 wk. For regression studies, rats were first fed the HF diet for 6 wk to induce hepatic steatosis and were then treated with niacin (0.5% in the diet) while on the HF diet for 6 wk. The findings indicated that inclusion of niacin at 0.5% and 1.0% doses in the HF diet significantly decreased liver fat content, liver weight, hepatic oxidative products, and prevented hepatic steatosis. Niacin treatment to rats with preexisting hepatic steatosis induced by the HF diet significantly regressed steatosis. Niacin had no effect on the mRNA expression of fatty acid synthesis or oxidation genes (including sterol-regulatory element-binding protein 1, acetyl-CoA carboxylase 1, fatty acid synthase, and carnitine palmitoyltransferase 1) but significantly inhibited mRNA levels, protein expression, and activity of diacylglycerol acyltrasferase 2, a key enzyme in triglyceride synthesis. These novel findings suggest that niacin effectively prevents and causes the regression of experimental hepatic steatosis. Approved niacin formulation(s) for other indications or niacin analogs may offer a very cost-effective opportunity for the clinical development of niacin for treating NAFLD and fatty liver disease.

Elevated plasma lipoprotein(a) (LPA) levels are recognized as an independent risk factor for cardiovascular diseases. Our knowledge on LPA metabolism is incomplete, which makes it difficult to develop LPA-lowering medications. Nicotinic acid (NA) is the main drug recommended for the treatment of patients with increased plasma LPA concentrations. The mechanism of NA in lowering LPA is virtually unknown. To study this mechanism, we treated transgenic (tg) APOA mice with NA and measured plasma APOA and hepatic mRNA levels. In addition, mouse and human primary hepatocytes were incubated with NA, and the expression of APOA was followed. Feeding 1% NA reduced plasma APOA and hepatic expression of APOA in tg-APOA mice. Experiments with cultured human and mouse primary hepatocytes in addition to reporter assays performed in HepG2 cells revealed that NA suppresses APOA transcription. The region between -1446 and -857 of the human APOA promoter harboring several cAMP response element binding sites conferred the negative effect of NA. In accordance, cAMP stimulated APOA transcription, and NA reduced hepatic cAMP levels. It is suggested that cAMP signaling might be involved in reducing APOA transcription, which leads to the lowering of plasma LPA.

The anti-dyslipidemic drug niacin has recently been shown to reduce the hepatic expression and plasma levels of CETP. Since liver macrophages contribute to hepatic CETP expression, we investigated the role of macrophages in the CETP-lowering effect of niacin in mice. In vitro studies showed that niacin does not directly attenuate CETP expression in macrophages. Treatment of normolipidemic human CETP transgenic mice, fed a Western-type diet with niacin for 4 weeks, significantly reduced the hepatic cholesterol concentration (-20%), hepatic CETP gene expression (-20%), and plasma CETP mass (-30%). Concomitantly, niacin decreased the hepatic expression of CD68 (-44%) and ABCG1 (-32%), both of which are specific markers for the hepatic macrophage content. The decrease in hepatic CETP expression was significantly correlated with the reduction of hepatic macrophage markers. Furthermore, niacin attenuated atherogenic diet-induced inflammation in liver, as evident from decreased expression of TNF-alpha (-43%). Niacin similarly decreased the macrophage markers and absolute macrophage content in hyperlipidemic APOE*3-Leiden.CETP transgenic mice on a Western-type diet. In conclusion, niacin decreases hepatic CETP expression and plasma CETP mass by attenuating liver inflammation and macrophage content in response to its primary lipid-lowering effect, rather than by attenuating the macrophage CETP expression level., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Niacin reduces plasma triglycerides, but it may increase free fatty acids and insulin resistance during long-term treatment. We examined the effect of extended-release niacin on liver fat content in Chinese patients with dyslipidemia and whether the common diacylglycerol acyltransferase-2 (DGAT2) polymorphisms influenced this effect. The 39 patients (baseline liver fat content: 12.8 ± 7.6%, triglycerides: 3.30 ± 1.67 mmol/l) were treated with niacin, gradually increasing the dose to 2 g/day for a total of 23 weeks. The liver fat content and visceral/subcutaneous fat was measured before and after treatment. Subjects were genotyped for the DGAT2 rs3060 and rs101899116 polymorphisms. There were significant (P < 0.001) reductions in plasma triglycerides (-34.9 ± 37.6%), liver fat content (-47.2 ± 32.8%), and visceral fat (-6.3 ± 15.8%, P < 0.05) after niacin treatment. Mean body weight decreased by 1.46 ± 2.7% (1.17 ± 2.44 kg, P < 0.001) during the study, but liver fat changes remained significant after adjustment for age, gender, and body weight changes [mean absolute change (95% CI): -6.1% (-8.0, -4.3), P < 0.001]. The DGAT2 variant alleles were associated with a smaller reduction in liver fat content in response to niacin after adjustment for other covariates (P < 0.01). These findings suggest that niacin treatment may reduce liver fat content in Chinese patients with dyslipidemia and that the mechanism may involve inhibition of DGAT2. However, the findings might have been confounded by the small but significant reductions in body weight during the study. Future large randomized controlled trials are needed to verify these findings.

Nicotinic acid (niacin) can suppress lipolysis, but responses to dietary niacin have been inconsistent in cattle. Our aim was to determine if 24 g/d of encapsulated niacin (EN; providing 9.6g/d of bioavailable nicotinic acid) alters lipid metabolism and productivity of transition cows. Beginning 21 d before expected calving, primiparous (n = 9) and multiparous (n = 13) cows (body condition score of 3.63 ± 0.08) were sequentially assigned within parity to EN (12 g provided with ration twice daily) or control through 21 d postpartum. Liver biopsies were collected on d -21, -4, 1, 7, and 21 relative to parturition. Blood samples were collected on d -21, -14, -7, -4, 1, 4, 7, 14, and 21 relative to parturition. On d 7 postpartum, a caffeine clearance test was performed to assess liver function, and on d 21 to 23 postpartum, blood samples were collected every 8h to monitor posttreatment nonesterified fatty acid (NEFA) responses. Data were analyzed using mixed models with repeated measures over time. A treatment × time × parity effect was observed on prepartum dry matter intake (DMI), which was caused by a 4 kg/d decrease in DMI of EN-treated multiparous cows compared with control multiparous cows during the final 4 d prepartum. A significant increase in plasma nicotinamide concentration occurred in EN-treated cows on d -7 and 21 relative to parturition. Prepartum glucose concentration decreased in treated animals, with no difference in plasma insulin concentration. Treatment × time × parity effects were detected for NEFA and β-hydroxybutyrate concentrations during the postpartum period. Plasma NEFA peaked at 1,467 ± 160 μM for control animals compared with 835 ± 154 μM for EN-treated animals. After treatments ended on d 21, no evidence was found for a plasma NEFA rebound in either parity group. A treatment × parity × time interaction was detected for liver triglyceride content, indicating a tendency for less liver triglyceride in EN-treated primiparous cows, but caffeine clearance rates were not affected by treatment. No treatment effects were observed for body condition score, body weight, energy balance, or milk or milk component production. A high dose of EN can decrease postpartum plasma NEFA concentration, but may also decrease prepartum DMI., (Copyright © 2011 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

We examined the effect of dietary nicotinic acid (NA) variations before and after oxidative stress (OS) treatment on the antioxidant defence system, function and morphology of the liver along with Zn status in rats. OS was generated by three intraperitoneal injections of tert-butyl hydroperoxide in the first week for the pre-exposure group and in the third week for the post-exposure group, respectively. These groups were further divided into subgroups and fed on a diet with marginally deficient Zn (10 mg Zn/kg diet) and NA variations as NA deficient, normal and excess with 10, 30 and 1000 mg NA/kg diet, respectively. Aspartate aminotransferase and alanine aminotransferase levels were elevated in rats with OS coupled with the Zn- and NA-deficient diet, which decreased towards normal with excess dietary NA. Excess NA supplementation in the OS pre-exposure group resulted in nearly preserved hepatic architecture with normal hepatocytes, whereas maximum tissue destruction was evident in the post-exposure group with NA deficiency. Dose-dependent improvement in the antioxidant defence system, enhanced reduced glutathione levels, lowered lipid peroxidation and higher hepatic Zn levels were observed with NA supplementation. The effect was more prominent in the pre-exposure group. In conclusion, dietary NA supplementation improves hepatic Zn uptake and results in hepatoprotection against OS-induced damage in rats.

Peroxisome proliferator-activated receptor (PPAR) γ coactivator-1β (PGC-1β) is a transcriptional coactivator that induces hypertriglyceridemia in response to dietary fats through activating hepatic lipogenesis and lipoprotein secretion. The expression of PGC-1β is regulated by free fatty acids. Here we show that PGC-1β regulates plasma triglyceride metabolism through stimulating apolipoprotein C3 (APOC3) expression and elevating APOC3 levels in circulation. Remarkably, liver-specific knockdown of APOC3 significantly ameliorates PGC-1β-induced hypertriglyceridemia in mice. Hepatic expression of PGC-1β and APOC3 is reduced in response to acute and chronic treatments with nicotinic acid, a widely prescribed drug for lowering plasma triglycerides. Adenoviral-mediated knockdown of PGC-1β or APOC3 in the liver recapitulates the hypolipidemic effect of nicotinic acid. Proteomic analysis of hepatic PGC-1β transcriptional complex indicates that it stimulates APOC3 expression through coactivating orphan nuclear receptor ERRα and recruiting chromatin-remodeling cofactors. Together, these studies identify PGC-1β as an important regulator of the APOC3 gene cluster and reveal a mechanism through which nicotinic acid achieves its therapeutic effects., (Copyright © 2010 Elsevier Inc. All rights reserved.)

A novel dietary supplement composed of three well-known phytochemicals, namely, Salvia officinalis (sage) extract, Camellia sinensis (oolong tea) extract, and Paullinia cupana (guarana) extract, and two prominent vitamins (thiamine and niacin) was designed to provide nutritional support by enhancing metabolism and maintaining healthy weight and energy. The present study evaluated the safety of this dietary supplement (STG; S=sage; T=tea; G=guarana) and assessed changes in target organ antioxidant enzymes (liver, kidneys and heart), serum chemistry profiles and organ histopathology in Fisher 344 rats. Adult male and female Fisher 344 rats were fed control (no STG) or STG containing (1X and 7X, 1X=daily human dose) diets and sacrificed after 2 and 4 months. Serum chemistry analysis and histopathological examination of three vital target organs disclosed no adverse influence on protein, lipid and carbohydrate profiles, genomic integrity of the liver and/or the tissue architecture. However, analysis of the most important antioxidant components in the liver, kidney and heart homogenates revealed a dramatic increase in total glutathione concentrations, glutathione peroxidase and superoxide dismutase enzyme activities. Concomitantly, oxidative stress levels (malondialdehyde accumulation) in these three organs were less than control. Organ specific serum markers (ALT/AST for the liver; CPK/AST for the heart; BUN/creatinine for kidneys) and the genomic integrity disclosed no STG-induced alteration. Some of the serum components (lipid and protein) showed insignificant changes. Overall, STG-exposed rats were more active, and the results suggest that STG exposure produces normal serum chemistry coupled with elevated antioxidant capacity in rats fed up to seven times the normal human dose and does not adversely influence any of the vital target organs. Additionally, this study reiterates the potential benefits of exposure to a pharmacologically relevant combination of phytochemicals compared to a single phytochemical entity.

Glycogen content of white and red skeletal muscles, cardiac muscle, and liver was investigated in conditions where changes in plasma levels of non-esterified fatty acids (NEFA) occur. The experiments were performed in fed and 12 and 48 h-fasted rats. The animals were also submitted to swimming for 10 and 30 min. Glycogen content was also investigated in both pharmacologically induced low plasma NEFA levels fasted rats and pharmacologically induced high plasma NEFA levels fed rats. The participation of Akt and glycogen synthase kinase-3 (GSK-3) in the changes observed was investigated. Plasma levels of NEFA, glucose, and insulin were determined in all conditions. Fasting increased plasma NEFA levels and reduced glycogen content in the liver and skeletal muscles. However, an increase of glycogen content was observed in the heart under this condition. Akt and GSK-3 phosphorylation was reduced during fasting in the liver and skeletal muscles but it remained unchanged in the heart. Our results suggest that in conditions of increased plasma NEFA levels, changes in insulin-stimulated phosphorylation of Akt and GSK-3 and glycogen content vary differently in liver, skeletal muscles, and heart. Akt and GSK-3 phosphorylation and glycogen content are decreased in liver and skeletal muscles, but in the heart it remain unchanged (Akt and GSK-3 phosphorylation) or increased (glycogen content) due to consistent increase of plasma NEFA levels.

High-density lipoprotein (HDL) plays an important protective role against atherosclerosis, and the anti-atherogenic properties of HDL include the promotion of cellular cholesterol efflux and reverse cholesterol transport (RCT), as well as antioxidant, anti-inflammatory and anticoagulant effects. RCT is a complex pathway, which transports cholesterol from peripheral cells and tissues to the liver for its metabolism and biliary excretion. The major steps in the RCT pathway include the efflux of free cholesterol mediated by cholesterol transporters from cells to the main extracellular acceptor HDL, the conversion of free cholesterol to cholesteryl esters and the subsequent removal of cholesteryl ester in HDL by the liver. The efficiency of RCT is influenced by the mobilization of cellular lipids for efflux and the intravascular remodelling and kinetics of HDL metabolism. Despite the increased cardiovascular risk in people with type 2 diabetes, current knowledge on RCT in diabetes is limited. In this article, abnormalities in RCT in type 2 diabetes mellitus and therapeutic strategies targeting HDL and RCT will be reviewed.

Objective: Niacin potently decreases plasma triglycerides and LDL-cholesterol. In addition, niacin is the most potent HDL-cholesterol-increasing drug used in the clinic. In the present study, we aimed at elucidation of the mechanism underlying its HDL-raising effect., Methods and Results: In APOE*3Leiden transgenic mice expressing the human CETP transgene, niacin dose-dependently decreased plasma triglycerides (up to -77%, P<0.001) and total cholesterol (up to -66%, P<0.001). Concomitantly, niacin dose-dependently increased HDL-cholesterol (up to +87%, P<0.001), plasma apoAI (up to +72%, P<0.001), as well as the HDL particle size. In contrast, in APOE*3Leiden mice, not expressing CETP, niacin also decreased total cholesterol and triglycerides but did not increase HDL-cholesterol. In fact, in APOE*3Leiden.CETP mice, niacin dose-dependently decreased the hepatic expression of CETP (up to -88%; P<0.01) as well as plasma CETP mass (up to -45%, P<0.001) and CETP activity (up to -52%, P<0.001). Additionally, niacin dose-dependently decreased the clearance of apoAI from plasma and reduced the uptake of apoAI by the kidneys (up to -90%, P<0.01)., Conclusions: Niacin markedly increases HDL-cholesterol in APOE*3Leiden.CETP mice by reducing CETP activity, as related to lower hepatic CETP expression and a reduced plasma (V)LDL pool, and increases HDL-apoAI by decreasing the clearance of apoAI from plasma.

The effects of co-administration of nicotinic acid (NA) and alcohol (Alc) on liver function in male Wistar rats were evaluated. The rats were randomized into five groups: (i) Olive oil (Oil), (ii) Alc+Oil, (iii) NA+Oil, (iv) NA+Alc+Oil, and (v) Controls (fed only normal rat chow). Alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (AP), total cholesterol and triglycerides (TG) were measured. Liver histopathology was also assessed. The Alc+Oil group had higher TG levels compared with the NA+Alc+Oil group and all other groups, as well. NA+Oil group had higher levels of AP compared with Alc+Oil and Oil groups. The NA+Oil group had higher ALT levels compared with the Oil group. The Oil group had lower ALT levels compared with the control group. The Alc+Oil group had higher AST levels compared with the NA+Alc+Oil group, as well as with all other groups. Liver histopathology was within the normal range. A moderate amount of Alc daily together with NA is safe in rats. The NA and Alc co-administration reduces the TG and AST levels in rats, compared with the administration of Alc alone., (Copyright (c) 2005 Prous Science. All rights reserved.)

Our objective was to examine very low density lipoprotein-triglyceride (VLDL-TG) kinetics after chronic and acute administration of nicotinic acid (NA). Incorporation of [1,2,3,4-(13)C(4)]palmitate and [2-(13)C(1)]glycerol into VLDL-TG was measured in five healthy, normolipidemic women. Each subject was studied twice; the 4-day hospital stays were separated by 1 mo, during which time doses of NA were increased to 2 g/day (500 mg, 4 times/day). During posttreatment study, 500 mg of NA were administered acutely at 0800. Under baseline postabsorptive conditions, incorporation curves from (13)C-labeled free fatty acid (FFA) and (13)C-labeled glycerol were superimposable, and VLDL-TG kinetics were in agreement (t(1/2) = 1.4 +/- 0.3 and 1.3 +/- 0.3 h, and production rates = 27.2 +/- 6.1 and 28.5 +/- 5.3 g/day, respectively). In the postabsorptive state after chronic NA therapy, VLDL-TG concentrations and production rates were lower despite a trend toward elevated plasma FFA concentrations and fluxes. After the acute dose of NA, plasma FFA concentrations and flux fell dramatically, and there was a virtual halt to VLDL-TG production, which continued throughout the 6-h period after NA, despite a marked rebound overshoot in serum FFA concentrations and flux after hour 2. Plasma homocysteine concentrations increased 68% (P < 0.001) in the NA phase, consistent with chronic increased transmethylation demand. We conclude that 1) NA acutely and chronically decreases VLDL-TG production rate in normal women; 2) the acute effect on VLDL-TG production is associated with an initial suppression of lipolysis but persists for several hours after the antilipolytic action of NA has abated and is observed in the basal postabsorptive state, when lipolytic rates are not reduced; and 3) the effect of NA on VLDL-TG production, therefore, cannot be completely explained by its antilipolytic actions.

Bile acid sequestrants, fibric acid derivatives (fibrates), hydroxy-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors ("statins"), and niacin are able to increase HDL-C serum concentrations to varying degrees. Bile acid sequestrants are the least effective, whereas niacin is the most powerful agent for increasing HDL-C levels. Because of 2 alternate metabolic pathways of niacin breakdown, flushing or hepatotoxicity can occur in patients taking niacin. These effects can be mediated in a carefully designed formulation of niacin that releases the drug at a predictable rate. Niacin has few drug interactions and is a relatively inexpensive means of increasing HDL-C. A combination formulation that combines niacin plus a statin has shown promise in ongoing clinical trials.

Aims/background: We report in this study that, 10 mM nicotinamide can stimulate the proliferation of primary rat hepatocytes in serum-free Dulbecco's modified Eagle's medium supplemented with 10 ng/ml epidermal growth factor and that small hepatocyte colonies appear from 4 to 5 days after plating. We examined the effects of nicotinamide-related agents on the growth and differentiation of primary rat hepatocytes and on the appearance of small hepatocyte colonies., Methods: As nicotinamide is an aqueous vitamin named niacin and known to act as an inhibitor of poly (ADP-ribose) polymerase (PARP), we therefore chose to examine the effects on hepatocytes of three nicotinamide-related agents, nicotinic acid (NA) which is also a niacin, 3-aminobenzamide (3-AB) which is a strong inhibitor of PARP but is not a niacin, and 3-acetylpyridine (3-AP) which is a weak inhibitor of PARP and also not a niacin. To examine their effects on the growth of the cells and on the formation of the colony, immunocytochemistry for BrdU was carried out. Expression of albumin, tryptophan 2,3-dioxygenase (TO), and connexin 32 (Cx32) mRNAs were used as marks of hepatic differentiation. Intracellular NAD+ content was also measured., Results: At concentration of 10 mM, NA could not enhance the proliferation of mature hepatocytes but induced the appearance of small hepatocyte colonies. At concentration of 5 mM, 3-AB enhanced the proliferation of the hepatocytes but did not induce small hepatocyte colonies. On the other hand, although 10 mM 3-AP remarkably inhibited the DNA synthesis of the cells, the expression not only of albumin but also of TO and Cx32 mRNAs in the cells was well maintained for more than one week. The intracellular NAD+ concentration was correlated with the proliferation of the hepatocytes., Conclusion: These results suggest that the intracellular NAD+ content may be correlated with the proliferation of primary hepatocytes and that the supplementation of niacin in the medium may be important for the appearance of small hepatocyte colonies.

NAD+ biosynthesis from tryptophan in the presence of nicotinic acid or vice versa by rat hepatocytes was investigated. In the control hepatocytes, NAD+ synthesis from tryptophan was not affected by nicotinic acid from 0.026 to 0.26 mM. NAD+ synthesis from nicotinic acid was slightly inhibited with varying concentrations of tryptophan from 0.1 to 1.0 mM. In the clofibrate-treated hepatocytes, NAD+ synthesis from tryptophan was greatly increased (234% of the control), while that from nicotinic acid was decreased (71.2% of the control). Both, NAD+ synthesis from tryptophan and that from nicotinic acid were decreased by the coexisting nicotinic acid or tryptophan. Total amount of NAD+ synthesized from tryptophan and nicotinic acid at their physiological concentrations was significantly higher than that in the control hepatocytes as a result of a large increase of NAD+ synthesized from tryptophan. When the metabolic flux of 0.1 or 0.5mM tryptophan was investigated, the glutarate pathway was suppressed in the clofibrate-treated hepatocytes, the quinolinic acid-NAD+ flux being elevated. Similarly to clofibrate, DEHP and CPP revealed an increase in NAD+ synthesis from tryptophan. Mutual relationship of NAD+ biosyntheses from tryptophan and nicotinic acid in rat hepatocytes is discussed and the relevance with peroxisomal proliferation is suggested.

In this study, our previous finding that nicotinic acid activates tryptophan 2,3-dioxygenase as strongly as tryptophan was investigated in further detail. This study focused on the role of the adrenals in the activation process. Adrenalectomy abolished the activation due to nicotinic acid, but not the activation caused by tryptophan. The role of corticoids and/or adrenomedullary hormones in the enzyme activation was studied, by supplementing these hormones in adrenalectomized rats using minipumps implanted under the skin. The results showed that the enhanced activity of tryptophan 2,3-dioxygenase caused by nicotinic acid was partly restored by adrenaline following adrenalectomy but not by corticosterone supplementation. The results were supported by further experiments in which the rats were treated with adrenaline or corticosterone intraperitoneally before nicotinic acid administration. The conclusion that adrenaline participates in the regulation of tryptophan 2,3-dioxygenase should promote further study to determine whether adrenaline is a general modulator of this enzyme. This experimental model generated new information on the activation mechanism of tryptophan 2,3-dioxygenase by nicotinic acid.

Chronic administration of thiazopyr in the diet at dose levels of 1000 and 3000 ppm, but not 100 ppm, has demonstrated an increase in thyroid follicular cell tumors in male Sprague-Dawley rats. In the studies reported here we have evaluated the mechanism of thiazopyr-induced thyroid tumors by studying the effect of thiazopyr on a number of endpoints that indicate hypothalamic-pituitary-thyroid homeostasis. At a dose level of 3000 ppm, thiazopyr caused a marked depression in circulating levels of T4 as soon as 7 days after commencement of treatment. Concurrent with this decrease in T4 was an increase in TSH levels, an increase in thyroid and liver weights, a three- to sixfold increase in hepatic T4-uridine diphosphate glucuronosyl transferase (UDPGT) activity, and increases in thyroid follicular cell hypertrophy and hyperplasia. Dose-related changes associated with thiazopyr treatment were significant increases in liver weight, thyroid weight, and hepatic T4-UDPGT activity at high doses. Increased levels of serum TSH, T3, and rT3, decreased levels of T4, and an increased incidence of thyroid follicular cell hypertrophy and hyperplasia were observed 56 days after the initiation of 3000 ppm thiazopyr. All the changes, except thyroid weight, were partially or completely reversible upon removal of thiazopyr from the diet. Increased thyroid T4 elimination, primarily via increased hepatic conjugation by T4-UDPGT, resulting in decreased serum T4, appeared to be responsible for the increased TSH levels. The sustained increase in TSH by thiazopyr appears responsible for the stimulation of the thyroid follicular cells resulting in follicular cell hypertrophy, hyperplasia, and ultimately neoplasia. In summary, evidence is presented for a hormonally mediated, threshold-dependent process for the development of thyroid follicular cell tumors from high-dose thiazopyr administration in male rats. This mechanism is not considered to be relevant to humans, since the thyroid of humans is much less sensitive to this pathogenic phenomenon than rodents.

The distal enzymatic step in the process of glucose output is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) complex. The recently cloned catalytic unit of this complex has been shown to be regulated by insulin, dexamethasone, cAMP, and glucose. Using a combination of intralipid and/or nicotinic acid infusions and a pancreatic clamp technique, we maintained plasma free fatty acids (FFAs) at three different levels (0.26 +/- 0.07, 0.56 +/- 0.09, and 1.59 +/- 0.12 mmol/l) in the presence of well-controlled hormonal and metabolic conditions. An increase in the plasma FFA concentration within the physiological range caused a rapid, greater than threefold increase in the mRNA and protein levels of the catalytic subunit of Glc-6-Pase in the liver. These data indicate that the in vivo gene expression of Glc-6-Pase in the liver is regulated by circulating lipids independent of insulin and thus that prolonged hyperlipidemia may contribute to the increased production of glucose via increased expression of this protein.

Poly(ADP-ribose) is a homopolymer of ADP-ribose units synthesized from NAD+ on nuclear acceptor proteins and is known to be involved in DNA repair. It is not known whether large oral doses of the clinically utilized NAD precursors nicotinic acid or nicotinamide affect poly(ADP-ribose) metabolism or the cellular response to DNA damage. In our first study, using Fischer-344 rats, 2 wk of dietary nicotinic acid supplementation (500 and 1000 mg/kg diet) caused elevated levels of NAD+ in the blood, liver, heart and kidney, while nicotinamide caused elevated levels only in the blood and liver, compared with controls fed a diet containing 30 mg/kg nicotinic acid. Both nicotinic acid and nicotinamide, at 1000 mg/kg diet, caused elevations in liver NAD+, by 44 and 43%, respectively. Only nicotinamide, however, elevated liver poly(ADP-ribose) (63% higher than control group). Following treatment with the hepatocarcinogen diethylnitrosamine, higher levels of hepatic NAD+ were observed in rats fed both nicotinic acid and nicotinamide at 1000 mg/kg diet, but only nicotinic acid supplementation caused a greater accumulation of hepatic poly(ADP-ribose) (61% higher than control group). Neither of the dietary treatments significantly affected the proportion of the liver occupied by placental glutathione-S-transferase positive foci. These results show that poly(ADP-ribose) synthesis is not directly responsive to hepatic NAD+ levels during niacin supplementation, and that the mechanisms of action of nicotinic acid and nicotinamide are different. The observed changes in poly(ADP-ribose) metabolism do not appear to cause any change in susceptibility to chemically induced carcinogenesis in this organ.

Poly(ADP-ribose) is synthesized on nuclear proteins in response to DNA damage and plays an important role in DNA repair. Niacin and tryptophan are dietary precursors to NAD+, which is the substrate for poly(ADP-ribose) synthesis. This study examined the influence of niacin status on poly(ADP-ribose) metabolism and carcinogenesis. Diets devoid of added niacin, with different levels of tryptophan, were used to produce moderate and severe niacin deficiencies in male Fischer-344 rats. Control rats were pair fed niacin-replete diets. After a 21-day feeding period, rats were injected with diethylnitrosamine (DEN) (Expt 1, 200 mg/kg ip; Expt 2, 100 mg/kg ip). In Experiment 1, blood and liver NAD+ and liver poly(ADP-ribose) were measured over the next 15 hours. Whereas blood and liver NAD+ were decreased by niacin deficiency, blood NAD+ was not affected by DEN. Liver NAD+ decreased significantly in response to DEN treatment in the pair-fed groups, but it did not change in the niacin-deficient groups. Unexpectedly, at 10 hours postinjection, liver poly(ADP-ribose) accumulation was greater (p < 0.05) in the niacin-deficient than in the pair-fed rats (n = 9), despite lower initial NAD+ levels and a lack of NAD+ disappearance in niacin-deficient livers. In Experiment 2, livers were examined for the presence of altered hepatic foci three months after DEN exposure. There were no significant differences in the percentage of liver occupied by foci between the niacin-deficient and pair-fed groups (n = 8). These results indicate that niacin-deficient rats were able to accumulate higher concentrations of hepatic poly(ADP-ribose) in response to DEN and did not show elevated susceptibility to initiation of altered hepatic foci.

Acute elevation in plasma free fatty acid (FFA) levels in nonpregnant rabbits simulates some aspects of insulin resistance observed in late pregnancy. The aim of this study was therefore to evaluate whether lowered FFA induced by nicotinic acid (NA) infusion can improve insulin action in pregnant rabbits. To assess insulin sensitivity we applied euglycemic hyperinsulinemic clamp without (control study) or with concomitant NA infusion (NA study; infusion started 150 min before the glucose clamp) in conscious virgin and pregnant rabbits after an 18-h fast. A primed constant infusion of [3-3H]glucose was used to measure total glucose disposal and hepatic glucose production. Hyperinsulinemia, in association with NA infusion (NA study) brings about a significantly greater stimulation of total glucose disposal in both pregnant (approximately 30%) and nonpregnant (approximately 35%) rabbits compared with the control study. A more pronounced inhibition of hepatic glucose production occurred in NA study in pregnant rabbits (approximately 30 vs. approximately 10%) but it did not reach a statistical significance, whereas there was a total inhibition in nonpregnant rabbits. We conclude that acute decrease in plasma FFA levels mainly improves insulin action at the site of glucose utilization. These results therefore suggest that elevated FFA would contribute to the peripheral insulin resistance in late pregnancy.

Adenylate cyclase activity in isolated rat liver plasma membranes was inhibited by NADH in a concentration-dependent manner. Half-maximal inhibition of adenylate cyclase was observed at 120 microM concentration of NADH. The effect of NADH was specific since adenylate cyclase activity was not altered by NAD+, NADP+, NADPH, and nicotinic acid. The ability of NADH to inhibit adenylate cyclase was not altered when the enzyme was stimulated by activating the cyclase was not altered when the enzyme was stimulated by activating the Gs regulatory element with either glucagon or cholera toxin. Similarly, inhibition of Gi function by pertussis toxin treatment of membranes did not attenuate the ability of NADH to inhibit adenylate cyclase activity. Inhibition of adenylate cyclase activity to the same extent in the presence and absence of the Gpp (NH) p suggested that NADH directly affects the catalytic subunit. This notion was confirmed by the finding that NADH also inhibited solubilized adenylate cyclase in the absence of Gpp (NH)p. Kinetic analysis of the NADH-mediated inhibition suggested that NADH competes with ATP to inhibit adenylate cyclase; in the presence of NADH (1 mM) the Km for ATP was increased from 0.24 +/- 0.02 mM to 0.44 +/- 0.08 mM with no change in Vmax. This observation and the inability of high NADH concentrations to completely inhibit the enzyme suggest that NADH interacts at a site(s) on the enzyme to increase the Km for ATP by 2-fold and this inhibitory effect is overcome at high ATP concentrations.

The effect of changes in plasma non-esterified fatty acid concentration (NEFA) on plasma glucose concentration, hepatic glucose production (Ra), and glucose disposal (Rd) rates was determined in 14 patients with Type 2 diabetes. Seven patients had relatively mild fasting hyperglycaemia (less than 10.0 mmol l-1), whereas the remaining seven had relatively severe fasting hyperglycaemia (greater than 14.0 mmol l-1). Each patient was infused from 2000 to 0800 h with 3-3H-glucose on two occasions, with or without neutral fat emulsion and heparin (mild hyperglycaemia group), or with or without nicotinic acid (severe hyperglycaemia group). Plasma NEFA concentration increased from 0.33 +/- 0.06 (+/- SE) to 4.78 +/- 0.42 mmol l-1 in response to the lipid and heparin infusion, but plasma glucose concentration (7.8 +/- 0.7 vs 7.4 +/- 0.8 mmol l-1), Ra (0.44 +/- 0.02 vs 0.46 +/- 0.02 mmol m-2 min-1), and Rd (0.42 +/- 0.02 vs 0.46 +/- 0.02 mmol m-2 min-1) were unchanged. Nicotinic acid decreased plasma NEFA concentration from 0.54 +/- 0.15 to 0.23 +/- 0.08 mmol l-1, but plasma glucose (15.0 +/- 1.0 vs 15.5 +/- 1.4 mmol l-1), Ra (0.74 +/- 0.07 vs 0.68 +/- 0.07 mmol m-2 min-1), and Rd (0.73 +/- 0.07 vs 0.68 +/- 0.07 mmol m-2 min-1) were unchanged. The results indicate that acute changes in plasma NEFA concentration did not lead to any change in overnight glucose production or disposal rates.

Tryptophan is one of the strongest activators of tryptophan 2,3-dioxygenase, the rate-limiting enzyme in the pathway which degrades tryptophan. One of the metabolites thus formed is nicotinic acid, widely administered as a drug--often at high doses--and a vitamin. This study determined whether nicotinic acid also has a potency to activate tryptophan 2,3-dioxygenase and, if so, by what mechanism, whether changes in plasma tryptophan result, and if such activation is permanent. The results showed that nicotinic acid activated the enzyme almost as strongly as tryptophan. The results confirmed the activation to be of the "substrate" type, i.e., at least partly due to increased tryptophan concentrations in the liver. In repeated nicotinic acid administration plasma tryptophan levels did not diminish, despite the high activation of tryptophan 2,3-dioxygenase (high flux of tryptophan through the kynurenine pathway). However, the activation disappeared after 11 days of treatment. The powerful and sustained activation of tryptophan 2,3-dioxygenase by nicotinic acid may at least partly explain some of its side effects, such as glucose intolerance. However, mental disturbance as a side effect of diminished brain levels of tryptophan is not supported by these findings.

Sites of cAMP and ATP binding which regulate acetyl-CoA-carboxylase phosphorylation rate characterized under conditions of lipogenesis intensification and nicotinic acid action on this enzyme 1500 fold purified and containing proteinkinase activity. The acetyl-CoA-carboxylase preparation contains only one type of the cAMP binding sites which possess higher capacity under the action of nicotinic acid in vivo. A decrease of the cAMP binding under the conditions of lipogenesis intensification is induced by diminution of the cAMP binding site capacity without changing the binding constant value. It is established that [gamma-32P]ATP is incorporated in enzyme with Km value equal for two states under study. It this case the [gamma-32P]ATP incorporation rate is much higher for acetyl-CoA-carboxylase produced from chicken liver under the action of nicotinic acid.

Protein kinase strong-associated with acetyl-CoA-carboxylase is isolated from the liver of chicken and 300-fold purified with alimentary intensification of lipogenesis and under the effect of nicotinic acid against this background. The obtained enzymes are studied comparatively. It is found that their preparations are phosphorylated with different rate, have two pH optima and differ in the sensitivity to cAMP and to thermostable protein inhibitor. The hydrophobic chromatography was used to separate components of the acetyl-CoA-carboxylase-protein kinase complex and to reveal in the chicken liver cAMP-dependent and cAMP-independent protein kinases highly specific to acetyl-CoA-carboxylase and strongly bound with it.

Pharmacological doses of niacin and its analogues were given intraperitoneally to rats with and without coadministration of a hepatocarcinogenic dose of diethylnitrosamine (DEN), and their effects on the induction of ornithine decarboxylase (ODC, EC 4.1.1.17) activity in the rat liver were studied. The induction of ODC activity by DEN was inhibited by 74.3, 85.5, 94.6, 97.6, 72.6 and 55.2% by nicotinamide, nicotinic acid, 3-hydroxymethylpyridine, beta-picoline, pyridine-3-aldehyde and ethylnicotinate respectively. When given alone, these analogues did not induce ODC activity. All these compounds are known to have a niacin effect. DEN-induced ODC activity was also inhibited by 84.0, 93.3, 52.8 and 75.9% by 6-aminonicotinamide, picolinic acid, pyridine-3-sulfonic acid and thionicotinamide, respectively, but, peculiarly, they induced ODC activity by their administration alone. These niacin analogues are known to have anti-niacin effects. Tryptophan, N'-methylnicotinamide and isonicotinic acid hydrazide did not affect the DEN-induced ODC activity but could induce ODC by themselves. Tryptophan belongs to the former group and isonicotinic acid hydrazide to the latter group. The reason for these discrepancies is discussed.

Nicotinic acid in vivo affects the citrate demand for acetyl-CoA-carboxylase activation in the chicken liver under conditions of alimentary lipogenesis stimulation. Stoichiometry of the citrate binding with the dissociation constant of the enzyme-allosteric activator complex is determined under experimental conditions. Endogenic phosphorylation of acetyl-CoA-carboxylase completely correlates with its inactivation and depends on the citrate level. cAMP is established to have an activating effect on phosphorylation of acetyl-CoA-carboxylase of test animals.

Acetyl-CoA-carboxylase is isolated and purified to a homogeneous state from the chicken liver with alimentary lipogenesis stimulation. Under the action of nicotinic acid in vivo the specific enzyme activity is shown to decrease considerably followed by some variations in its properties. According to the results obtained during ultracentrifugation and PAAG electrophoresis nicotinic acid causes partial enzyme deaggregation with simultaneous increase of its phosphorylation. The latter is accompanied by a rise in the content of phosphate labile to alkali on acetyl-CoA-carboxylase subunits. Nicotinic acid in vivo has practically no effect on acetyl-CoA-carboxylase synthesis and decay rate. Its inhibiting action is induced by stimulation of enzyme phosphorylation.

Nicotinic acid and nicotinamide (100 mg/kg) increase the activity of the rat liver microsomal uridine diphosphate-glucuronyltransferase by 55 and 73.8%. Administration of nicotinamide in combination with ziksorin or phenobarbital enhanced the enzyme-inducing effects of the latter.

Stimulation of fatty acid biosynthesis by feeding high-carbohydrate diet to chickens after fasting induces a 38% and 23% rise in the 3':5'-cyclo-nucleotide phosphodiesterase and adenylate cyclase activities in the liver tissue, respectively. Administration of nicotinic acid (150 mg per 1 kg of weight) to chickens with stimulated biosynthesis of fatty acids leads to a sharp (70% at an average) decrease of 3':5'-cyclo-nucleotide phosphodiesterase activity, the adenylate cyclase activity under these conditions remains practically unchanged. The participation of the adenylate cyclase system in regulating the fatty acid biosynthesis under mentioned conditions, the role of phosphodiesterase are discussed.

An investigation was made into the effects of conditioned running (1 h and 2 h at 20 m min-1), which accelerates lipolysis, on the concentrations of tryptophan (Trp) in plasma, liver and brain and on 5-hydroxytrptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in brain. Running caused time-dependent increases in plasma free Trp and brain Trp of the rat, leading to increased brain 5-HT turnover as revealed by higher amounts of its metabolite, 5-HIAA. The ratio of brain Trp to plasma free Trp was decreased after 2 h of running. Liver Trp content rose only after 3 h of running, while liver unesterified fatty acid (UFA) concentrations remained unmodified. A comparison between food deprivation and running (both of which promote lipolysis) was performed. Running for 2 h affected to the same extent plasma Trp disposition when compared with 24 h food deprivation. Nevertheless, the ratio of brain Trp to plasma free Trp was decreased in the food-deprived rats, when compared to the runners. Nicotinic acid, which inhibits fat catabolism, completely abolished the plasma UFA increase induced by 1 h of running. The drug did not affect plasma free Trp, brain Trp, 5-HT or 5-HIAA but enhanced plasma total Trp level. Naloxone, an opiate antagonist, which decreased running-induced lipolysis, did not alter plasma Trp disposition. Desipramine, an antidepressant compound, affected only peripheral Trp concentrations of the runners. Plasma free and total Trp concentrations were increased in desipramine-treated runners, compared with saline-treated runners. In addition, desipramine increased the ratio of brain Trp to plasma free Trp of the runners. Brain 5-HT and 5-HIAA were increased in both desipramine-treated controls and runners. 9 The results suggest that running, which like food deprivatiQn accelerates lipolysis, increases brain Trp content and then 5-HT turnover. Comparison of these two physiological situations suggests that effectiveness of brain Trp entry is much more altered by fasting.

The pyruvate dehydrogenase and ATP-citrate(pro-3S)lyase activity in the liver after administration of nicotinic acid to chickens against a background of stimulated lypogenesis is shown to increase in the period of maximum fall of the acetyl-CoA-carboxylase activity. Affinity of ATP-citrate (pro-3S)-lyase to citrate at this time is lowered, the content of citrate and isocitrate is elevated and CoASac is decreased to some extent. A conclusion is made on the absence of substrate limitation of acetyl-CoA-carboxylase with administration of nicotinic acid to chickens under conditions of the fatty acid biosynthesis intensification.

The paper deals with the analysis of data available in literature and those of the authors' own investigations concerning the structure, properties and regulation of acetyl-CoA-carboxylase. Nicotinic acid is one of the factors regulating the enzyme activity in the animal liver. It inhibits the acetyl-CoA-carboxylase activity through two mechanisms--allosteric regulation and covalent modification. A comparative characteristic of the studied enzyme preparations has shown that administration of nicotinic acid to animals leads to phosphorylation of acetyl-CoA-carboxylase which affects its structure. A complex with homogenic acetyl-CoA-carboxylase is found to contain cAMP-independent and cAMP-dependent protein kinase. The phosphorylation is controlled by citrate competing with nicotinic acid for the coupling sites.