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The metabolism of apolipoprotein B-100 was studied in three patients with familial hyperchylomicronemia (type I hyperlipoproteinemia) using a very low density lipoprotein (VLDL) dual-tracer technique. Radioiodinated VLDL1 (Sf 60-400) and VLDL2 (Sf 20-60) were injected and their catabolism and rate of the transfer of apoB into VLDL2, intermediate density lipoprotein (IDL) (Sf 12-20), and low density lipoprotein (LDL) (Sf 0-12) were compared in patients and in five normolipidemic controls. The rates of delipidation of large triglyceride-rich VLDL1 to VLDL2 (0.26-0.54 pools/day vs. 2.5-5.2 pools/day in controls) and VLDL1 direct catabolism (0.33-0.92 pools/day vs. 4.2-14.7 pools/day in controls) were found to be significantly reduced in type I patients resulting in a tenfold increase of VLDL1 pool size. ApoB synthesis into this density interval was, however, normal as was that into smaller VLDL2. the circulating apoB mass in VLDL2 was not increased. In fact, apart from a modest decrease in the rate of VLDL2 delipidation to IDL and LDL, the behavior of apoB in this density interval was similar in hyperchylomicronemic and normal subjects. Likewise, the transfer of apoB through the IDL and LDL density ranges was not significantly different from normal. Pool sizes of these fractions, however, were reduced, the latter significantly (354-491 mg vs. 1,160-2,505 mg in controls) due to increased direct catabolism in hyperchylomicronemic patients. The results of this study indicate that lipoprotein lipase deficiency primarily affects VLDL1 metabolism, both its delipidation and direct removal from plasma. Lipolysis further down the delipidation cascade is not dependent on this enzyme. Hypercatabolism rather than a failure of synthesis of IDL and LDL was responsible for the decreased pools for both lipoproteins.

The molecular basis of familial chylomicronemia (type I hyperlipoproteinemia), a rare autosomal recessive trait, was investigated in six unrelated individuals (five of Spanish descent and one of Northern European extraction). DNA amplification by polymerase chain reaction (PCR) followed by single strand conformation polymorphism (SSCP) analysis allowed rapid identification of the underlying mutations. Six different mutant alleles (three of which are previously undescribed) of the gene encoding lipoprotein lipase (LPL) were discovered in the five LPL-deficient patients. These included an 11 bp deletion in exon 2, and five missense mutations: Trp 86 Arg (exon 3), His 136 Arg (exon 4), Gly 188 Glu (exon 5), Ile 194 Thr (exon 5), and Ile 205 Ser (exon 5). The Trp 86 Arg mutation is the only known missense mutation in exon 3. The other missense mutations lie in the highly conserved "central homology region" in close proximity with the catalytic site of LPL. These and other previously reported missense mutations provide insight into structure/function relationships in the lipase family. The missense mutations point to the important role of particular highly conserved helices and beta-strands in proper folding of the LPL molecule, and of certain connecting loops in the catalytic process. A nonsense mutation (Arg 19 Term) in the gene encoding apolipoprotein C-II (apoC-II), the cofactor of LPL, was found to underlie chylomicronemia in the sixth patient who had normal LPL but was apoC-II-deficient.

Primary familial forms of chylomicronemia can lead to acute life-threatening complications, especially acute pancreatitis. The main aim of therapy is to avoid this so-called chylomicronemia syndrome. In 12 patients with primary chylomicronemia due to familial hypertriglyceridemia, the addition of 2.16 g omega-3 fatty acids over 4 weeks and 4.32 g for 8 weeks resulted in a decrease of serum triglyceride levels from 1,624 +/- 333 to 894 +/- 241 mg/dL after 12 weeks. Cholesterol and triglyceride levels in the chylomicron fraction were reduced concomitantly, the apolipoprotein B-100/B-48 ratio increased, very--low-density lipoprotein (VLDL) triglycerides, VLDL cholesterol, and total cholesterol levels decreased, and low-density lipoprotein (LDL) cholesterol showed a tendency to increase, but this finding did not reach significance. High-density lipoprotein (HDL) cholesterol levels remained unchanged, as did the levels of apolipoproteins A-I, A-II, and E, and lipoprotein(a). Apolipoprotein B levels decreased significantly. The decrease of triglyceride levels to still-elevated concentrations was accompanied by a substantial decrease in plasma and whole-blood viscosity and erythrocyte aggregation, which reached normal values. As in chylomicronemia, complications usually occur at triglyceride levels higher than 1,500 mg/dL; patients can still profit from treatment with omega-3 fatty acids, even though triglyceride levels are still substantially elevated. No clinically relevant side effects occurred, with the exception of the manifestation of diabetes mellitus in one patient, which could be reversed after discontinuation of treatment.

In humans with the lipoprotein lipase deficiency disorder large amounts of chylomicrons and large very low-density lipoprotein (VLDL) accumulate in plasma. In spite of this, atherosclerosis does not seem to develop at an accelerated rate, suggesting that these lipoproteins do not promote atherogenesis. In humans with dysbetalipoproteinemia remnant lipoproteins (intermediate density lipoprotein (IDL) plus beta-VLDL) accumulate in plasma and these particles may therefore be the factor causing accelerated atherosclerosis in this disorder. Epidemiological studies in humans suggest that IDL or remnant lipoproteins are predictors of the severity or progression of atherosclerosis. Similar studies in the St. Thomas' Hospital rabbit strain, an animal model with genetically elevated plasma levels of VLDL, IDL and low-density lipoprotein (LDL), showed that IDL or remnant lipoproteins were better predictors of the extent of atherosclerosis than were LDL or VLDL. Studies of lipoprotein/arterial wall interactions have demonstrated that the larger the lipoprotein particle, the lower the influx into intima. Very large VLDL and chylomicrons do not seem to enter intima. Although high-density lipoprotein (HDL) enters intima faster than other lipoproteins, the small HDL particles seem to penetrate the entire arterial wall and leave via lymphatics and vasa vasorum in the outer media and adventitia. In contrast, LDL, and possibly also IDL and smaller VLDL, may only leave the intima via the lumen of the artery. In conclusion, a substantial body of evidence suggests that remnant lipoproteins (IDL and smaller VLDL) share with LDL the potential for promoting atherosclerosis, whereas very large VLDL and chylomicrons do not seem to have this effect.

The chemical mismatch method has been utilized to screen for mutations in the apoC-II gene of a patient with familial chylomicronemia and apoC-II deficiency. Cleavage of heteroduplexes formed between normal and patient DNA strands with hydroxylamine and osmium tetroxide readily localized a mutation near base 2660 of the mutant apoC-II. Sequence analysis of PCR amplified patient DNA in the mismatched region localized by this method identified the substitution of a thymidine (T) for a cytosine (C) at base 2668 in exon 2 of the patient's gene within a CpG dinucleotide. The C to T transition in the apoC-IIParis2 gene leads to the introduction of a premature termination codon (TGA) at a position corresponding to amino acid-19 of the signal peptide of apoC-II and the formation of a new Nla III restriction enzyme site absent in the normal apoC-II gene. Consistent with the history of consanguinity in this kindred, amplification of DNA isolated from the proband's parents by the polymerase chain reaction and digestion with Nla III established that the proband is a true homozygote for this genetic defect. Analysis of the patient's plasma by two-dimensional gel electrophoresis and immunoblotting failed to detect any plasma apoC-II. Thus, we have identified a novel mutation in the apoC-II gene of a patient with apoC-II deficiency from a Paris kindred presenting with severe hypertriglyceridemia and chylomicronemia.(ABSTRACT TRUNCATED AT 250 WORDS)

This chapter has outlined the reasons for the development of marked elevations of triglycerides that can be associated with the chylomicronemia syndrome. The clinical features of the syndrome have been discussed, with particular emphasis on chylomicron-induced pancreatitis, since this can be life-threatening. An approach to the diagnosis and management of this syndrome has been provided, with emphasis on the need for long-term follow-up and compliance to keep plasma triglycerides to a reasonable level and thereby avoid recurrent clinical complications due to chronic chylomicronemia.

To determine the molecular basis for type I hyperlipoproteinemia in two Austrian families, the lipoprotein lipase (LPL) gene of two patients exhibiting LPL deficiency was analyzed by Southern blotting and by direct genomic sequencing of DNA amplified by polymerase chain reaction (PCR). All exons of the LPL gene except part of the noncoding region of exon 10, all splice donor and acceptor sites, as well as 430 basepairs of the 5'-region including the promotor were sequenced. A homozygous substitution of adenine for guanine in the fifth exon at cDNA position 818 of the LPL gene was found in both patients. Our sequencing strategy largely ruled out a linkage disequilibrium of the identified nucleotide change with another defect potentially causing the clinical phenotype. The base change described abolishes a normally present AvaII restriction site allowing the identification of carriers of the mutant allele by AvaII digestion of PCR fragments of exon 5; three members of the two families were homozygous for this mutation and ten members were heterozygous. The activity of LPL in postheparin plasma was almost completely absent in homozygotes and about half normal in heterozygotes. The loss of activity was related to LPL protein structure. This mutation alters the amino acid sequence at residue 188 from Gly to Glu. The conformational preferences of the protein chain around position 188 were calculated with the use of a knowledge-based computerized method. The most probable conformation is a beta-turn formed by residues 189-192. The mutation seems to destabilize the beta-turn and/or a yet larger domain critical for substrate alignment.

Newborn infants with inborn errors of lipid metabolism are relatively rare. There are only a few reports of hyperlipoproteinemia in infancy. In a six-day-old "healthy" newborn infant with a weight of 2960 gm a milky blood serum was found. Diagnosis of primary hyperlipoproteinemia type I was established by exclusion of other diseases and by analysis of blood lipids. Lack of lipoprotein lipase activity in postheparin plasma confirmed the diagnosis. The parents of the patient were normolipemic but showed reduced lipoprotein lipase activity. Thus, they were recognized as heterozygotes. A low fat (6-19 gm fat/d, 2-3 gm fat/kg bodyweight/d) diet with a high proportion of medium-chain triglycerides (78% MCT) lowered the initially extremely high serum triglyceride level (3674 mg/dl) significantly (214-711 mg/dl). Until the age of 7 months body weight and length followed the 10. percentile, head circumference followed the 50. percentile. Unexpectedly the patient died of pulmonary infection at the age of 8 months.

In patients with familial lipoprotein lipase deficiency (FLPL-d) and glycogen storage disease type I (GSD-I), hypertriglyceridaemia (1445 +/- 247 and 1082 +/- 312 mg dl-1, n = 5 per group) was associated primarily with reduced extrahepatic lipoprotein lipase (LPL) activity (0.33 +/- 0.33 and 1.69 +/- 0.38 mumol FFA ml-1 h-1) when compared with controls (4.83 +/- 0.90). Hypercholesterolaemia was characterized by elevated LDL cholesterol (191 +/- 30 and 344 +/- 34 vs. 115 +/- 5 mg dl-1 in controls P less than 0.01) and low HDL cholesterol (12 +/- 2 and 22 +/- 2 vs. 56 +/- 3 in controls, P less than 0.001). In order to ascertain the role of LPL in the interconversion and remodelling of lipoproteins in these disorders, we analysed lipid and lipoprotein profiles before and following in vitro incubation of patient plasma with purified milk LPL (EC 3.1.1.34) for 6 h at 37 degrees C. The efficiency of exogenous LPL in vitro was demonstrated by the extent of hydrolysis of chylomicrons and of VLDL-TG in both groups. Concomitant with the disappearance of TG-rich lipoprotein particles, a consistent per cent increment of IDL (99.2 +/- 30.8 and 43.9 +/- 70.5), LDL (152.8 +/- 36.2 and 137.0 +/- 36.1) and of HDL2 (144.8 +/- 29.4 and 99.8 +/- 18.7) was observed in both groups of patients. The enhancement of the latter fractions contrasted with the decline of HDL3 mass concentration (25.4 +/- 7.7 and 51.4 +/- 5.8%), suggesting that a major shift of HDL3----HDL2 occurs following in vitro lipolysis by LDL. Simultaneous compositional and morphological changes of individual lipoprotein particles were noted, confirming the dynamic movement and exchange of neutral lipids and proteins. Specificity of LPL results was demonstrated by experiments in which incubation of the whole plasma at 37 degrees C without exogenous lipolytic enzyme did not cause any substantial changes. The present study, therefore, demonstrates a correction of the major lipoprotein abnormalities associated with FLPL-d and GSD-I by exogenous LPL. No substantial difference was noted between primary (FLPL-d) and secondary (GSD-I) hyperlipidaemias. These studies allow us to conclude that a simple in vitro system, utilizing an exogenous source of LPL and plasma from patients, may serve as a suitable model for the study of the metabolic relationships of lipoproteins. However, in view of the fact that the extent of lipolysis achieved in vitro did not differ between FLPL-d and GSD-I, it may not be able to separate primary from secondary hyperlipaemias.

A monoclonal antibody to lipoprotein lipase (LPL) has been used in an enzyme-linked immunosorbent assay (ELISA) for LPL protein mass. Measurement of LPL immunoreactive mass in pre- and postheparin plasma distinguished three classes of abnormalities in patients with classical deficiency of lipoprotein lipase activity. The class I defect consisted of the absence of LPL immunoreactive homodimer in pre- and postheparin plasma compatible with a potential 'null allele'. Patients with a class II defect had almost no LPL immunoreactive mass in preheparin plasma but showed an increase in their LPL mass of 68 +/- 23 ng ml-1 (mean +/- SD) after heparin. Patients with the class III defect had considerable amounts of LPL immunoreactive material in preheparin plasma (159 +/- 190 ng ml-1). Heparin administration, however, caused very little additional release of LPL into the plasma (16 +/- 51 ng ml-1). Thus although both class II and class III patients had an LPL protein with abnormal catalytic activity, class III patients also appeared to have a defect in heparin binding of LPL. To test this hypothesis, postheparin plasma of classes II and III patients was analysed by heparin-Sepharose chromatography. In contrast to class II patients, the LPL immunoreactive mass of class III patients did not show affinity for the heparin and eluted in the column void volume, suggesting the class III defect is also associated with a defect in heparin binding.

Five of six children with severe familial primary hypertriglyceridemia in the first three to five years of life had what we believe is a new clinical feature, intermittent swelling of the limbs and scrotum. This was associated with higher levels of plasma triglyceride (greater than 2,000 mg/dL) and, in some instances, venous stasis of the affected part. All children had severe elevation of plasma chylomicron levels and mild elevation of very low-density lipoprotein levels. Four of the subjects were siblings who demonstrated heterogeneity in plasma lipase activities after heparin sodium administration. The clinical features and the lipase heterogeneities suggest that these patients possibly have an unusual variant of primary hypertriglyceridemia.

Plasma lipids, lipoproteins, tissue lipoprotein lipase (LPL) and hepatic lipase (H-TGL) were studied in 7 patients with familial hyperchylomicronemia from four different families. Their first-degree relative were also studied. The patients were heterogeneous for the genetic defect; LPL activity was absent in five patients (LPL deficiency) but normal in two. However, these two did not have apo C-II, the physiological activator of LPL (C-II deficiency). There were no significant differences in the clinical picture between patients with LPL deficiency and C-II deficiency. In both mutants, marked hypertriglyceridemia was due to an accumulation of lipoproteins of density less than 1.006 g/ml. The LDL fraction was very reduced and abnormal in composition, presenting a CH/TG ratio of 0.5. The plasma apolipoprotein B (apo B) level was low (67 +/- 5.5 mg/dl) and was transported mainly in the VLDL fraction (26 +/- 3.2 mg/dl) rather than in the LDL fraction (15 +/- 1.4 mg/dl). Very low levels of cholesterol and apolipoprotein A-I in HDL subfractions HDL2 and HDL3 were also recorded. Only 3 out of the 24 first-degree relatives of patients with LPL deficiency showed even a small increase in plasma triglycerides, but 15 had low or low to normal LPL values. H-TGL levels were normal in all subjects. The 4 first-degree relatives of C-II deficiency patients showed normal levels of plasma lipids. LPL and H-TGL, and 2 children of 1 patient showed normal distribution of apo C peptides in their VLDL. A block in chylomicron catabolism, due to the absence of LPL or apo C-II, may lead to a massive accumulation of lipoproteins with a density less than 1.006 g/ml, and a drastic reduction in the LDL and HDL fractions. Low LPL values in the first-degree relatives of LPL deficiency patients might represent a biochemical marker for healthy carriers of LPL deficiency.

This report describes studies on the plasma and milk lipid composition of a patient with primary Type I hyperlipoproteinemia who had been followed through her second pregnancy. Post-partum she lactated, supplying milk for assay. It was abnormal in the low content of its total lipid and in the bizarre composition of its fatty acids. The proportion of long chain fatty acids was unusually low, and that of medium chain fatty acids unusually high. Furthermore, the fatty acids of the patient's milk differed greatly from those of her plasma triglycerides. This was in marked contrast to normal nursing mothers' milk, in which the fatty acid composition is comparable to that of plasma triglycerides. The patient's milk fatty acids were shorter in chain length and deficient in essential fatty acids. During the time of lactation, the patient remained hyperlipidemic and her post-heparin plasma had no lipolytic activity. These data and the differences between the plasma and milk fatty acids suggested that in the patient the circulating triglyceride fatty acids did not enter the mammary gland. Without preformed fatty acids entering it from plasma or adipose tissue, the lactating breast apparently synthesized fatty acids de novo. These newly synthesized fatty acids were of medium, rather than long chain length. This accounted for the abundance of medium chain length triglycerides in the patient's milk. The studies suggested that the deficit of lipoprotein lipase in Type I hyperlipoproteinemia extended to the mammary gland.

Because there are no characteristic clinical or biochemical manifestations, the heterozygote state for lipoprotein lipase (LPL) deficiency has been difficult to detect. Measurements of postheparin plasma LPL activity and of LPL mass were performed in six families of probands with LPL deficiency to characterize the heterozygote state. LPL mass was measured in a sandwich enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (5D2) that had been produced against bovine milk LPL. Thirteen obligate heterozygotes from these families had reduced LPL activity and mass below the 95th percent confidence limits of 34 normal controls, while one obligate heterozygote had LPL activity and mass between the 90th and 95th percent confidence limits. Potential heterozygotes in these families were identified as normal (n = 8) or heterozygotes (n = 6) by comparison to the 95th percent confidence limits of the controls. Some relatives in four of the six families exhibited mild hyperlipidemia, similar to the pattern seen in familial combined hyperlipidemia (FCHL). The hyperlipidemia segregated with the heterozygote state for LPL deficiency in these families (p less than 0.03). High density lipoprotein (HDL) cholesterol was significantly reduced in the heterozygotes for LPL deficiency (p less than 0.01). The measurement of LPL activity and mass allows identification of the heterozygote state for LPL deficiency, which is characterized by variable expressions of hyperlipidemia and reduced HDL cholesterol. These results suggest that the heterozygote state for LPL deficiency may form one subset of FCHL.

Primary Type I Hyperlipoproteinaemia is the rarest phenotype of the inherited hyperlipidaemias. A study of the plasma lipids, lipoprotein distribution and apoprotein concentrations has been carried out in a propositus and in three generations of her immediate family, as has associated post heparin lipolytic activity (PHLA). The propositus presented in infancy with spontaneous bruising and abdominal pain. She has gross chylomicronaemia in the presence of depressed PHLA, which is due to deficiency of lipoprotein lipase. Apo Al and B levels are depressed in concert with low density and high density lipoproteins. Introduction of a low fat diet has resulted in loss of symptoms but plasma lipids remain abnormal. Within her immediate family, six of nine members have depressed PHLA. Two such members have elevated plasma triglycerides, one associated with hypercholesterolaemia and peripheral vascular disease. Low PHLA in this family is then associated with different lipoprotein phenotypes.

Defective lipoprotein lipase (LpL) was found in the postheparin plasma (PHP) of a patient with severe hypertriglyceridaemia. The patient was a 14-year-old girl with a maximum plasma triglyceride (TG) level of 3600 mg d-1 who had been suffering from recurrent pancreatitis. The patient's LpL purified from the PHP by heparin-Sepharose and phenyl-Sepharose chromatographies hydrolysed tributryrin, but not triolein emulsified with Triton X-100 and phosphatidylcholine (PC), or in chylomicrons, whereas normal LpL hydrolysed these substrates. Moreover, unlike normal LpL, LpL from the patient did not associate with VLDL, as shown by Sepharose 4B column chromatography. The patient's LpL hydrolysed triolein emulsified with lysophospholipid at a normal rate in the presence of apolipoprotein CII. These findings suggest that this patient has LpL with a normal catalytic site for tributyrin but with a defect in lipid interface recognition resulting in loss of ability to recognize VLDL or chylomicrons, but not of triolein emulsified with lysophospholipid.

Primary hyperlipoproteinaemia (hyperchylomicronaemia) with a slight increase in very low density lipoprotein) is described in 20 cats. Fasting hyperlipaemia, lipaemia retinalis and peripheral neuropathies were the most frequently detected clinical signs. The disease is thought to be inherited as an autosomal recessive trait but the exact mode of inheritance has not been determined. Affected cats showed reduced lipoprotein lipase activity measured after heparin activation compared with the response in normal cats. Plasma triglyceride and cholesterol were increased in all the cats with the major proportion of triglyceride and cholesterol being present in chylomicrons. The peripheral nerve lesions were caused by compression of nerves by lipid granulomata. It is probable that the lipid granulomata result from trauma because the nerves most often affected were at sites like the spinal foraminae where they were susceptible to trauma.

Serum lipoprotein pattern, apoproteins and two postheparin triglyceride lipases were analyzed in a patient with familial lipoprotein lipase (LPL) deficiency and her family. Serum of the patient showed extreme hyperchylomicronemia and her postheparin plasma LPL activity was distinctly decreased. None of heterozygotes had any type of hyperlipoproteinemia. The mother and brother of the patient had moderately decreased LPL activity. There were no consistent changes in hepatic triglyceride lipase (H-TGL) activity among heterozygotes. These results suggest that assay of LPL may be helpful for detection of heterozygotes in familial LPL deficiency.

The underlying pathophysiological defect was studied in four siblings with familial hyperchylomicronemia. Deficiency of apolipoprotein C-II and E-3 was identified. In addition, these subjects had markedly decreased LPL activity in postheparin plasma. Addition of normal plasma to the assay as source for apoC-II enhanced LPL activity only to a limited extent. In contrast with previously reported patients with apoC-II deficiency, a far less pronounced effect of intravenous infusion of normal plasma was seen in one of the siblings, probably due to the combined deficiency of apoC-II and LPL. Plasma VLDL-TG turnover rate was not decreased in one of the siblings with apoC-II and LPL deficiency, suggesting different metabolic pathways for chylomicrons and VLDL. Family study confirmed an autosomal recessive mode of inheritance both for apoC-II and for apoE-3 deficiency. The mode of inheritance for LPL deficiency could not be established exactly.

Patients with type 1 hyperlipoproteinaemia present with relatively normal plasma levels of very-low-density lipoprotein (VLDL) together with marked hyperchylomicronaemia. Since the aetiological basis for the type 1 phenotype is a severe reduction in peripheral lipoprotein lipase (LPL) activity, the discrepancy between the two triglyceride-rich lipoprotein fractions is paradoxical. In order to account for these observations we have previously proposed (G.M.B) that the hepatic secretion of lipoproteins in these patients is characterized by the production of chylomicron-like particles rather than VLDL. The implications of this hypothesis were examined in 3 patients with type 1 hyperlipoproteinaemia. A high-carbohydrate diet led to the enhancement of hyperchylomicronaemia in 2 of the 3 subjects. The severity of the defect in peripheral LPL activity correlated with the tendency to hyperchylomicronaemia, and in vitro assay suggested that hepatic lipase was capable of hydrolysing chylomicron triglyceride when present at the high concentrations characteristic of type 1 hyperlipoproteinaemia. These results were compatible with the above hypothesis implicating the liver as a partial determinant of the type 1 phenotype.

In normal subjects, apolipoprotein E (apo E) is present on very low density lipoproteins (VLDL) (fraction I) and on particles of a size intermediate between VLDL and low density lipoproteins (LDL) (fraction II). The major portion of apo E is, however, on particles smaller than LDL but larger than the average high density lipoproteins (HDL) (fraction III). To investigate the possible role of the vascular lipases in determining this distribution of apo E among the plasma lipoproteins, we studied subjects with primary deficiency of either hepatic lipase or of lipoprotein lipase and compared them with normal subjects. Subjects with familial hepatic triglyceride lipase deficiency (n = 2) differ markedly from normal in that fraction II is the dominant apo E-containing group of lipoproteins. When lipolysis of VLDL was enhanced in these subjects upon release of lipoprotein lipase by intravenous heparin, a shift of the apo E from VLDL into fractions II and III was observed. In contrast, apolipoproteins CII and CIII (apo CII and CIII, respectively) did not accumulate in intermediate-sized particles but were shifted markedly from triglyceride rich lipoproteins to HDL after treatment with heparin. In subjects with primary lipoprotein lipase deficiency (n = 4), apo E was confined to fractions I and III. Release of hepatic triglyceride lipase by heparin injection in these subjects produced a shift of apo E from fraction I to III with no significant increase in fraction II. This movement of apo E from large VLDL and chylomicron-sized particles occurred with little hydrolysis of triglyceride and no significant shift of apo CII or CIII into HDL from triglyceride rich lipoproteins. When both lipoprotein lipase and hepatic triglyceride lipase were released by intravenous heparin injection into normal subjects (n = 3), fraction I declined and the apo E content of fraction III increased by an equivalent amount. Either moderate or no change was noted in the intermediate sized particles (fraction II). These data strongly support the hypothesis that fraction II is the product of the action of lipoprotein lipase upon triglyceride rich lipoproteins and is highly dependent on hepatic triglyceride lipase for its further catabolism. In addition, the hydrolysis by hepatic triglyceride lipase of triglyceride rich lipoproteins in general results in a preferential loss of apo E and its transfer to a specific group of large HDL.

Familial chylomicronemia is a rare genetic disorder attributable to the absence of lipoprotein lipase activity or the absence of apo-CII, i.e., the cofactor for the same enzyme. Plasma lipoproteins were analyzed by zonal ultracentrifugation under rate flotation conditions in four patients with lipoprotein lipase deficiency and two patients with apo-CII deficiency. Lipoproteins of density less than 1.006 gm/ml, and particularly lipoproteins with Sf greater than 100, were present in very high concentrations. Low levels of density greater than 1.006 gm/ml lipoproteins were observed. This fraction was composed of some different and discrete lipoprotein populations: intermediate-density lipoproteins (in three of six patients, density = 1.006 to 1.019 gm/ml); low-density lipoprotein LDL2 (in all patients, density = 1.019 to 1.045 gm/ml); low-density lipoprotein LDL3 (in all patients, density = 1.045 to 1.063 gm/ml); high-density lipoprotein HDL2 (in four of six patients); and high-density lipoproteins HDL3 (in all patients). LDL3 was never observed in normal participants by means of zonal ultracentrifugation; this subclass of low-density lipoproteins seems to correspond to LDL particles of very low Sf (2 to 5) previously identified by analytical ultracentrifugation in patients with severe hypertriglyceridemia. LDL3 was isolated by means of zonal ultracentrifugation as a single and discrete peak in all patients. Lipoproteins of density greater than 1.006 gm/ml were rich in triglycerides and poor in cholesterol in comparison with normal lipoproteins. The heterogeneity of low-density lipoproteins (particularly the appearance of LDL3), low levels of total high-density lipoproteins, and lower HDL3 flotation rate than normal are typical aspects of serum lipoproteins in these patients. No significant differences in the lipoprotein profiles of the patients with lipoprotein lipase deficiency in comparison with patients with apo-CII deficiency were found. In both groups of patients, the plasma lipoproteins profile and the altered lipoprotein composition could be related to the impaired catabolism of triglyceride-rich lipoproteins caused by the absence of lipoprotein lipase activity.

The peripheral production of high density lipoprotein (HDL) cholesterol and of the subclasses HDL2 and HDL3 was assessed by measurement of the arteriovenous fluxes across the human forearm, at rest and after 20 min isometric exercise in the forearm. Eight subjects were studied twice--fasting and after a high-fat meal--and one other subject was studied only after fat loading. In the fasted state the net fluxes of HDL2 and HDL3 cholesterol were slightly negative in the resting forearm, but they became positive during exercise, indicating greater production during short-term muscular activity. The effect of exercise, particularly that on HDL3 cholesterol, was greatly increased by a high-fat meal; the difference in HDL3 cholesterol arteriovenous flux between rest and exercise was significant (-0.06 [SEM 0.05] vs 0.51 [0.17] mumol/100 ml forearm/min). By contrast, there was no peripheral production of HDL2 or HDL3 cholesterol during exercise in two patients with lipoprotein lipase deficiency. These findings suggest that formation of HDL3 during lipolysis by lipoprotein lipase in the muscle capillary bed is influenced by the supply of chylomicrons and other lipoprotein substrates for this enzyme. Muscle blood flow may therefore be an important determinant of HDL production by this mechanism. The effect of exercise in raising HDL cholesterol, and the inverse relation between exercise and coronary heart disease, may be partly the result of this process.

Lipoprotein lipase (LPL) and hepatic lipase (HL) are enzymatic activities involved in lipoprotein metabolism. The purpose of this study was to analyze the physicochemical modifications of plasma lipoproteins produced by LPL activation in two patients with apoC-II deficiency syndrome and by HL activation in two patients with LPL deficiency. LPL activation was achieved by the infusion of normal plasma containing apoC-II and HL was released by the injection of heparin. Lipoproteins were analyzed by ultracentrifugation in a zonal rotor under rate flotation conditions before and after lipase activation. The LPL activation resulted in: a reduction of plasma triglycerides; a reduction of fast-floating very low density lipoprotein (VLDL) concentration; an increase of intermediate density lipoprotein (IDL), which maintained unaltered flotation properties; an increase of low density lipoproteins (LDL) accompanied by modifications of their flotation rates and composition; no significant variations of high density lipoprotein (HDL) levels; and an increase of the HDL flotation rate. The HL activation resulted in: a slight reduction of plasma triglycerides; a reduction of the relative triglyceride content of slow-floating VLDL, IDL, LDL2, and HDL3 accompanied by an increase of phospholipid in VLDL and by an increase of cholesteryl ester in IDL; and a reduction of the HDL flotation rate. These experiments in chylomicronemic patients provide in vivo evidence that LPL and HL are responsible for plasma triglyceride hydrolysis of different lipoproteins, and that LPL is particularly involved in determining the levels and physicochemical properties of LDL. Moreover, in these patients, the LPL activation does not directly change the HDL levels, and LPL or HL does not produce a step-wise conversion of HDL3 to HDL2 (or vice versa) but rather modifies the flotation rates of all the HDL molecules present in plasma.

The anomalous finding that very low density lipoprotein levels are relatively normal in patients with familial hyperchylomicronaemia has never been satisfactorily explained, particularly in view of the marked reduction or absence of peripheral lipoprotein lipase activity characteristic of this condition. I propose that the discrepancy between the plasma levels of the two triglyceride-rich lipoprotein fractions in these patients is due to the secretion by the liver of triglyceride in the form of chylomicron-like particles, rather than as very low density lipoprotein. The proposed "switch" in the spectrum of lipoproteins secreted by the liver is probably contingent upon the activity of the hepatic lipase present on the liver cell plasma membrane.

1. A variant very-low-density lipoprotein was associated with severe hypertriglyceridaemia. Urea-polyacrylamide gel electrophoresis of the tetramethylurea-soluble apolipoproteins of these very-low-density lipoproteins (VLDL) showed that the apolipoprotein C-II content was less than 10% of that in VLDL from hypertriglyceridaemic (3-120 mmol/l) controls. 2. VLDL were incubated with bovine milk lipoprotein lipase (LPL) and a 9,10-3H-labelled triglyceride emulsion. The VLDL deficient in apolipoprotein C-II were a poor activator of LPL, compared with the effect of VLDL with normal content of apolipoprotein C-II obtained from either normal or hypertriglyceridaemic sera. 3. The efficacies of various VLDL as substrates fo activated LPL were examined. Apolipoprotein C-II-deficient VLDL were a poor substrate for the activated enzyme compared with normal or hypertriglyceridaemic VLDL, and compared wtih an artificial triglyceride emulsion. 4. The abnormal VLDL were obtained from a subject with an IgG3 lambda myeloma protein. Intravenous infusion of normal plasma containing apolipoprotein C-II was followed by rapid, complete, but short-lived (5-10 days) clearance of serum triglyceride. The effect was observed on three occasions until treatment of the myeloma was effective. 5. The monoclonal protein behaved as a cryoglobulin, and formed large particle complexes with triglyceride-rich lipoproteins, especially at temperatures below 37 degrees C. The apolipoprotein C-II deficiency, and consequent hypertriglyceridaemia, may be secondary to an autoantibody directed against apolipoprotein C-II. VLDL from relatives with hypertriglyceridaemia, but without myeloma, had normal apolipoprotein content, activated LPL, and were efficient substrates for the enzyme.

The binding of T4, T3, and rT3 to plasma lipoproteins was investigated in normal subjects and patients with abnormal lipoprotein metabolism. Gel filtration on Sepharose CL-6B demonstrated iodothyronine binding to all lipoprotein classes. In the total lipoprotein fraction (density less than 1.210 g/mL), high density lipoproteins (HDL) were the major binders, accounting for 92% of lipoprotein-bound T4, 99% of lipoprotein-bound T3, and 55% of lipoprotein-bound rT3. The estimated iodothyronine binding in normal plasma to HDL, low density lipoproteins (LDL), and very low density lipoproteins (VLDL) was 3%, 0.2%, and 0.03% for T4, 6%, 0.05%, and 0.02% for T3, and 0.1%, 0.1%, and 0.01% for rT3, respectively. These estimates may be low owing to possible dissociation during chromatography and the short incubation period used to avoid changes in lipoprotein structure. In VLDL and LDL deficiency (abetalipoproteinemia), HDL deficiency (Tangier disease), LDL excess (type IIa hyperlipoproteinemia), and VLDL excess (type III, IV, and V hyperlipoproteinemia), the distribution of iodothyronines reflected the lipoprotein abnormality. Variations resulting from altered distribution within HDL subclasses were also found. Binding was saturable, with approximate dissociation constants for VLDL, LDL, and HDL of 10(-5)-10(-6) mol/L. We conclude that thyroid hormones bind specifically to apolipoproteins, although additional binding by solubilization in the lipid components of the lipoproteins may also occur.

Studies were undertaken to investigate the mechanism of the marked accumulation of an apoE-poor very low density lipoprotein (VLDL) subfraction in untreated Type IV and IIb hypertriglyceridemic subjects. Heparin-Sepharose chromatography was used to separate large VLDL (Sf 60-400) from fasted subjects, into an apoE-poor, unbound fraction and an apoE-rich, bound fraction. As a percent of total VLDL protein, the apoE-poor fraction comprised 40 +/- 4% of total VLDL in hypertriglyceridemic subjects versus 25% in normal subjects. Compared to the apoE-rich, bound fraction, this apoE-poor material was found to have a 5-fold lower ratio of apoE to apoC (0.20 +/- 0.06 vs 0.91 +/- 0.18, P less than 0.005), but a 1.5-fold higher ratio of triglyceride to protein (11.41 +/- 0.85 vs 7.97 +/- 0.77, P less than 0.01). In addition, the apoE-poor fraction was found to be enriched 2-fold in apoB-48 (10.30 +/- 2.41% vs 5.73 +/- 1.59% of total apoB, P less than 0.005) compared to the apoE-rich fraction, suggesting that the apoE-poor fraction contains more chylomicron remnants. The amount of this apoE-poor VLDL was markedly reduced following a reduction in VLDL triglyceride levels (a decrease from 40 +/- 4% to 21 +/- 2% of VLDL protein following a 50% reduction in VLDL triglyceride levels). The large VLDL from Type I, III, and V hyperlipoproteinemic subjects subfractionated using heparin-Sepharose showed an equal distribution of apoE between the two fractions in contrast with the Type IV and IIb subjects. The separation of VLDL from Type I, III, and V subjects using heparin-Sepharose involves a mechanism other than apoE binding. Separation in the latter likely results from apoB-100 binding to heparin, as opposed to apoE binding of VLDL from Type IV and IIb subjects.

Hepatic lipase deficiency produces significant distortion in the plasma lipoprotein profile. Particles with reduced electrophoretic mobility appear in very low density lipoprotein (VLDL). Intermediate density lipoprotein (IDL) increases markedly in the circulation and plasma low density lipoprotein (LDL) levels fall. At the same time there is a mass redistribution within the high density lipoprotein (HDL) spectrum leading to dominance in the less dense HDL2 subfraction. The present study examines apolipoprotein B turnover in a patient with hepatic lipase deficiency. The metabolism of large and small very low density lipoproteins was determined in four control subjects and compared to the pattern seen in the patient. Absence of the enzyme did not affect the rate at which large very low density lipoproteins were converted to smaller particles within this density interval (i.e., of VLDL). However, subsequent transfer of small very low density lipoproteins to intermediate density particles was retarded by 50%, explaining the abnormal accumulation of VLDL in the patient's plasma. Despite this, intermediate density particles accumulated to a level 2.4-times normal because their subsequent conversion to low density lipoprotein has been almost totally inhibited. Consequently, the plasma concentration of low density lipoprotein was only 10% of normal. On the basis of these observations, hepatic lipase appears to be essential for the conversion of small very low density and intermediate density particles to low density lipoproteins. The pathways of direct plasma catabolism of these species were not affected by the enzyme defect. In vitro studies were performed by adding purified hepatic lipase to the patient's plasma.(ABSTRACT TRUNCATED AT 250 WORDS)