Search

3-Hydroxyhexobarbital dehydrogenase (3HBD) catalyzes NAD(P)⁺-linked oxidation of 3-hydroxyhexobarbital into 3-oxohexobarbital. The enzyme has been thought to act as a dehydrogenase for xenobiotic alcohols and some hydroxysteroids, but its physiological function remains unknown. We have purified rabbit 3HBD, isolated its cDNA, and examined its specificity for coenzymes and substrates, reaction directionality and tissue distribution. 3HBD is a member (AKR1C29) of the aldo-keto reductase (AKR) superfamily, and exhibited high preference for NADP(H) over NAD(H) at a physiological pH of 7.4. In the NADPH-linked reduction, 3HBD showed broad substrate specificity for a variety of quinones, ketones and aldehydes, including 3-, 17- and 20-ketosteroids and prostaglandin D₂, which were converted to 3α-, 17β- and 20α-hydroxysteroids and 9α,11β-prostaglandin F₂, respectively. Especially, α-diketones (such as isatin and diacetyl) and lipid peroxidation-derived aldehydes (such as 4-oxo- and 4-hydroxy-2-nonenals) were excellent substrates showing low K(m) values (0.1-5.9 μM). In 3HBD-overexpressed cells, 3-oxohexobarbital and 5β-androstan-3α-ol-17-one were metabolized into 3-hydroxyhexobarbital and 5β-androstane-3α,17β-diol, respectively, but the reverse reactions did not proceed. The overexpression of the enzyme in the cells decreased the cytotoxicity of 4-oxo-2-nonenal. The mRNA for 3HBD was ubiquitously expressed in rabbit tissues. The results suggest that 3HBD is an NADPH-preferring reductase, and plays roles in the metabolisms of steroids, prostaglandin D₂, carbohydrates and xenobiotics, as well as a defense system, protecting against reactive carbonyl compounds., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Isatin is an endogenous indole present in mammalian tissues and fluids. This study was designed to test the effect of both acute stress (AS) and gender on isatin's levels in rat serum, heart and brain, using a HPLC-UV detection method. Basal brain isatin levels were higher in females than in males. AS resulted in increased isatin levels in male serum, heart and brain, and in female serum and heart, but not in the brain. The percentage increases were greater in males than in females in all three sources. After stress, there were significant correlations between isatin levels in the serum and the heart. However, there were no correlations between brain levels and those in the other sources. These results consolidate previous evidence that isatin levels are altered by stress and suggest that brain isatin is under separate control from that in the periphery.

Tribulin is the name given to a family of endogenous nonpeptide substances which inhibit monoamine oxidase (MAO) and benzodiazepine binding. It is widely distributed in mammalian tissues and body fluids, and exhibit some species and tissue variations. Several components selectively inhibiting MAO A, MAO B, central and peripheral benzodiazepine binding (tribulins A, B, BZc and BZp, respectively) have been recognised. Tribulin A represents some tissue-specific metabolites of trace amines, whereas isatin is the major component of tribulin B. Tribulin content increases in brain under conditions of stress and anxiety and is reduced under sedation. Changes in tribulin content in the brain are accompanied by corresponding changes in the content of monoamines and their acidic metabolites, and also by altered susceptibility of MAO to specific mechanism-based inhibitors. This suggests that tribulin is involved in MAO inhibitory regulation in vivo.

A peripheral 5-HT(2A) receptor-mediated hemodynamic change prompted formation of indole-2,3-dione, an endogenous inhibitor of atrial natriuretic peptide (ANP) receptor binding and G protein-mediated intracellular signaling (IC(50): 0.4 microM). This effect was significantly suppressed by dexamethasone, indomethacin and the 5-HT(2) receptor antagonists, ketanserin or ritanserin. 5-HT(2A) receptor-mediated acute hemodynamic change was not modified significantly by indomethacin, prazosin or propranolol pretreatment. A tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, but not a dopamine beta hydroxylase inhibitor, diethyldithiocarbamate, abolished the 5-HT(2A) receptor-mediated increase in indole-2,3-dione. Exogenous indole-2,3-dione induced a significant increase in plasma catecholamine levels and decrease in urine volume. A 5-HT(2A) receptor-mediated decrease in capillary flow may have caused an inflammatory process and peripheral sympathetic activation via ANP signaling inhibition. 3,4-dihydroxyphenylalanine (DOPA)/dopamine may contribute to the progression of inflammation or the generation of a precursor of indole-2,3-dione. The observation that indole-2,3-dione abolished angiotensin AT(1) receptor-mediated NADPH activation in both human umbilical vein endothelial cells and smooth muscle cells at 20 microM may suggest that sulfhydryl-reactive indole-2,3-dione could influence mitochondrial function and cellular redox states via flavoenzyme inhibition and/or regulation of dehydrogenase-oxidase conversion.

Brain tribulin activity in rats with an inherited predisposition to audiogenic epilepsy was studied after seizures of different intensity were induced by an electric bell. Weak seizures (from 0 to 2 arbitrary units) did not produce any changes in endogenous inhibitory activity towards either monoamine oxidase (MAO) A or B. Moderate seizures were characterized by increases in both MAO A and MAO B inhibitory activity (up to 1.9-fold). Complete tonic epileptiform seizures with total areflexia (4 arbitrary units) induced further augmentation (up to 2.5-fold) of MAO A but not of MAO B inhibitory activity. This dissociation between the two inhibitory activities points to the existence of a separate MAO A-inhibiting component of brain tribulin which is different from isatin.