Disease relevance of CYB5A

• Congenital methemoglobinemia with a deficiency of cytochrome b5 [1].
• The human CYP17 expressed in Escherichia coli [Imai, Globerman, Gertner, Kagawa and Waterman (1993) J. Biol. Chem. 268, 19681-19689] was also purified to homogeneity and was found to catalyse the hydroxylation of pregnenolone and progesterone without requiring cytochrome b5 [2].
• This MAb recognized homologous cytochrome b5 of human liver microsomes and in homogenates of TK- cells infected with recombinant vaccinia virus encoding human cytochrome b5 [3].
• In-vitro evidence for the regulation of 17,20-lyase activity by cytochrome b5 in adrenocortical adenomas from patients with Cushing's syndrome [4].
• Cytochrome b5 concentrations were greater in the two adenomas with high 17,20-lyase activity than in the other adenomas, while no significant difference in Red activity was observed among all the adenomas [4].

Psychiatry related information on CYB5A

• These MAbs to cytochrome b5 will be useful for the identification, quantification, and purification of cytochrome b5 from animal and human tissues, and for understanding its role in cytochrome P450 catalyzed drug metabolism and carcinogen activation with respect to tissue, organ and individual differences [3].
• Chronic alcohol consumption caused significant increases of cytochrome P-450 and cytochrome b5 contents of liver and kidney microsomes [5].

High impact information on CYB5A

• Contrary to predictions of recent theoretical analysis, 1H NMR spectroscopy indicates that there is no major movement of cytochrome c residue Phe82 on binding to cytochrome b5 [6].
• Several issues remain to be resolved regarding the catalytic activity of the P-450 3A4 protein, including rate-limiting steps and the need for cytochrome b5, divalent cations, and acidic phospholipid systems for optimal activity [7].
• However, the cytochrome b5 content and the activities of the reduced forms of nicotinamide adenine dinucleotide phosphate- and nicotinamide adenine dinucleotide-cytochrome c reductases did not differ between the two groups [8].
• We restricted expression to the endoplasmic reticulum or mitochondria by exchanging the Bcl-2 carboxy-terminal insertion sequence for an equivalent sequence from cytochrome b5 or ActA, respectively [9].
• Microsomes, peroxisomes, and outer mitochondrial membranes, all containing endogenous cytochrome b5, incorporated large amounts of the hemoprotein in such a way that it was reducible by an inherent NADH cytochrome b5 reductase [10].

Chemical compound and disease context of CYB5A

• These results immunohistochemically support the functional association of b5 with androgen production through interaction with P450c17 and the previous finding that higher concentrations of cytochrome b5 are associated with greater production of adrenal androgens in adrenocortical adenomas from patients with Cushing's syndrome [11].
• All of the adenomas were composed mainly of compact cells, which exhibited immunoreactive staining for cytochrome b5 as well as for P450c17 and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) [11].
• Biphasic effects of the flavonoids quercetin and naringenin on the metabolic activation of 2-amino-3,5-dimethylimidazo[4,5-f]quinoline by Salmonella typhimurium TA1538 co-expressing human cytochrome P450 1A2, NADPH-cytochrome P450 reductase, and cytochrome b5 [12].
• Subcellular fractions prepared from human normal granulocytes and leukemic cells from patients with chronic myeloid leukemia (CML), were examined for 7-ethoxycoumarin O-deethylase activity as well as cytochrome P-450 and cytochrome b5 contents [13].
• Electrochemical studies performed with the V45I/V61I mutant of cytochrome b5, and with a derivative in which the heme propionates have been converted into the corresponding dimethyl ester groups, clearly demonstrate that dehydration of the heme edge contributes to the modulation of the reduction potential of cytochrome b5 [14].

Biological context of CYB5A

• The combination of cytochrome b5 and phosphorylation was not additive [15].
• Suppressing cytochrome b5 expression in human adrenal NCI-H295A cells by >85% with RNA interference had no effect on 17alpha-hydroxylase activity but reduced 17,20 lyase activity by 30% [15].
• The human cytochrome b5 gene and two of its pseudogenes are located on chromosomes 18q23, 14q31-32.1 and 20p11.2, respectively [16].
• Chromosomal localization of a cytochrome b5 gene to human chromosome 18 and a cytochrome b5 pseudogene to the X chromosome [17].
• Using a combination of standard cDNA library screening techniques and the Polymerase Chain Reaction (PCR) we have isolated and sequenced DNA fragments corresponding to the human reticulocyte cytochrome b5 mRNA [18].

Anatomical context of CYB5A

• We expressed human P450c17, human P450-oxidoreductase (OR), and/or human cytochrome b5 in Saccharomyces cerevisiae and analyzed the 17alpha-hydroxylase and 17,20-lyase activities of the resulting yeast microsomes [19].
• Cytochrome b5 augments the 17,20-lyase activity of cytochrome P450c17 in vitro, but this has not been demonstrated in membranes, and the mechanism of this action is unknown [19].
• Cytochrome b5 is preferentially expressed in the fetal adrenal and postadrenarchal adrenal zona reticularis; the basis of this tissue-specific, developmentally regulated transcription of the b5 gene is unknown [20].
• Using very high stringency hybridization conditions for the Southern blot hybridization analysis of hamster-human cell hybrid DNA, we were able to map the human cytochrome b5 gene and two of its pseudogenes (psgb(5)1 and psgb(5)2) unambiguously to chromosomes 18, 14, and 20 [16].
• The human liver and reticulocyte cytochrome b5 mRNAs are products from a single gene [18].

Associations of CYB5A with chemical compounds

• Regulation of 17,20 lyase activity by cytochrome b5 and by serine phosphorylation of P450c17 [15].
• These residues are invoked to mediate protein-protein interaction between cytochrome b5 and CYP17, which 'awakens' the lyase activity of the enzyme required for androgen formation [21].
• It was found that the mutation of Lys83, Arg347 and Arg358 produced proteins that were deficient in their responsiveness to cytochrome b5, and the effect was most pronounced for the two arginine mutants (Arg347-->His and Arg358-->Gln) which have been found in male patients suffering from genital ambiguity [21].
• It is specific for NADH and reduces artificial substrates such as dichlorophenol and tetrazolium derivatives, but not natural substrates such as methemoglobin, cytochrome b5 or lipoate [22].
• The purified enzyme was characterized by a high specificity for NADH as electron donor (Km = 6.4 mumol/L v Km greater than 1.6 mmol/L for NADPH) and exhibited a maximal turnover of ca. 30,000 min-1 at 22 degrees C with either ferricyanide or cytochrome b5 (Km = 10 nmol/L) as electron acceptor [23].

Physical interactions of CYB5A

• Modulation of CYP1A1-mediated oxidation of carcinogenic azo dye Sudan I and its binding to DNA by cytochrome b5 [24].
• Lysine mutagenesis identifies cationic charges of human CYP17 that interact with cytochrome b5 to promote male sex-hormone biosynthesis [25].
• The electrostatic potential of the surface near the flavin-protruding side (dimethylbenzene end of the flavin ring) of NADH-cytochrome b5 reductase was positive over a wide area while that of the surface near the heme-binding site of cytochrome b5 was negative [26].

Enzymatic interactions of CYB5A

• Like the pig CYP17, the human CYP17 also catalysed the cytochrome b5-dependent direct cleavage of pregnenolone into the delta 5,16-steroid, but unlike it the human enzyme did not cleave progesterone at all [2].
• In recombinant CYP from human B-lymphoblast cells or baculovirus-infected insect cells and in reconstituted systems, a requirement of cytochrome b5 (b5) for POD activities catalysed by CYP2E1 and CYP3A4 was observed [27].

Regulatory relationships of CYB5A

• These data provide essential evidence in support of a functional role for cytochrome b5 in the regional control of 17alpha-hydroxylase and 17,20-lyase activities of P450c17 and thereby adrenal C19 steroid secretion by the primate adrenal gland [28].
• Thus, formation of the putative carcinogen 16alpha-OHE1 is catalysed by CYP3A4 and 3A5 and stimulated by cytochrome b5 [29].
• 9-fold, 8-fold, and 30-fold rate increases were found respectively for nifedipine 1,4-oxidation, lidocaine N-deethylation and testosterone 6 beta-hydroxylation between P450 NF25-containing yeast microsomes from the basic strain and from the strain that both overexpresses yeast NADPH-P450 reductase and expresses human cytochrome b5 [30].
• Interestingly, cytochrome b5 markedly stimulated the CYP2E1-mediated two-electron oxidation of the first three substrates, while it had almost no effect on the presumed one-electron reduction of carbon tetrachloride [31].
• Third, methionine synthase can be activated in vitro by a two-component redox system comprised of soluble cytochrome b5 and P450 reductase [32].

Other interactions of CYB5A

• When expressed in bacteria, human P450c17 required either cytochrome b5 or phosphorylation for 17,20 lyase activity [15].
• NCB5OR is the first example of an animal flavohemoprotein containing cytochrome b5 and chrome b5 reductase cytodomains [33].
• Assessment of porcine and human 16-ene-synthase, a third activity of P450c17, in the formation of an androstenol precursor. Role of recombinant cytochrome b5 and P450 reductase [34].
• These results suggest that the guanabenz N-hydroxylation is mediated via CYP1A2, whereas the corresponding guanoxabenz N-reduction is catalyzed by an enzyme system composed of cytochrome b5, NADH cytochrome b5-reductase, and benzamidoxime reductase [35].
• Both P-450-ALC and P-450-C effectively demethylate N-nitrosodimethylamine (NDMA) at low substrate concentrations (0.5 mM), especially in the presence of cytochrome b5 [36].

Analytical, diagnostic and therapeutic context of CYB5A

• Affinity chromatography of microsomal enzymes on immobilized detergent-solubilized cytochrome b5 [37].
• Resonance Raman spectroscopy of NCB5OR presents typical signatures of a six-coordinate low-spin heme similar to those found in other cytochrome b5 proteins [33].
• Magnetic circular dichroism studies of low-spin cytochromes. Temperature dependence and effects of axial coordination on the spectra of cytochrome c and cytochrome b5 [38].
• Expression of immunodetectable cytochrome b5 was highest in the liver, next highest in the kidney, and quite low in the other tissues examined with MAb 1-17-1 by Western blotting [3].
• All of the MAbs bound strongly to rat cytochrome b5 as measured by radioimmunoassay (RIA) [3].

References