title: Regulation of thiol environment of the N-demethylation and ring hydroxylation of N,N-dimethyl-4-aminoazobenzene (DAB) by rat liver microsomes.
abstract: A study was conducted on the regulation by thiol environment of microsomal metabolism of the azo dye hepatocarcinogen, N,N-dimethyl-4-aminoazobenzene (DAB). Physiological concentrations of glutathione (GSH) stimulated N-demethylation and ring hydroxylation of the dye in normal and phenobarbital (PB)-treated microsomes. However, little effect of GSH was seen with microsomes from beta-naphthoflavone (BNF)-treated rats. The synthetic thiol, dithiothreitol (DTT), stimulated ring-hydroxylation of DAB but inhibited N-demethylation at all concentrations in control nd PB-induced microsomes. A biphasic response to DTT was obtained with BNF microsomes; inhibition of N-demethylation was seen only at low concentrations (0.1 mM) and a return to control values occurred at higher concentrations. DTT inhibition was shown to be specific for the first N-demethylation step, whereas the second was slightly stimulated at concentrations greater than 3.0 mM. Agents which alkylate [N-ethylmaleimide (NEM), p-hydroxymercuribenzoate] or oxidize [5,5'-dithiobis(nitrobenzoic acid) or Ellman's reagent] protein SH groups inhibited DAB metabolism. Inhibition of microsomal NADPH-cytochrome c reductase activity by p-hydroxymercuribenzoate required an order of magnitude more inhibitor than was needed to block DAB metabolism. This suggests that DAB metabolism requires viable SH groups other than those involved in NADPH-cytochrome c reductase activity. NEM, in contrast, inhibited the N-demethylation of DAB and NADPH-cytochrome c reductase at approximately the same concentrations. Ring-hydroxylation was stimulated by high (greater than 1 mM) concentrations of NEM, implying a different enzymic mechanism for this pathway.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.forename: W G
authors.initials: WG
authors.lastname: Levine
mesh_terms: D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000032:analysis; D003640:Dealkylation; D004228:Dithionitrobenzoic Acid / Q000494:pharmacology; D004229:Dithiothreitol / Q000494:pharmacology; D005978:Glutathione / Q000494:pharmacology; D006900:Hydroxylation; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase / Q000032:analysis; D010634:Phenobarbital / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013438:Sulfhydryl Compounds / Q000494:pharmacology*; D010128:p-Aminoazobenzene / Q000031:analogs & derivatives / Q000378:metabolism; D004124:p-Dimethylaminoazobenzene / Q000378:metabolism*
chemical_list: D013438:Sulfhydryl Compounds; D010128:p-Aminoazobenzene; C005202:N-methyl-4-aminoazobenzene; D003577:Cytochrome P-450 Enzyme System; D004228:Dithionitrobenzoic Acid; D004124:p-Dimethylaminoazobenzene; D009251:NADPH-Ferrihemoprotein Reductase; D005978:Glutathione; D004229:Dithiothreitol; D010634:Phenobarbital