Metabolism of N-hydroxy-2-acetylaminofluorene and N-hydroxy-phenacetin by guinea pig liver microsomal enzymes

Purification and characterization of guinea-pig liver microsomal deacetylase involved in the deacetylation of the O-glucoside of N-hydroxyacetanilide

A microsomal deacetylase that catalyses the deacetylation of the O-glucoside of N-hydroxyacetanilide (GHA) was purified from guinea-pig liver. The activity was located exclusively in the microsomes and not detected in the cytosol. The purified GHA deacetylase was a trimeric protein with a molecular mass of 160+/-10 (S.D.) kDa composed of subunits of 53+/-2 kDa; its pI was 4.7. The N-terminal amino acid sequence of GHA deacetylase was similar to those reported for guinea-pig and rat liver microsomal carboxylesterases. The GHA deacetylase showed a comparable hydrolytic activity towards p-nitrophenyl acetate (PNPA), although the activities towards N-hydroxyacetanilide, acetanilide and some endogenous acylated compounds were very low or not detectable. The deacetylase activity towards GHA was inhibited by organophosphates but not by p-chloromercuribenzoate, suggesting that GHA deacetylase can be classified as a B-esterase. The enzyme exhibited a positive homotropic co-operativity towards GHA. The values of the Hill coefficient, the half-saturating concentration ([S]0.5) for GHA, and Vmax were 1.59+/-0.03, 5.51+/-0.07 mM and 32.5+/-1.4 micromol/min per mg respectively, at the optimum pH of 8.5. The bell-shaped pH dependence of the Vmax/[S]0.5 profile indicated pKa values attributed to histidine and lysine residues. The study of stoichiometric inhibition by di-isopropyl fluorophosphate and kinetic analysis with the Monod-Wyman-Changeux model suggests that GHA deacetylase has six substrate binding sites and three catalytically essential serine residues per enzyme molecule.

Further studies on the comutagenic activity of cigarette smoke condensate

The comutagenic effect exerted by cigarette smoke condensate (CSC) was investigated. In vitro experiments with Salmonella typhimurium strains TA98 and TA98/1.8DNP6 indicated that CSC specifically enhances the mutagenicity of polyaromatic amines such as 2-aminofluorene, 2-acetylaminofluorene, 4-acetylaminofluorene and 2-aminoanthracene. The pattern of comutagenicity of CSC was shown to differ from that of norharman, a tobacco-related known comutagenic substance. Both black and blond tobacco CSCs proved to interact synergistically with 2-aminoanthracene mutagenicity. Chemical fractionation of CSC indicates the occurrence of comutagenic substance(s) in both neutral and basic components. Further in vitro experiments with 2-acetylaminofluorene metabolites and derivatives suggest that the comutagenic effect of CSC could involve later step(s) in the metabolic activation of fluorenylamines, i.e., the conversion of hydroxylamines into ultimate reactive species. The possible occurrence of a synergistic interaction of CSC with chemical mutagens in vivo was evaluated. Administration of 2-aminoanthracene/CSC mixtures, previously shown to be comutagenic in vitro, failed to demonstrate a synergistic effect in SCE induction in bone marrow cells of mice. This apparent discrepancy may rely on divergences in the activation pathways of polycyclic amines in vitro and in vivo.

Effect of ring substituents on the transketolase-catalyzed conversion of nitroso aromatics to hydroxamic acids

Transketolase catalyzed the conversion of eight different aromatic C-nitroso compounds into the corresponding N-glycolyl derived hydroxamic acids. Three of the nitroso compounds were also found to be converted in part to the arylhydroxylamines by a reductive process. A correlation was found for the rates of production of these metabolites with the electronegativities of substituent groups that were present on the aromatic ring. The rates of reaction of these substituted nitroso substrates with transketolase and D-fructose-6-phosphate were found to decrease in the order 4-NO2 much greater than 4-CF3 greater than 3-CF3, unsubstituted greater than 4-Cl greater than 4-CH3, 4-phenyl greater than 4-OC2H5. N,N-Dimethyl-p-nitrosoaniline was not metabolized by transketolase under the conditions employed for the other substrates. Those substrates possessing the strong electron-withdrawing groups 4-NO2, 4-CF3 and 3-CF3 were the only substrates that were found to undergo enzymatic reduction to the hydroxylamines as a competing process. A mechanism was proposed that involves a redox reaction between the nitroso substrate and the enzymatic intermediate "active glycolaldehyde" at the active-site of transketolase.