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2-Amino-3-methyl-9H-pyrido[2,3-b]indole (MeAαC) and some metabolites were investigated for mutagenicity in mammalian cell lines and bacterial strains engineered for the expression of human enzymes. MeAαC induced gene mutations (studied at thehprtlocus) in Chinese hamster V79-derived cells co-expressing cytochrome (CYP) 1A2 and sulphotransferase (SULT) 1A1 even at a concentration of 30 nM, but was inactive in cells co-expressing CYP1A2 andN-acetyltransferase (NAT) 1 or 2. MeAαC, tested in the presence of rat liver post-mitochondrial fraction, showed strongly enhanced mutagenicity in aSalmonella typhimuriumstrain expressing human SULT1A1 compared with the control (recipient) strain TA1538/1,8-DNP (deficient in endogenous acetyltransferase). Mutagenicity was also enhanced, although to a lesser extent, when NAT2 was expressed in the latter strain. The metabolite, 2-hydroxylamino-3-methyl-9H-pyrido[2,3-b]indole (N-OH-MeAαC) was a direct mutagen to strains TA1538 and TA1538/ 1,8-DNP. This mutagenicity was strongly enhanced in corresponding strains expressing SULT1A1. A moderate enhancement was observed when SULT1A2, SULT1B1, SULT1C2 or NAT2 were expressed in strain TA1538. The remaining enzymes studied (SULT1A3, 1C1, 1E1, 2A1, 2B1a, 2B1b, 4A1 and NAT1) did not indicate any activation ofN-OH-MeAαC. Preliminary mutagenicity experiments in SULT-expressingS.typhimuriumstrains were conducted with other hydroxylated metabolites of MeAαC. The phenols, 6- and 7-hydroxy-MeAαC, were inactive under the conditions studied. The benzylic alcohol, 2-amino-3-hydroxymethyl-9H-pyrido[2,3-b]indole, was mutagenic in a strain expressing SULT1A1, but its activity was much weaker than that ofN-OH-MeAαC. Thus,N-hydroxylation (e.g. mediated by CYP1A2) and sulpho conjugation (primarily mediated by SULT1A1) was the dominating activation pathway of MeAαC in model systems engineered for human enzymes. Some other SULT forms as well as NAT2 were also capable of activatingN-OH-MeAαC, although with much lower efficiency than SULT1A1. Another minor activation pathway involved benzylic hydroxylation followed by sulpho conjugation by SULT1A1.

carcinogenesis

Bioactivation of the heterocyclic aromatic amine 2-amino-3-methyl-9              H              -pyrido [2,3-              b              ]indole (MeAαC) in recombinant test systems expressing human xenobiotic-metabolizing enzymes