METABOLISM OF 2-AMINO-ALPHA-CARBOLINE - A FOOD-BORNE HETEROCYCLIC AMINE MUTAGEN AND CARCINOGEN BY HUMAN AND RODENT LIVER-MICROSOMES AND BY HUMAN CYTOCHROME P4501A2

2-Amino-alpha-carboline (A alpha C) is a mutagenic and carcinogenic he terocyclic amine that is formed as a pyrolysis product during the high temperature cooking of food and the burning of tobacco. Human, rat, a nd mouse hepatic microsomes each catalyzed the NADPH-dependent oxidati on of A alpha C to form six products separable by HPLC. The two major metabolites, which together accounted for similar to 85% of the total metabolism, were characterized by UV, fluorescence, proton magnetic re sonance, and mass spectral analyses as 3-hydroxy-A alpha C and 6-hydro xy-A alpha C. The remaining 15% were judged to be N-hydroxy-A alpha C and its oxidation products, based on chromatographic and spectral comp arisons with a standard, whose synthesis and characterization are also described. Although the proportions of each metabolite were similar a cross species and individuals, the overall rate of metabolism of A alp ha C by human hepatic microsomes showed a wide interindividual variati on (37-fold), with a mean activity that was comparable with that obser ved with rat or mouse liver microsomes. alpha-Naphthoflavone, a select ive inhibitor for cytochromes P4501A1 and P4501A2, strongly inhibited formation of both ring-hydroxylation and N-oxidation products by human , rat, or mouse liver hepatic microsomes. In addition, A alpha C oxida tion was strongly correlated (r = 0.98; p < 0.001) with the oxidation of 4-aminobiphenyl, a known selective substrate for human and rodent c ytochromes P4501A2. Immunoblot analyses confirmed the presence of cyto chromes P4501A2, and not P4501A1, in human liver microsomes. Additiona l studies using recombinant human cytochromes P450 show that high cata lytic activity for A alpha C metabolism was associated with human cyto chrome P4501A2. Lower, but significant activity was also noted for P45 01A1 and P4502C10, which could have important implications for the met abolic activation of A alpha C in extrahepatic tissues. Neither A alph a C metabolism nor immunoreactive cytochrome P4501A2 (or P4501A1) was detected in human pancreatic microsomes. Although further carcinogenic ity and biomarker studies for A alpha C are needed, the high rate of A alpha C metabolism by human liver cytochrome P4501A2 suggests that hu mans with the rapid P4501A2 phenotype with may be more susceptible tha n rodents to this heterocyclic amine carcinogen.