Activation of the hepatocarcinogen aflatoxin B1 by forms of human hepatic cytochromes P-450: Some do and some don't

Abstract

Twelve forms of human hepatic cytochrome P-450 were expressed in hepatoma cells by means of recombinant vaccinia viruses. The expressed P-450's were analyzed for their abilities to activate the potent hepatocarcinogens aflatoxin B1 to metabolites having mutagenic or DNA-binding properties. Five forms, P-450's IA2, IIA3, IIB7, IIIA3, and IIIA4, activated aflatoxin B1 to mutagenic metabolites as assessed by the production of His revertants of Salmonella typhimurium in the Ames test. The same P-450's catalyzed conversion of aflatoxin B1 to DNA-bound derivatives as judged by an in situ assay in which the radiolabeled carcinogen was incubated with cells expressing the individual P-450 forms. Seven other human P-450's, IIC8, IIC9, IID6, IIE1, IIIA5, and IVB1, did not significantly activate aflatoxin B1 as measured by both the Ames test and the DNA-binding assay. Moreover, polyclonal anti-rat liver P-450 antibodies that crossreact with individual human P-450's IA2, IIA3, IIIA3, and IIIA4 each inhibited aflatoxin B1 activation catalyzed by human liver S-9 extracts. Inhibition ranged from as low as 10% with antibody against IIA3 to as high as 65% with antibody against IIIA3 and IIIA4. These results establish the metabolic activation of aflatoxin B1 in human liver involves the contribution of multiple forms of P-450.