Human hepatic CYP1A1 and CYP1A2 content, determined with specific anti-peptide antibodies, correlates with the mutagenic activation of PhIP

Abstract

Mono-specific antibodies targeted to human CYP1A1 and CYP1A2 have been produced by immunizing rabbits with protein conjugates of short synthetic peptides corresponding to residues 290–297 and 284–296 respectively, of these enzymes. The antibody targeted to CYP1A1 bound in immunoblotting to the recombinant protein expressed in yeast but did not bind to any human hepatic microsomal protein, whereas the antibody targeted to CYP1A2 bound only to this enzyme in immunoblotting of human hepatic microsomal fractions and did not recognize recombinant human CYP1A1. The intensity of hepatic microsomal CYP1A2 immunoreactivity (n = 5) correlated significantly with a number of activities characteristic of this enzyme: phenacetin O-deethylase (POD), ethoxyresorufin O-deethylase (EROD) and methoxyresorufin O-demethylase activities and the ability to activate the dietary carcinogen 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), to a mutagen. The anti-CYP1A2 anti-peptide antibody consistently inhibited both POD and EROD activities, but inhibition was incomplete (28%). In view of the known (>90%) contribution of CYP1A2 to these activities and the correlation with antibody binding, this is consonant with an epitope for the anti-CYP1A2 anti-peptide antibody that forms the edge of a functionally important proinhibitory surface region previously identified in rat cytochromes CYP1A. CYP1A2 immunoreactivity determined by immunoblotting correlated significantly with the ability of human hepatic microsomal fractions to activate 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP), another dietary carcinogen, to a mutagen. It is concluded that CYP1A1 is absent from human liver and that CYP1A2 is likely to be a major catalyst in the hepatic activation of PhIP.