In vivo assay for somatic point mutations induced by genotoxic carcinogens: incorporation of [35S]methionine into a rat liver cytochrome b5 normally lacking sulphur-containing amino acids.

The trypsin fragments of rat liver microsomal cytochrome b5 (Tb5) lack both methionine (met) and cysteine (cys), i.e., the sulphur-containing amino acids. Tb5 should therefore contain no 35S-radioactivity after isolation from animals treated with [35S]met or [35S]cys. If, however, the nucleic acids coding for this polypeptide have been damaged by a genotoxic carcinogen, a miscoding could result in an incorporation of met or cys into the polypeptide so that Tb5 could now be 35S-radiolabelled. Two experiments are described, the first one where a toxic regimen of N-nitrosomorpholine (NNM) to rats resulted in a significant increase of 35S-radioactivity in the Tb5 of liver microsomes, and a second experiment with a non-toxic regimen of N,N-diethylnitrosamine (DENA), where no increase was observable.