The steady state kinetics and isotope effects were examined for demethylation of dimethylnitrosamine and phenylmethylnitrosamine, as well as their deuterated analogs, using the S-9 fraction, the microsomal pellet, and postmicrosomal supernatant from rat livers. The isotope effect (ratio of maximal rates for the deuterated and light substrates) using the S-9 from Long-Evans rat livers was found to be 1.82 for dimethylnitrosamine and 5.38 for phenylmethylnitrosamine. Phenobarbital was shown to induce dimethylnitrosamine demethylase activity in the microsomal pellet of both Long-Evans and Sprague-Dawley rats but to repress this activity in the postmicrosomal supernatant in the Long-Evans rats, while markedly increasing it in the Sprague-Dawley rats. It was also found that there was nitrosamine demethylase activity in the so-called “pH 5 enzymes” and in the supernatant from that preparation. The latter activity shows substantially different characteristics from that found in the other fractions.
Supported by grants from the USPHS (1-RO1-CA22110) and the National Science Foundation (CHE 76-24095).