Influence of microsomal and cytosolic fractions from rat, mouse, and hamster liver on the mutagenicity of dimethylnitrosamine in the Salmonella plate incorporation assay.

Dimethylnitrosamine (DMN) was mutagenic in the Salmonella plate incorporation assay (Ames test) at a level of 10 mumol/plate (3.7 mM) in the presence of hamster liver S-9. Mutagenicity of DMN at this level was not observed when the S-9 was derived from mouse or rat liver, although the mouse liver and hamster liver S-9 had similar DMN demethylase activities. Both mouse and rat liver S-9 inhibited the mutagenicity of DMN mediated by hamster liver S-9; the inhibitory factor was contained in the microsomal fraction. Mouse or rat liver microsomes did not inhibit the DMN demethylase activity of hamster liver S-9. The microsomal inhibitor from rat or mouse liver was stable at 60 but was inactivated at 70 degrees. DMN demethylase from both rat and mouse liver was inactivated at 60 degrees. Although the DMN demethylase activity of hamster liver S-9 was contained in the microsomal fraction, DMN mutagenesis under conditions of the assay required the presence of both microsomal and cytosolic (S-105) fractions; the cytosols from hamsters, mice, and rats were all effective. The cytosolic factor required for DMN mutagenesis was sensitive to trypsin and was not dialyzable. The presence of an inhibitor of DMN activation in rat and mouse microsomes may account for, or contribute to, the failure of liver S-9 preparations from these species to activate DMN to a mutagen under standard conditions of the Ames test. The requirement for the cytosolic fraction may indicate that DMN demethylase is not sufficient for the activation of DMN to a mutagen under the conditions used in these studies.