Suppression by methionine and choline of onco-fetal patterns of liver tRNA methyltransferase activities in carcinogen-treated rats.

When male Fischer rats were fed Purina chow supplemented with 2% D,L-methionine and 1% choline chloride, the rapid increase in N2-guanine tRNA methyltransferase II (NMG2) activity otherwise seen in response to cancer-promoting doses (0.02% in the diet) of 2-acetylaminofluorene (AAF) was prevented, and the increase in NMG2 activity otherwise caused by carcinogenic doses of AAF (0.06% in the diet) was decreased by 50%. In addition, the return of NMG2 activity to a normal level after completion of a 3-week regimen of 0.06% AAF was accelerated in animals fed the methionine plus choline supplemented diet. As shown earlier in this laboratory, liver tRNA methylating enzyme activities are shifted rapidly to an onco-fetal pattern in rats receiving methyl-deficient diets. This pattern is characterized by selectively elevated NMG2 activity while the activities of other base-specific tRNA methylating enzymes are relatively unchanged. Our combined results indicate that the exogenous supply of methyl groups is a factor in regulating NMG2 activity and can modulate at least one response of animals to carcinogens.