Comparison of methyltransferase activities of pair-fed rats given adequate or methyl-deficient diets.

The short-term effects of a lipotrope-deficient (methyl-deficient) diet on tRNA and protein methyltransferase activities have been studied using pair-fed male Fischer rats. The activity of liver N2-guanine tRNA methyltransferase II (NMG2) of animals receiving the methyl-deficient diet (MDD) for 2 weeks was found to be elevated more than 2-fold. This is in agreement with the results of earlier experiments in which the animals were fed ad libitum. These data indicate that the effects of lipotrope-deficient diets on NMG2 activity observed in the earlier studies can be attributed to the nature of the diet, and not to differences in caloric intake. In the same pair-fed animals, very little effect of MDD on the activity of NMG2 of either brain or spleen was observed. In liver, the activity of one of the enzymes that catalyze protein methylation--protein methylase I (S-adenosyl-methionine: protein-arginine N-methyltransferase)--was significantly elevated in response to the lipotrope-deficient diet. In contrast, the activities of protein methylase II (S-adenosylmethionine: protein-carboxy-O-methyltransferase), from control and experimental animals did not differ significantly. Lipotrope-deficient diets are thus seen to induce, within a short period of time, selective changes in the activities of some, but not all, of the liver enzymes that catalyze the methylation of tRNA and protein.