Elevated methylation capacity of selected transfer RNA methyltransferases from 9,10-dimethyl-1,2-benzanthracene-induced rat mammary tumors.

A comparisonof the transferRNA(tRNA)methyltransferases from rat mammary gland and mammary tumors was carried out to explore possible tumor-specific differences in enzyme activ ity, substrate specificity, and capacity to methylatebacterial tRNA substrates. Enzymes from 9,10-dimethyl-1,2-benzan thracene-induced mammary tumors had specific activities 2to 3-fold higher than those from normal mammary glands or proliferating glands from pregnant rats; however, the relative proportions of individual methyltransferases from tumor en zymes were not markedly different from those found with nonneoplastic mammary tissue. When purified isoaccepting tRNA species were used as substrates, mammary tumor enzymes yielded a higher extent of methylation than did normal or pregnant mammary enzymes. These higher extents for tumor enzymes were not due to formation of new methylated bases. The formation of N2-meth ylguanosine at position 10 in purified tRNA's showed the most striking increase with the tumor enzymes. To explore the question of whether tumor-specific differ ences were due to differences in the tRNA methyltransferases from neoplastic versus nonneoplastic mammary tissues or to the presence of other components in the enzyme extracts, N2guanine methyltransferases were purified by diethylaminoethyl Sephadex chromatography, and their ability to methylate tRNA to capacity was examined. Nucleoside analysis of methyl-14Clabeled phenylalanine-specific tRNA indicated that a more com plete methylation of the guanosine at position 10 persists with tumor enzymes compared to enzymes from normal or late pregnant mammary gland. Therefore, the difference in extent of methylation between tumor and nontumor N2-guanine meth yltransferases appears to be a property of the enzymes and is not due to stimulatory or inhibitory factors in the enzyme extracts.