Elevated Methylation Capacity of Selected Transfer RNA Methyltransferases from 9,10-Dimethyl-1,2-benzanthracene-induced Rat Mammary Tumors¹

A comparison of the transfer RNA (tRNA) methyltransferases from rat mammary gland and mammary tumors was carried out to explore possible tumor-specific differences in enzyme activity, substrate specificity, and capacity to methylate bacterial tRNA substrates. Enzymes from 9,10-dimethyl-1,2-benzanthracene-induced mammary tumors had specific activities 2- to 3-fold higher than those from normal mammary glands or proliferating glands from pregnant rats; however, the relative proportions of individual methyltransferases from tumor enzymes 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-methylguanosine at position 10 in purified tRNA's showed the most striking increase with the tumor enzymes.

To explore the question of whether tumor-specific differences 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, N²-guanine methyltransferases were purified by diethylaminoethyl-Sephadex chromatography, and their ability to methylate tRNA to capacity was examined. Nucleoside analysis of methyl-¹⁴C-labeled phenylalanine-specific tRNA indicated that a more complete 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 methyltransferases appears to be a property of the enzymes and is not due to stimulatory or inhibitory factors in the enzyme extracts.