Further investigation of the increased transfer ribonucleic acid methylase activity in tumours of the mouse colon

Abstract

Extracts prepared from tumors of the mouse colon induced by 1,2 dimethylhydrazine were considerably more active in catalyzing the methylation of tRNA than were extracts from normal colon. The enhanced activity was observed when both unfractionated methyl deficient tRNA and purified tRNA preparations from yeast and bacteria were used as substrates for methylation. The methylated bases produced in these reactions were identified. There were no differences between the products of the reaction catalyzed by extracts of tumor and normal colon. The increased activity of tRNA methylases was not due to the presence in the extracts of stimulatory or inhibitory molecules of low molecular weight such as polyamines or S adenosylhomocysteine. Other enzymes concerned with tRNA metabolism (RNA polymerase, ATP tRNA adenylyltransferase, aminoacyl tRNA ligases) were also increased in activity in the tumor tissue. The extent of methylation of a limiting amount of tRNA was greater when tumor extracts were compared with controls, but in no case was it possible to achieve a stoicheiometric methylation of the purified tRNA preparations used as substrates, and the tumor extracts were not able to methylate the tRNA obtained from normal mouse colon. It is concluded that there was no evidence for changes in the specificity of these enzymes during neoplastic growth.