RESUMEN
An increased tRNA methylase activity (100 %) accompanied by a 40 % decrease in the regulatory glycine methyltransferase activity was demonstrated in livers of mice fed on the carcinogenic (thioacetamide) diet, long before the onset of malignant transformation. Shortterm treatment with thioacetamide and phenobarbital independently, also brought about a significant increase in the rat liver tRNA methylase activity. A significant increase in the tRNA methylase activity was observed in the mammary glands of pregnant as well as lactating mice as against the negligible enzyme activity in the normal mammary glands of C3H and CBA mice, whereas a large increase in the tRNA methylase activity was evident in the spontaneously induced mammary tumours in these strains. Hepatic tRNA methylase activity was shown to remain unaffected in rats during various physiological stress conditions. It is suggested that elevation in the tRNA methylase activity may be one of the prerequisites during malignant transformation. A considerable increase in the tRNA methylase activity in host tissues of the tumour-bearing mice was also demonstrated.