Further studies on the pathways of synthesis of thymine compounds by lymphatic tissues and tumors.

Previous investigations have shown that the incorporation in vitro of formate, formaldehyde, or the beta carbon of serine into thymine compounds of normal lymphatic tissues and tumors is greatly increased when deoxycytidine, deoxyuridine, or cytidine are added to the incubation medium (1, 2). The present study demonstrates that in the above tissues (a) methyl cytosine deoxyribonucleoside is not an obligatory intermediate in the conversion of deoxycytidine to thymidine, and (b) the methyl group of methionine may function to a limited degree as a thymine precursor. The conversion of methionine-C14H3 to thymine compounds is inhibited by anaerobic conditions or the presence of nonlabeled formaldehyde, suggesting that the methionine methyl group is oxidized before transfer to the thymidine precursor.