Formation of ribothymidine from thymine and ribonucleosides by the cell-free extract of tumors and rat tissues.

Formation of ribothymidine by the ribose exchange reaction between thymine and uridine with the cell-free extract of mouse Ehrlich ascites tumor cells was demonstrated. Since phosphate ions appear to be not required for this reaction, perhaps it proceeds by the mechnism of direct exchange of nucleoside N-ribosyltransferase. The transfer activity was found in the precipitates when the crude extract was fractionated with 30-60% saturated ammonium sulfate. Ribothymidine formation was also demonstrated between thymine and ribonucleosides other than uridine with this tumor extract. Production of ribothymidine from thymine and uridine was detected also by the use of extracts from lung, brain, and regenerating liver of normal rats, and from newborn rats (whole body). An extract of Rhodamine sarcoma exhibited the ribose exchange activity, while that of human gastric cancer did not.