Relative activities of thymidylate synthetase and thymidine kinase in human mammary tumours.

Thymidylate synthetase (TS) and thymidine kinase (TK) are known to catalyse the methylation of dUMP for the de novo synthesis of dTMP and the phosphorylation of thymidine for the salvage synthesis of dTMP in the pyrimidine pathway, respectively. High activities of TS and TK have been observed in rapidly proliferating tissues. In the present study, both enzyme activities were measured in 8 specimens of normal mammary tissues, 12 fibroadenomas and 10 adenocarcinomas in the human breast. The average activities of TS in fibroadenomas and adenocarcinomas were approximately 5- and 7-fold that in normal mammary tissues, respectively. The average activities of TK in fibroadenomas and adenocarcinomas were 4- and 14-fold that in normal mammary tissues, respectively. Thus, DNA synthesis in benign and malignant mammary tumors may be relatively predominant in de novo and salvage pathways, respectively.