Background: Thymidylate synthase (dTMP synthase, EC 2.1.1.45) is responsible for the de novo biosynthesis of thymidylate (dTMP) whereas thymidine kinase (TdR kinase, EC 2.7.1.21) is the salvage enzyme which leads to the production of dTMP even in presence of d TMP synthase inhibition. The current study was undertaken to compare the steady-state activities of d TMP synthase and TdR kinase in extracts of human epithelial ovarian carcinoma and normal ovaries.
Material and methods: Tissue extracts of epithelial ovarian carcinomas and normal ovaries were analyzed for activities of d TMP synthase (by a method that measures released 3H2O) and TdR kinase (by polyethyleneimine impregnated (PEI) cellulose plate radioassay), methods established in this laboratory.
Results: The dTMP synthase activity (mean +/- S.E.) in extracts of ovarian carcinomas (N = 11) was 0.198 +/- 0.069 and in extracts of normal ovaries (N = 12) it was 0.025 +/- 0.0004 nmol/hr/mg protein. By contrast, TdR kinase activity in extracts of ovarian carcinoma (N = 13) was 27.7 +/- 8.5 compared to 1.0 +/- 0.3 nmol/hr/mg protein in extracts of normal ovaries (N = 15).
Conclusion: The observed 140-fold higher TdR kinase activity suggests that DNA synthesis may continue despite inhibition of dTMP synthase with current schedules of 5-fluorouracil (5-FU) and leucovorin. The addition of azidothymidine, a competitive inhibitor of TdR kinase, to 5-FU and leucovorin might be a rational biochemical strategy to employ in patients with refractory ovarian carcinoma.