Deoxyuridine metabolism in cultured human lymphoblasts treated with methotrexate

The deoxyuridine suppression test and labeled deoxyuridine incorporation studies assume a stable level of deoxyuridine phosphorylase (thymidine:orthophosphate deoxyribosyltransferase, EC 2.4.2.4) activity. We report a large increase in deoxyuridine phosphorylase activity in three of five cultured lymphoblast lines treated with 10(-7) M methotrexate. In two of these lines, a parallel increase in tritiated deoxyuridine incorporation into RNA was seen following methotrexate treatment. The high basal deoxyuridine phosphorylase activity in another lymphoblast line was associated with 80% incorporation of tritiated deoxyuridine into RNA in untreated cells. Since methotrexate-induced changes in deoxyuridine phosphorylase activity were time dependent, the reliability of the deoxyuridine suppression test and labeled deoxyuridine incorporation into DNA as measures of thymidylate synthetase (EC 2.7.4.6) inhibition would also vary with time. Moreover, increases in deoxyuridine phosphorylase activity in methotrexate-treated cells may influence the metabolism of fluorouracil, a drug frequently used in combined treatment regimens.