Deoxyuridine metabolism in human megaloblastic marrow cells

Despite the clinical use of the 'deoxyuridine suppression test' to document vitamin B12 or folate deficiency its biochemical basis is unclear and currently disputed. Because of this the metabolism of deoxyuridine in marrow cells has been examined. In normoblastic marrow cells the ratio of radio-labelled deoxyuridine to thymidine incorporation into DNA approximates one and preincubation of cells with unlabelled deoxyuridine results in progressive reduction of uptake of both radio-labelled deoxynucleosides. In the same cells methotrexate significantly reduces the DNA incorporation of deoxyuridine but not that of thymidine. In megaloblastic marrow the ratio of radio-labelled deoxyuridine to thymidine uptake is less than one and the reduced deoxyuridine uptake is not significantly altered by either cyanocobalamin or folic acid. With megaloblastic samples the reduction by deoxyuridine of radio-labelled deoxyuridine uptake is less marked than that observed with normoblastic cells and to achieve similar results requires folic acid. These findings suggest that reduced deoxyuridylate conversion to deoxythymidylate by thymidylate synthetase is appropriate to explain the 'deoxyuridine suppression test' in megaloblastic marrow cells and that altered substrate requirements for this activity may occur in megaloblastic cells.