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In vitro DNA synthesis by megaloblastic bone marrow: Effect of folates and cobalamins on thymidine incorporation and de novo thymidylate synthesis

The de novo pathway of thymidylate synthesis (i.e., methylation of dUMP to dTMP) is directly folate dependent and indirectly vitamin B12 (cobalamins) dependent. In deficiency of these vitamins, this pathway is impaired, and exogenous deoxyuridine (dU) fails to suppress adequately in vitro incorporation of [3H]thymidine (3H‐TdR) into DNA via the salvage pathway (i.e., abnormal dU suppression). This abnormality is corrected by the addition of folate compounds (analogues) and/or vitamin B12 depending on the nature of the underlying deficiency. We studied the effects of addition of PteGlu, 5‐methyl THF (5‐CH3‐FH4), 5‐formyl‐THF (5‐CHO‐FH4), and hydroxy‐cobalamin (OH‐cbl) on 3H‐TdR incorporation into DNA and thymidine kinase activity (salvage pathway), and on [3H]deoxyuridine (3H‐dU) incorporation and dU suppression values (de novo pathway) in cultures of normal and megaloblastic bone marrows. The results showed that 3H‐TdR incorporation into DNA and the salvage enzyme, thymidine kinase, activity were greater and 3H‐dU incorporation into DNA less in megaloblastic cells as compared with normal cells. The addition of folates significantly reduced 3H‐TdR incorporation and thymidine kinase activity and enhanced 3H‐dU incorporation in folate and vitamin B12‐deficient cells except that 5‐CH3‐FH4 had no effect on vitamin B12‐deficient cells. None of these additives had any significant effect on normal cells. This study also showed that the addition of the deficient vitamin(s) to the “control tubes” in the dU suppression test is inappropriate, as these vitamins may at least partially correct the defect in cellular DNA synthesis caused by the deficiences of these vitamins and may mask these deficiencies in the results of the in vitro correction of the dU suppression abnormalities in mild cases of megaloblastic anemia.