Folate-deficient human lymphoblasts: changes in deoxynucleotide metabolism and thymidylate cycle activities

Peripheral blood lymphocytes of healthy volunteers cultured with phytohaemagglutinin in folate-deficient medium exhibit megaloblastic maturation with reduced intracellular folate content. We have employed this in vitro model for megaloblastic maturation to determine accompanying changes in cellular thymidylate cycle activities and deoxynucleotide levels. Folate-deficient cells exhibit a two-fold increase in thymidine kinase and thymidylate synthase activities. These increased activities were reduced to those of folate-replete cells by co-culture of folate-deficient cells with thymidine. Folate deficiency was accompanied by reduced cellular levels of thymidine triphosphate (TTP) and deoxyguanosine triphosphate (dGTP). Exogenous deoxyuridine produced no increase in the reduced levels of TTP of folate-deficient cells but effected a two-fold increase in cellular deoxycytidine triphosphate. Exogenous thymidine increased the reduced TTP levels of folate-deficient cells and corrected the reduced dGTP level; the increase in cellular TTP accompanying exogenous thymidine was more pronounced in folate-deficient cells. These in vitro findings are compatible with a block in de novo thymidylate synthesis and explain in part the reported in vivo changes for the deoxynucleotide pool in megaloblastic marrow cells due to folate or vitamin B12 deficiency.