Methylcobalamin corrects the deleterious in vitro effect of nitrous oxide on thymidylate synthetase.

In this study, cobalamin deficiency was produced in vitro by the use of nitrous oxide, known to inactivate the vitamin. In 14 sets of experiments, normal human lymphocytes stimulated with phytohemagglutinin on day 0 were exposed to nitrous oxide and oxygen on day 2. MeCbl was delivered later to half of the cells. Untreated cells served as a control. On day 3, the cells were harvested, the lymphocytes were lysed, and the obtained extracts were assayed for thymidylate synthetase. In 16 other experiments the same procedure was performed, and the incorporation of radioactive thymidine or deoxyuridine by the intact cells was measured. In additional experiments, a deoxyuridine suppression test of treated and untreated stimulated lymphocytes was also performed. The results indicate that nitrous oxide significantly reduces the activity of thymidylate synthetase and that this reduction is significantly corrected by MeCbl, suggesting a causative relation between the vitamin and the enzyme. However, there was no statistically significant effect of nitrous oxide demonstrated on the nucleoside incorporation nor on the deoxyuridine suppression test.