COBALAMIN INACTIVATION DECREASES PURINE AND METHIONINE SYNTHESIS IN CULTURED HUMAN LYMPHOBLASTS: 22

Abstract

The megaloblastic anemia of cobalamin deficiency appears secondary to decreased methionine synthetase activity. Decreased activity of this enzyme should cause 5-methyltetrahydrofolate to accumulate intracellularly. However, only some of the clinical and biochemical manifestations of cobalamin deficiency can be explained by the methylfolate trap. Since nitrous oxide (N2O) inhibits methionine synthetase activity by inactivating cobalamin, cultured lymphoblasts were treated with N2O to create a model of cobalamin deficiency; 4 h of N2O exposure reduced rates of methionine synthesis by 89%. Rates of purine synthesis were not significantly reduced by N2O when folate and methionine were present at 100 μM in the medium; however, at the physiologic methionine concentration of 10 μM, N2O decreased rates of purine synthesis by 33% and 57% in the presence of 100 μM folate and in the absence of folate, respectively. The dependency of rates of purine synthesis on methionine availability would be expected in cells with restricted methionine synthetic capacity because (a) methionine is the immediate precursor of S-adenosylmethionine, a potent inhibitor of 5-methyltetrahydrofolate synthesis; (b) methionine serves as a source of formate for purine synthesis; and (c) rates of purine synthesis are dependent on the intracellular availability of essential amino acids. It appears that cobalamin deficiency decreases purine synthesis by both methylfolate trapping and reduction of intracellular methionine synthesis.