Effect of methionine and nitrous oxide on homocysteine export and remethylation in fibroblasts from cystathionine synthase-deficient, cb1G, and cb1E patients.

We investigated the nitrous oxide-induced inactivation of methionine synthase and the concurrent homocysteine (Hcy) export in mutant fibroblasts with defects in the homocysteine catabolizing enzyme, cystathionine beta-synthase, or in methionine synthase, which carries out homocysteine remethylation. The fibroblasts were incubated in various concentrations of methionine to create conditions favoring methionine conservation or catabolism. In cystathionine beta-synthase-deficient cells, high medium methionine partly protected the enzyme against inactivation, as previously found in normal fibroblasts. The Hcy export rate at low methionine levels was low (0.2-0.6 nmol/h/10(6) cells), and increased 2-3-fold at high methionine levels. Nitrous oxide enhanced Hcy export rate at low methionine, so that in the presence of nitrous oxide, the Hcy export became less dependent of methionine. In cb1G cells, the enzyme inactivation was moderate and independent of medium methionine. The Hcy export rate was intermediate (0.5-0.8 nmol/h/10(6) cells) at low methionine levels, and increased moderately (< 2-fold) at high methionine levels or following nitrous oxide exposure. In cb1E mutants, the enzyme activity was not affected by nitrous oxide, and the Hcy export was high (0.8-1.6 nmol/h/10(6) cells) and independent of methionine and nitrous oxide. These data suggest that Hcy remethylation and cystathionine beta-synthase activity are major determinants of Hcy export at low and high methionine, respectively. The low susceptibility of methionine synthase to nitrous oxide in the presence of high methionine or in cb1G or cb1E mutants is probably related to low catalytic turnover.