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 s ynthase and the concurrent homocysteine (Hcy) export in mutant fibrobl asts with defects in the homocysteine catabolizing enzyme, cystathioni ne beta-synthase, or in methionine synthase, which carries out homocys teine remethylation. The fibroblasts were incubated in various concent rations of methionine to create conditions favoring methionine conserv ation or catabolism. In cystathionine beta-synthase-deficient cells, h igh medium methionine partly protected the enzyme against inactivation , as previously found in normal fibroblasts. The Hcy export rate at lo w methionine levels was low (0.2-0.6 nmol/h/10(6) cells), and increase d 2-3-fold at high methionine levels. Nitrous oxide enhanced Hcy expor t rate at low methionine, so that in the presence of nitrous oxide, th e Hcy export became less dependent of methionine. In cb1G cells, the e nzyme inactivation was moderate and independent of medium methionine. The Hcy export rate was intermediate (0.5-0.8 nmol/h/10(6) cells) at l ow methionine levels, and increased moderately (<2-fold) at high methi onine 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 a nd nitrous oxide. These data suggest that Hcy remethylation and cystat hionine beta-synthase activity are major determinants of Hcy export at low and high methionine, respectively. The low susceptibility of meth ionine synthase to nitrous oxide in the presence of high methionine or in cb1G or cb1E mutants is probably related to low catalytic turnover .