Redundancy in the pathway for redox regulation of mammalian methionine synthase. Reductive activation by the dual flavoprotein, novel reductase 1

Methionine synthase is an essential cobalamin-dependent enzyme in mammals that catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine to give tetrahydrofolate and methionine. It is oxidatively labile and requires for its sustained activity an auxiliary repair system that catalyzes a reductive methylation reaction. Genetic and biochemical studies have demonstrated that the soluble dual flavoprotein oxidoreductase, methionine synthase reductase, serves as a redox partner for methionine synthase in an NADPH-dependent reaction. However, three reports suggest the possibility of redundancy in this redox pathway. First, a hyperhomocysteinemic patient has been reported who has an isolated functional deficiency of methionine synthase but appears to be distinct from the cblE and cblG classes of patients with defects in methionine synthase reductase and methionine synthase, respectively. Second, another dual flavoprotein oxidoreductase with significant homology to methionine synthase reductase, NR1, has been described recently, but its function is unknown. Third, methionine synthase can be activated in vitro by a two-component redox system comprised of soluble cytochrome b₅ and P450 reductase. In this study, we demonstrate a function for human NR1 in vitro. It is able to fully activate methionine synthase in the presence of soluble cytochrome b₅ with a V_max of 2.8 ± 0.1 μmol min^-1 mg^-1 protein, which is comparable with that seen with methionine synthase reductase. The K_actNR1 is 1.27 ± 0. 16 μM, and a 20-fold higher stoichiometry of reductase to methionine synthase is required for NR1 versus methionine synthase reductase, suggesting that it may represent a minor pathway in the cell, assuming that the two proteins are present at similar levels.