DEFECTS IN AUXILIARY REDOX PROTEINS LEAD TO FUNCTIONAL METHIONINE SYNTHASE DEFICIENCY

Methionine synthase catalyzes a methyl transfer reaction from methylte trahydrofolate to homocysteine to form methionine and tetrahydrofolate and is dependent on methylcobalamin, a derivative of vitamin B-12, fo r activity, Due to the lability of the intermediate, cob(I) alamin, th e activity of methionine synthase is additionally dependent on a redox activation system. In bacteria, two flavoproteins, NADPH-flavodoxin r eductase and flavodoxin, shuttle electrons from NADPH to methionine sy nthase, Their mammalian counterparts are unknown, and a putative intri nsic thiol oxidase activity of the mammalian methionine synthase has b een proposed to be involved, We demonstrate that the mammalian methion ine synthase can be activated in an NADPH-dependent reaction and requi res a minimum of two redox proteins, This model is consistent with our results from biochemical complementation studies between cblG and cbl E cell lines and mutation detection analysis in cblG cell lines, These demonstrate that the cblG; cell line has defects affecting methionine synthase directly, whereas the cblE cell line has defects in the redo x proteins, We have also identified a P1173L mutation in the activatio n domain of methionine synthase in the cblG cell line WG1505.