Involved in the reductive regeneration of cob(I)alamin cofactor required for the maintenance of methionine synthase in a functional state. H(+) + 2 methylcob(III)alamin-[methionine synthase] + NADP(+) + 2 S-adenosyl-L-homocysteine = 2 cob(II)alamin-[methionine synthase] + NADPH + 2 S-adenosyl-L-methionine