Sustained activity of mammalian methionine synthase (MS) requires MS reductase (MSR), but there have been few studies of the interactions between these two proteins. In this study, recombinant human MS (hMS) and MSR (hMSR) were expressed in baculovirus-infected insect cells and purified to homogeneity. hMSR maintained hMS activity at a 1:1 stoichiometric ratio with a K(act) value of 71 nM. Escherichia coli MS, however, was not activated by hMSR. Moreover, hMS was not significantly active in the presence of E. coli flavodoxin and flavodoxin reductase, which maintain the activity of E. coli MS. These results indicate that recognition of MS by their reductive partners is very strict, despite the high homology between MS from different species. The effects of hMSR on the formation of hMS holoenzyme also were examined by using crude extracts of baculovirus-infected insect cells containing hMS apoenzyme (apoMS). In the presence of MSR and NADPH, holoenzyme formation from apoMS and methylcobalamin was significantly enhanced. The observed stimulation is shown to be due to stabilization of human apoMS in the presence of MSR. Apoenzyme alone is quite unstable at 37 degrees C. MSR also is able to reduce aquacobalamin to cob(II)alamin in the presence of NADPH, and this reduction leads to stimulation of the conversion of apoMS and aquacobalamin to MS holoenzyme. Based on these findings, we propose that MSR serves as a special chaperone for hMS and as an aquacobalamin reductase, rather than acting solely in the reductive activation of MS.