Cytochrome b5/cytochrome b5 reductase complex in rat liver microsomes has NADH-linked aquacobalamin reductase activity.

To study the mammalian system for synthesis of cobalamin coenzymes, rat liver microsomal NADH-linked aquacobalamin reductase was characterized. Microsomal NADH-linked aquacobalamin reductase, which was solubilized with 10 g/L sodium deoxycholate, showed identical elution behavior to NADH-cytochrome c reductase (cytochrome b5/cytochrome b5 reductase complex) on DEAE-Toyopearl 650 column chromatography. By mixing the purified cytochrome b5 with cytochrome b5 reductase, cob(II)alamin was immediately formed from aquacobalamin and NADH. These results provide evidence that the NADH-linked aquacobalamin reductase activity is derived from the cytochrome b5/cytochrome b5 reductase complex in rat liver microsomes. Some properties of the cytochrome b5/cytochrome b5 reductase complex in the form of NADH-linked aquacobalamin reductase were studied. The inhibition studies with cobalamin analogues suggested that hydrophobicity of the corrin ring of cobalamin molecule is involved in binding of cobalamin to the cytochrome b5/cytochrome b5 reductase complex.