Occurrence and tissue distribution of both NADH- and NADPH-linked aquacobalamin reductases in some vertebrates

To elucidate the synthesis of cobalamin coenzymes in view of comparative biochemistry, tissue distribution of activity of aquacobalamin reductase [EC 1.6.99.8] catalyzing the reduction of hydroxocobalamin to cob(II)alamin was studied in some vertebrates. Activity of NADH- or NADPH-linked aquacobalamin reductase (or both) was found in some tissues of monkey, rat, cow, hog, chicken, fish, and frog. The liver homogenates of these vertebrates contained both NADH- and NADPH-linked enzymes. Monkey and rat livers accounted for more than 80% of total activities of both NADH- and NADPH-linked enzymes. Ratios of activities of the liver NADH- and NADPH-linked enzymes were above 1.0 in some land animals (monkey, rat, cow, and hog) but below 1.0 in water animals (sea and freshwater fishes). These different ratios are presumably due to the difference in the metabolisms of cobalamin and/or in the other cellular components between land and water animals.