MITOCHONDRIAL NADH-LINKED OR NADPH-LINKED AQUACOBALAMIN REDUCTASE-ACTIVITY IS LOW IN HUMAN SKIN FIBROBLASTS WITH DEFECTS IN SYNTHESIS OF COBALAMIN COENZYMES

Mammalian livers have been reported to contain NADH- and NADPH-linked aquacobalamin reductases, which are distributed in both mitochondria a nd microsomes. The four aquacobalamin reductase isozymes have been pur ified and characterized from rat liver. It is unclear which aquacobala min reductase among the four reductase isozymes participates in the sy nthesis of cobalamin coenzymes. To clarify the physiological roles of the aquacobalamin reductase isozymes, human mutant fibroblasts (cblC a nd cblA cells) with defects in cobalamin reductases involved in the co enzyme synthesis were used. In the cblC cells, the activity of the mit ochondrial NADH-linked aquacobalamin reductase was reduced significant ly, compared with normal human fibroblasts but the mitochondrial NADPH -linked enzyme was not. The reduced specific activity of the NADH-link ed enzyme was not due to reduction in levels of the enzyme, but in its affinity for NADH. Although there was not a significant difference in the mitochondrial NADH-linked enzyme activity between normal and cblA cells, the activity of the mitochondrial NADPH-linked enzyme was not detectable in the mutant cells. These results indicate that the defect s in the mitochondrial NADH- and NADPH-linked aquacobalamin reductases underlie cblC and cblA disorders, respectively.