THE ROLE OF CYTOCHROME B(5) IN 4-ALPHA-METHYL-OXIDATION AND C5(6) DESATURATION OF PLANT STEROL PRECURSORS

The electron donors for the membrane-bound sterol-4 alpha-methyl-oxida ses and sterol C5(6)-desaturase of plant sterol biosynthesis have not been previously identified. The requirement of cytochrome b(5) to shut tle reducing equivalents from NAD(P)H to 4,4-dimethylsterol-4 alpha-me thyl oxidase (4,4-DMSO), 4 alpha-methylsterol-4 alpha-methyl oxidase ( 4 alpha-MSO), and Delta(7)-sterol-C5(6) desaturase (5-DES) was investi gated using a purified preparation of IgG raised against plant cytochr ome b(5). The activities of 4,4-DMSO, 4 alpha-MSO, and 5-DES, three ox idative reactions not mediated by cytochrome P-450, were strongly and completely inhibited by the antibody in a microsomal preparation from maize. In addition the IgG also inhibited NADH-dependent cytochrome c reduction in the same preparation. These results strongly suggest that membrane-bound cytochrome b(5) of maize microsomes is an obligatory e lectron carrier from NAD(P)H to 4,4-DMSO, 4 alpha-MSO, and 5-DES. (C) 1997 Academic Press.