The role of cytochrome b5 in 4alpha-methyl-oxidation and C5(6) desaturation of plant sterol precursors

Biochem Biophys Res Commun. 1997 Jul 18;236(2):434-7. doi: 10.1006/bbrc.1997.6974.


The electron donors for the membrane-bound sterol-4alpha-methyl-oxidases and sterol C5(6)-desaturase of plant sterol biosynthesis have not been previously identified. The requirement of cytochrome b5 to shuttle reducing equivalents from NAD(P)H to 4,4-dimethylsterol-4alpha-methyl oxidase (4,4-DMSO), 4alpha-methylsterol-4alpha-methyl oxidase (4alpha-MSO), and delta7-sterol-C5(6) desaturase (5-DES) was investigated using a purified preparation of IgG raised against plant cytochrome b5. The activities of 4,4-DMSO, 4alpha-MSO, and 5-DES, three oxidative 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 b5 of maize microsomes is an obligatory electron carrier from NAD(P)H to 4,4-DMSO, 4alpha-MSO, and 5-DES.

MeSH terms

  • Cytochromes b5 / metabolism*
  • Methylation
  • Microsomes / enzymology
  • Mixed Function Oxygenases / metabolism*
  • NADH Dehydrogenase / metabolism
  • Oxidation-Reduction
  • Oxidoreductases / metabolism*
  • Sterols / metabolism*
  • Zea mays / enzymology
  • Zea mays / metabolism*


  • Sterols
  • Cytochromes b5
  • Mixed Function Oxygenases
  • Oxidoreductases
  • methylsterol monooxygenase
  • sterol delta-5 desaturase
  • NADH Dehydrogenase