Involvement of cytochrome b5 and a cyanide-sensitive monooxygenase in the 4-demethylation of 4,4-dimethylzymosterol by yeast microsomes

Biochim Biophys Acta. 1981 Jan 26;663(1):194-202. doi: 10.1016/0005-2760(81)90205-8.


According to Ohba et al. (Ohba, M., Sato, R., Yoshida, Y., Nishino, T. and Katsuki, H. (1978) Biochem. Biophys. Res. Commun. 85, 21-27), yeast microsomes catalyze the removal of three methyl groups attached to the C-4 and C-14 positions of [1,7,15,22,26,30-14C]lanosterol (4,4,14 alpha-trimethyl-5 alpha-cholesta-8,24-dien-3 beta-ol) in the presence of NADPH, NAD+ and molecular oxygen, concomitant with the liberation of 14CO2 derived from C-30 (one of the two methyl groups at the C-4 position). In this process the methyl group at the C-14 position is first removed in a cyanide-insensitive reaction and then the two methyl groups at the C-4 position are removed by a cyanide-sensitive enzyme system. In this study it was found that the 14CO2 formation from the 14C-labeled lanosterol was inhibited by antibodies to yeast cytochrome b5 and by palmitoyl-CoA, a substrate of the cytochrome b5-containing fatty acyl-CoA desaturase system of yeast microsomes. However, neither the antibodies nor palmitoyl-CoA inhibited the conversion of lanosterol to 4,4-dimethyl zymosterol (4,4-dimethyl-5 alpha-cholesta-8,24-dien-3 beta-ol). It is concluded that cytochrome b5 and a cyanide-sensitive enzyme are involved in the 4-demethylation of 4,4-dimethylzymosterol, but not the 14 alpha-demethylation of lanosterol, by yeast microsomes. It is suggested that a cyanide-sensitive enzyme acts as the terminal 4-demethylase and cytochrome b5 transfers reducing equivalents from NADPH to the terminal enzyme, as in the case of fatty acyl-CoA desaturation. The cyanide sensitivity of the 4-demethylation was, however, much greater than that of the desaturation.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antimycin A / pharmacology
  • Chemical Phenomena
  • Chemistry
  • Cholestadienols*
  • Chromatography, Gas
  • Cyanides / pharmacology*
  • Cytochromes / metabolism*
  • Cytochromes b5
  • Kinetics
  • Lanosterol / metabolism*
  • Microsomes / enzymology*
  • Multienzyme Complexes*
  • NADH Dehydrogenase / metabolism
  • Oxidoreductases*
  • Potassium Cyanide / pharmacology*
  • Saccharomyces cerevisiae / enzymology*
  • gamma-Globulins / pharmacology


  • Cholestadienols
  • Cyanides
  • Cytochromes
  • Multienzyme Complexes
  • gamma-Globulins
  • Lanosterol
  • 4-methyl sterol demethylase
  • Antimycin A
  • 4,4-dimethyl-5-alpha-cholesta-(8,24)-dien-3-beta-ol
  • Cytochromes b5
  • Oxidoreductases
  • NADH Dehydrogenase
  • Potassium Cyanide