title: Involvement of cytochrome b5 and a cyanide-sensitive monooxygenase in the 4-demethylation of 4,4-dimethylzymosterol by yeast microsomes.
abstract: 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.
mesh_terms: D000968:Antimycin A / Q000494:pharmacology; D055598:Chemical Phenomena; D002621:Chemistry; D002775:Cholestadienols; D002849:Chromatography, Gas; D003486:Cyanides / Q000494:pharmacology*; D003580:Cytochromes / Q000378:metabolism*; D015786:Cytochromes b5; D007700:Kinetics; D007810:Lanosterol / Q000378:metabolism*; D008861:Microsomes / Q000201:enzymology*; D009097:Multienzyme Complexes; D009245:NADH Dehydrogenase / Q000378:metabolism; D010088:Oxidoreductases; D011190:Potassium Cyanide / Q000494:pharmacology*; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D005719:gamma-Globulins / Q000494:pharmacology
chemical_list: D002775:Cholestadienols; D003486:Cyanides; D003580:Cytochromes; D009097:Multienzyme Complexes; D005719:gamma-Globulins; D007810:Lanosterol; C023939:4-methyl sterol demethylase; D000968:Antimycin A; C013499:4,4-dimethyl-5-alpha-cholesta-(8,24)-dien-3-beta-ol; D015786:Cytochromes b5; D010088:Oxidoreductases; D009245:NADH Dehydrogenase; D011190:Potassium Cyanide