Plant sterol biosynthesis. Identification and characterization of two distinct microsomal oxidative enzymatic systems involved in sterol C4-demethylation.

Membrane-bound enzymatic systems obtained from maize embryos that catalyze the oxidative C4-monodemethylation of 4,4-dimethyl- and 4 alpha-methylsterols have been investigated. Enzymatic assay conditions have been developed for the first time to detect the C4-monodemethylated products formed. The properties of the microsomal systems have been established for co-factor requirements and kinetics. The demethylation process has been interrupted to demonstrate the formation of stable, oxygenated intermediates. In addition to the 3-keto and 3 beta-hydroxy-4-monodemethylated products formed, three new sterols have been identified. 3 beta-Hydroxy-4 beta,14 alpha-dimethyl-5 alpha-ergosta-9 beta,19-cyclo-24(24(1))-en-4 alpha-hydroxy methyl was identified for the first time as the immediate metabolite of 24-methylenecycloartanol by 4 alpha-methyl oxidase in addition to 3 beta-hydroxy-4 beta,14 alpha-dimethyl-5 alpha-ergosta-9 beta,19-cyclo- 24(24(1))-en-4 alpha-carboxylic-acid and 3 beta-hydroxy-5 alpha-stigmasta-7,24(24(1))-dien-4 alpha-carboxylic-acid, intermediates involved respectively in the oxidative demethylation of 24-methylenecycloartanol and 24-ethylidenelophenol. Proton nuclear magnetic resonance studies of enzymatically produced 3 beta-hydroxy-4 beta,14 alpha-dimethyl-5 alpha-ergosta-9 beta,19-cyclo-24(24(1))en-4 alpha-carboxylic acid indicate that the 4 alpha-methyl group of 24-methylenecycloartanol is oxidized and subsequently removed during its enzymatic conversion to cycloeucalenol. From a series of incubations with 25 natural or synthetic 4,4-dimethyl and 4 alpha-methylsterols, a high degree of substrate specificity for the oxidation at C4 of 4,4-dimethyl- and 4 alpha-methylsterols was determined. Our results indicate that oxidation of the 4 alpha-methyl group of the 4,4-geminal dimethylsterols requires the more flexible and presumably bent conformation of 9 beta,19-cyclopropylsterols and the absence of a delta 24(25) unsaturation, whereas the rigid planar conformation of delta 7-unsaturated sterols favors oxidation of 4 alpha-methylsterols. Distinct strict structural requirements for the oxidation of 4,4-dimethyl- and 4 alpha-methylsterols and different sensitivity toward cyanide ions and 3 beta,5 alpha,6 alpha-stigmastatriol, a novel inhibitor of 4 alpha-methylsterol C4 oxidase activity, are consistent with the conclusion that two distinct oxidative systems are involved in the removal of the first and second C4-methyl group of phytosterol precursors. Moreover, the present study directly establishes that during the conversion of cycloartenol to phytosterol one C4 dealkylation occurs before the removal of the 14 alpha-methyl group.