PLANT STEROL BIOSYNTHESIS - RECENT ADVANCES IN THE UNDERSTANDING OF OXIDATIVE DEMETHYLATIONS AT C4 AND C-14

This report describes recent work on the process of oxidative demethyl ation of methylsterols in photosynthetic organisms. Two distinctive me mbrane-bound enzyme systems obtained from maize embryos catalyze the r espective oxidative C14 and C4-monodemethylation of 14-methyl- and 4-m ethylsterols. Cytochrome P-450 catalyzes the oxidative removal of the 14 alpha-methyl group, but not the oxidative demethylation at C4 which involves the participation of a cyanide-sensitive enzyme. Substrate r ecognition studies directly establish that in higher plants the demeth ylations occur in the sequence 4, 14, 4 in contrast to animals and wil d type yeast where the sequence is 14, 4, 4. All reactions in the proc ess of plant sterol demethylation appear to proceed via a-face attack. These enzymes are good targets for inhibitors which should show usefu l herbicidal or fungicidal properties.