DEVELOPMENTAL REGULATION OF STEROL BIOSYNTHESIS IN ZEA-MAYS

Sixty-one sterols and pentacyclic triterpenes have been isolated and c haracterized by chromatographic and spectral methods from Zea mays (co rn). Several plant parts were examined; seed, pollen, cultured hypocot yl cells, roots, coleoptiles (sheaths), and blades. By studying reacti on pathways and mechanisms on plants fed radiotracers ([2-C-14]mevalon ic acid, [2-C-14]acetate, and [2-H-3]acetate), and stable isotopes (D2 O), we discovered that hydroxymethylglutaryl CoA reductase is not ''th e'' rate-limiting enzyme of sitosterol production. Additionally, we ob served an ontogenetic shift and kinetic isotope effect in sterol biosy nthesis that was associated with the C-24 alkylation of the sterol sid e chain. Blades synthesized mainly 24 alpha-ethyl-sterols, sheaths syn thesized mainly 24-methyl-sterols, pollen possessed an interrupted ste rol pathway, accumulating 24(28)-methylene-sterols, and germinating se eds were found to lack an active de novo pathway. Shoots, normally syn thesizing (Z)-24(28)-ethylidine-cholesterol, after incubation with deu terated water, synthesized the rearranged double-bond isomer, stigmast a-5,23-dien-3 beta-ol. Examination of the mass spectrum and H-1 nuclea r magnetic resonance spectrum of the deuterated 24-ethyl-sterol indica ted the Bloch-Cornforth route originating with acetyl-CoA and passing through mevalonic acid to sterol was not operative at this stage of de velopment. An alternate pathway giving rise to sterols is proposed,