Developmental regulation of sterol biosynthesis in Zea mays.
Sixty-one sterols and pentacyclic triterpenes have been isolated and characterized by chromatographic and spectral methods from Zea mays (corn). Several plant parts were examined; seed, pollen, cultured hypocotyl cells, roots, coleoptiles (sheaths), and blades. By studying reaction pathways and mechanisms on plants fed radiotracers ([2-(14)C]mevalonic acid, [2-(14)C]acetate, and [2-(3)H]acetate), and stable isotopes (D2O), we discovered that hydroxymethylgutaryl CoA reductase is not "the" rate-limiting enzyme of sitosterol production. Additionally, we observed an ontogenetic shift and kinetic isotope effect in sterol biosynthesis that was associated with the C-24 alkylation of the sterol side chain. Blades synthesized mainly 24 alpha-ethyl-sterols, sheaths synthesized mainly 24-methyl-sterols, pollen possessed an interrupted sterol pathway, accumulating 24(28)-methylene-sterols, and germinating seeds were found to lack an active de novo pathway. Shoots, normally synthesizing (Z)-24(28)-ethylidine-cholesterol, after incubation with deuterated water, synthesized the rearranged double-bond isomer, stigmasta-5,23-dien-3 beta-ol. Examination of the mass spectrum and 1H nuclear magnetic resonance spectrum of the deuterated 24-ethyl-sterol indicated the Bloch-Cornforth route originating with acetyl-CoA and passing through mevalonic acid to sterol was not operative at this stage of development. An alternate pathway giving rise to sterols is proposed.
Guo
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Department of Chemistry and Biochemistry, Texas Tech University, Lubbock 79409, USA.
Venkatramesh
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D000085:Acetates / Q000378:metabolism; D000478:Alkylation; D002250:Carbon Radioisotopes; D002851:Chromatography, High Pressure Liquid; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism; D013058:Mass Spectrometry; D008798:Mevalonic Acid / Q000378:metabolism; D008954:Models, Biological; D010084:Oxidation-Reduction; D010840:Phytosterols / Q000032:analysis / Q000096:biosynthesis* / Q000737:chemistry; D018517:Plant Roots / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D018520:Plant Shoots / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D012639:Seeds / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism; D012855:Sitosterols / Q000378:metabolism; D014018:Tissue Distribution; D003313:Zea mays / Q000737:chemistry / Q000254:growth & development / Q000378:metabolism*
D000085:Acetates; D002250:Carbon Radioisotopes; D010840:Phytosterols; D012855:Sitosterols; C025473:gamma-sitosterol; D006903:Hydroxymethylglutaryl CoA Reductases; D008798:Mevalonic Acid