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Developmental regulation of sterol biosynthesis in Zea mays
Author(s) -
Guo Dean,
Venkatramesh M.,
Nes W. David
Publication year - 1995
Publication title -
lipids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.601
H-Index - 120
eISSN - 1558-9307
pISSN - 0024-4201
DOI - 10.1007/bf02537823
Subject(s) - sterol , mevalonic acid , biosynthesis , stigmasterol , chemistry , biochemistry , mevalonate pathway , stereochemistry , cholesterol , enzyme , chromatography
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 (D 2 O), we discovered that hydroxymethylglutaryl 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 24α‐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‐3β‐ol. Examination of the mass spectrum and 1 H 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.