Stereochemical Aspects of the Biosynthesis of the Side Chain of 9β, 19-Cyclopropyl Sterols in Maize Seedlings Treated with Tridemorph

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Abstract
9β, 19-Cyclopropyl sterols such as 24-methyl pollinastanol accumulate dramatically in maize (Zea mays L. var LG 11) seedlings treated with Tridemorph (2,6-dimethyl-N-tridecyl-morpholine), a systemic fungicide (M. Bladocha, P. Benveniste, Plant Physiol 1983 41: 756-762). In contrast to the situation in control plants where 24-ethyl sterols predominate largely, 24-methyl sterols were more than 98% of total cyclopropyl sterols. In addition, 24-methyl cyclopropyl sterols were a mixture of (24-R)- and (24-S)-24-methyl epimers and are similar in that respect to the 24-methyl cholesterol of control plants. The presence of two epimers at C-24 has been previously explained by the operation of two routes (M. Zakelj, L. J. Goad, Phytochemistry 1983 22: 1931-1936). One may proceed via Δ24(28)- and Δ24(25)-sterols to produce the (24-R)-24-methyl epimer. The other route may involve reduction of either a Δ24(28)-, a Δ23-, or a Δ25-sterol intermediate to give the (24-S)-24-methyl epimer. Such intermediates have been searched for in excised Zea mays axes grown aseptically in the presence of Tridemorph and either [5-14C]mevalonic acid, or [Me-14C]-l-methionine. Whereas Δ24(28)- and Δ24(25)-cyclopropyl sterols were found in relatively large amounts, only traces of radioactivity were associated with Δ25-sterols. Gas chromatography/mass spectrometry analysis of the sterols from axes grown in the presence of [Me-2H3]-l-methionine showed that Δ24(28)-cyclopropyl sterols contained only two 2H atoms at C-28 as expected and that the 24-methyl pollinastanol fraction contained species with two 2H atoms and no species with three 2H atoms. These results indicate that both (24-R)- and (24-S)-epimers originate from a common Δ24(28) precursor. After incubation of the axis with [5-14C,(4-R)-4-3H1]mevalonic acid, the 24-methyl pollinastanol had a 3H:14C atomic ratio of 4:6 which is consistent with the intermediacy of a Δ24(25)-sterol. All these data are in accordance with a pathway where Δ24(28)-cyclopropyl sterols are isomerized to give Δ24(25)-cyclopropyl sterols which in turn would be reduced nonregiospecifically to yield both (24-R)- and (24-S)-24-methyl pollinastanols. A plausible mechanism for the reduction step is discussed.