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

9beta, 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 Delta(24(28))- and Delta(24(25))-sterols to produce the (24-R)-24-methyl epimer. The other route may involve reduction of either a Delta(24(28))-, a Delta(23)-, or a Delta(25)-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-(14)C]mevalonic acid, or [Me-(14)C]-l-methionine. Whereas Delta(24(28))- and Delta(24(25))-cyclopropyl sterols were found in relatively large amounts, only traces of radioactivity were associated with Delta(25)-sterols. Gas chromatography/mass spectrometry analysis of the sterols from axes grown in the presence of [Me-(2)H(3)]-l-methionine showed that Delta(24(28))-cyclopropyl sterols contained only two (2)H atoms at C-28 as expected and that the 24-methyl pollinastanol fraction contained species with two (2)H atoms and no species with three (2)H atoms. These results indicate that both (24-R)- and (24-S)-epimers originate from a common Delta(24(28)) precursor. After incubation of the axis with [5-(14)C,(4-R)-4-(3)H(1)]mevalonic acid, the 24-methyl pollinastanol had a (3)H:(14)C atomic ratio of 4:6 which is consistent with the intermediacy of a Delta(24(25))-sterol. All these data are in accordance with a pathway where Delta(24(28))-cyclopropyl sterols are isomerized to give Delta(24(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.