Abstract
Incorporation of [2- 14 C, (4R)-4- 3 H 1 ]-mevalonic acid into the sterols of Cyathula capitata (Amaran-thaceae) showed that the C-24 tritium of cycloartenol is eliminated during side chain alkylation in the formation of 24-ethyl-5 α -cholest-7-en-3 β -ol, (24S) –24-ethyl-cholesta-5,22-dien-3 β -ol and 24-ethylcholest-5-en-3 β -ol, but is retained probably at C-25 in (24Z)-5 α -stigmasta-7,24(28)-dien-3 β -ol. Cyasterone formed from the same substrate had a 3 H/ 14 C ratio 1 : 5. This tritium atom was retained in the nuclear fragment (probably at C-17) after chemical side chain cleavage of the derived 2-acetoxy-cyasterone. Negligible radioactivity was detected in the side chain fragment. Significant incorporation of [4- 14 C]-sitosterol and [7- 3 H 2 ]-fucosterol into cyasterone in C. capitata could not be detected under the conditions employed. The significance of these results is discussed in relation to the stage in phytosterol biosynthesis at which the cyasterone synthetic pathway branches.
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