The stereospecific biosynthesis of plant sterols and alpha- and beta-amyrin.

1. A preparation of pea seedlings has been obtained that will incorporate [2-(14)C]mevalonate into squalene, alpha- and beta-amyrin and the phytosterols. 2. The (14)C/(3)H ratio in alpha- and beta-amyrin biosynthesized in the presence of [2-(14)C,4R-(3)H]-mevalonate is the same as in the starting material and in squalene; this gives experimental support to the mechanism for the cyclization of squalene proposed by Ruzicka for the formation of these pentacyclic triterpenoids. 3. The (14)C/(3)H ratio for beta-sitosterol was 5:3, the same as that in cholesterol in liver. 4. As the absence of (3)H from C-3 in beta-sitosterol was demonstrated (3)H must be present in the side chain and thus the H at C-24 is not lost during alkylation of the side chain; it probably migrates to C-25.