The biosynthesis of beta-amyrin. Mechanism of squalene cyclization.

1. beta-Amyrin synthesized by pea seedlings in the presence of (3RS)-[2-(14)C,(4R)-4-(3)H(1)]mevalonic acid (for nomenclature see Cahn, Ingold & Prelog, 1956) was subjected to a series of degradations to locate the positions within the molecule of the incorporated tritium. 2. The location of five of the six labelled hydrogen atoms at C-3, C-9, C-18 and C-19 (two) confirms that the mechanism of cyclization of squalene expected from the biogenetic isoprene rule is functioning in vivo.