Biosynthesis of Gibberellins

The diterpenoid metabolites ( — )-kauren-19-al (III) and ( — )-kauren-19-oic acid (IV) have been identified as products of mevalonate metabolism in an endosperm homogenate of Echinocystis macrocarpa Greene (wild cucumber). These products are formed in addition to ( — )-kaurene (I) and ( — )-kauren-19-ol (II), which were identified earlier. The evidence indicates that kaurene is oxidized irreversibly to kaurenol, kaurenal, kaurenoic acid, and a mixture of at least four other unidentified acids, in that sequence, in endosperm homogenates. These reactions are localized in the microsomal pellet and require the addition of either a reduced pyridine nucleotide or a heated 105,000 x g supernatant fraction. The microsomal oxidations of kaurene to kaurenol and of kaurenol to kaurenal require air as well as a reduced pyridine nucleotide, with reduced nicotinamide adenine dinucleotide phosphate more effective than NADH. NADP+, NAD+, or flavin adenine dinucleotide could not serve as a coenzyme under either aerobic or anaerobic conditions. The conversion of kaurenol to kaurenal was inhibited by NADP+, p-chloromercuribenzoate, and β-diethylaminoethyldiphenylvalerate dihydrochloride (SKF 525-A). These properties suggest that both reactions are catalyzed by mixed function oxidases. 14C-Kaurenal was incorporated into gibberellic acid in washed suspensions of Fusarium moniliforme cells. This finding, and similar results for kaurene, kaurenol, and kaurenoic acid reported earlier, indicates an intermediate role for all these substances in gibberellin biosynthesis. The general implications of these results for gibberellin biosynthesis are discussed.