Biosynthesis of rice phytoalexin: Enzymatic conversion of 3 beta-hydroxy-9 beta-pimara-7,15-dien-19,6 beta-olide to momilactone A

Momilactone A, a major rice diterpene phytoalexin, could be synthesized by dehydrogenation at the 3-position of 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide in rice leaves. The presence of 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide in UV-irradiated rice leaves was confirmed by comparing the mass spectra and retention times after a GC/MS analysis of the natural and synthetic compounds. The soluble protein fraction from UV-irradiated rice leaves showed dehydrogenase activity to convert 3beta-hydroxy-9beta-pimara-7,15-dien-19,6betaolide into momilactone A. The enzyme required NAD(+) or NADP(+) as a hydrogen acceptor. The optimum pH for the reaction was 8. The K-m value to 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide was 36 mum when NAD(+) was supplied as a cofactor at a concentration of 1 mm. 3beta-Hydroxy-9beta-pimara-7,15-dien-19,6beta-olide and its dehydrogenase activity were induced in a time-dependent manner by UV irradiation.