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Biosci Biotechnol Biochem. 2002 Mar;66(3):566-70.

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

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United Graduate School of Agricultural Science, Tokyo University of Agriculture and Technology, Fuchu-shi, Japan.


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-19,6beta-olide into momilactone A. The enzyme required NAD+ or NADP+ as a hydrogen acceptor. The optimum pH for the reaction was 8. The Km value to 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide was 36 microM when NAD+ was supplied as a cofactor at a concentration of 1 mM. 3fl-Hydroxy-9beta-pimara-7,15-dien-19,6beta-olide and its dehydrogenase activity were induced in a time-dependent manner by UV irradiation.

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