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Natural Products

Biochemical Characterization of a Spearmint Mutant That Resembles Peppermint in Monoterpene Content ¹

Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164-6340, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser, Washington 99350-9687

A radiation-induced mutant of Scotch spearmint (Mentha x gracilis) was shown to produce an essential oil containing principally C3-oxygenated p-menthane monoterpenes that are typical of peppermint, instead of the C6-oxygenated monoterpene family characteristic of spearmint. In vitro measurement of all of the enzymes responsible for the production of both the C3-oxygenated and C6-oxygenated families of monoterpenes from the common precursor (–)-limonene indicated that a virtually identical complement of enzymes was present in wild type and mutant, with the exception of the microsomal, cytochrome P-450-dependent (–)-limonene hydroxylase; the C6-hydroxylase producing (–)-trans-carveol in the wild type had been replaced by a C3-hydroxylase producing (–)-trans-isopiperitenol in the mutant. Additionally, the mutant, but not the wild type, could carry out the cytochrome P-450-dependent epoxidation of the ,-unsaturated bond of the ketones formed via C3-hydroxylation. Although present in the wild type, the enzymes of the C3-pathway that convert trans-isopiperitenol to menthol isomers are synthetically inactive because of the absence of the key C3-oxygenated intermediate generated by hydroxylation of limonene. These results, which clarify the origins of the C3- and C6-oxygenation patterns, also allow correction of a number of earlier biogenetic proposals for the formation of monoterpenes in Mentha.

¹ This investigation was supported in part by a National Science Foundation grant (DCB 8803504), by a grant from the Washington Mint Commission/Mint Industry Research Council, and by Projects 0268 and 0281 from the Agricultural Research Center, Washington State University, Pullman, WA 99164.

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