Plant Physiology 69:1013-1017 (1982)
© 1982 American Society of Plant Biologists
Articles
Metabolism of Monoterpenes: Conversion of l-Menthone to l-Menthol and d-Neomenthol by Stereospecific Dehydrogenases from Peppermint (Mentha piperita) Leaves 1,2
Robert Kjonaas,
Charlott Martinkus-Taylor and
Rodney Croteau3
Institute of Biological Chemistry, and Biochemistry/Biophysics Program, Washington State University, Pullman, Washington 99164
The monoterpene ketone l-menthone is specifically converted to l-menthol and l-menthyl acetate and to d-neomenthol and d-neomenthyl- -D-glucoside in mature peppermint (Mentha piperita L. cv. Black Mitcham) leaves. The selectivity of product formation results from compartmentation of the menthol dehydrogenase with the acetyl transferase and that of the neomenthol dehydrogenase with the glucosyl transferase. Soluble enzyme preparations, but not particulate preparations, from mature peppermint leaves catalyzed the NADPH-dependent reduction of l-menthone to both epimeric alcohols, and the two dehydrogenases responsible for these stereospecific transformations were resolved by affinity chromatography on M trex Gel Red A. Both enzymes have a molecular weight of approximately 35,000, possess a Km for NADPH of about 2 x 10–5M, are very sensitive to inhibition by thiol-directed reagents, and are not readily reversible. The menthol dehydrogenase showed a pH optimum at 7.5, exhibited a Km for l-menthone of about 2.5 x 10–4M, and also reduced d-isomenthone to d-neoisomenthol. The neomenthol dehydrogenase showed a pH optimum at 7.6, exhibited a Km for l-menthone of about 2.2 x 10–5M, and also reduced d-isomenthone to d-isomenthol. These stereochemically distinct, but otherwise similar, enzymes are of key importance in determining the metabolic fate of menthone in peppermint, and they are probably typical of the class of dehydrogenases thought to be responsible for the metabolism of monoterpene ketones during plant development.
3 To whom inquiries should be made.
1 Supported in part by National Science Foundation Grant PCM 78-19417 and by grants from the Washington Mint Commission and Mint Industry Research Council. Scientific Paper 6021, Project 0268, College of Agriculture Research Center, Washington State University, Pullman, WA 99164.
2 Although the systematic name for l-menthone is (5R, 2S)-trans-5-methyl-2-(l-methylethyl)cyclohexanone, we have utilized here the more common nomenclature based on numbering of the p-menthane system (i.e. menthone = p-menthan-3-one), in which the methyl-substituted carbon is 1 R and the isopropyl-substituted carbon is 4 S.
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