Demonstration of the Intercellular Compartmentation of l-Menthone Metabolism in Peppermint (Mentha piperita) Leaves

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Abstract
The metabolism of l-menthone, which is synthesized in the epidermal oil glands of peppermint (M. piperita L. cv. Black Mitcham) leaves, is compartmented; on leaf maturity, this ketone is converted to l-menthol and l-methyl acetate in 1 compartment and to d-neomenthol and d-neomenthyl glucoside in a separate compartment. All enzymes involved in these reactions are soluble when prepared from whole-leaf homogenates. Mechanical separation of epidermal fragments from the mesophyll, followed by preparation of the soluble enzyme fraction from each tissue, revealed that the neomenthol dehydrogenase and the glucosyl transferase resided specifically in the mesophyll layer, whereas the menthol dehydrogenase and substantial amounts of the acetyl transferase were located in the epidermis, presumably within the epidermal oil glands. Apparently the compartmentation of menthone metabolism in peppermint leaves is intercellular, not intracellular.