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

The metabolism of l-menthone, which is synthesized in the epidermal oil glands of peppermint (Mentha piperita L. cv. Black Mitcham) leaves, is compartmented; on leaf maturity, this ketone is converted to l-menthol and l-menthyl acetate in one compartment, and to d-neomenthol and d-neomenthyl glucoside in a separate compartment. All of the 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. These results suggest that the compartmentation of menthone metabolism in peppermint leaves is intercellular, not intracellular.