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Soluble enzyme extracts from peppermint leaves, treated with polystyrene resin, were incubated with unlabeled substrates and were subsequently assayed by using capillary gas-liquid chromatography. Enzyme extracts from peppermint mesophyll cells were shown to isomerize (-)-isopiperitenone to piperitenone. Enzyme extracts from peppermint epidermal cells converted (-)-isopipertenone to (+)-pulegone, (-)-menthone, (-)-menthol and piperitenone, and (+)-pulegone to (-)-menthone and (-)-menthol. However, enzyme extracts cells harvested from suspension culture and callus did not catalyze the conversion of (-)-isopiperitenone and (+)-pulegone to any other monoterpenes. We thus decided to examine the metabolic capability of the cultured cells of peppermint with whole intact cells by feeding several exogenous substrates to the suspension cultures. Conversion of (-)-limonene into any other monoterpenes was not observed with the suspension cultures. (+)-Pulegone and (-)-menthone were converted into (+)-isomenthone and (-)-menthol, respectively. The experiments confirmed that the suspension retained most of the menthol biosynthesis machinery in the cell except a few loci. (-)-Isopiperitenone was transformed into (-)-pulegone and two unidentified products. Their molecular weight was 166 and mass spectrum of one of the products was close to that of mintlactone. To confirm (-)-isopiperitenone was a precursor of the mintlactone-like compound, deuterium labeled (-)-isopiperitenone was fed to peppermint suspension cell cultures.
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