Monoterpene biosynthesis: specificity of the hydroxylations of (-)-limonene by enzyme preparations from peppermint (Mentha piperita), spearmint (Mentha spicata), and perilla (Perilla frutescens) leaves

Microsomal preparations from the epidermal oil glands of Mentha piperita, Mentha spicata, and Perilla frutescens leaves catalyze the NADPH- and O2-dependent allylic hydroxylation of the monoterpene olefin (-)-limonene at C-3, C-6, and C-7, respectively, to produce the corresponding alcohols, (-)-trans-isopiperitenol, (-)-trans-carveol, and (-)-perillyl alcohol. These transformations are the key steps in the biosynthesis of oxygenated monoterpenes in the respective species, and the responsible enzyme systems meet most of the established criteria for cytochrome P450-dependent mixed function oxygenases. The reactions catalyzed are completely regiospecific and, while exhibiting only a modest degree of enantioselectivity, are highly specific for limonene as substrate. Of numerous monoterpene olefins tested, including several positional isomers of limonene, only the 8,9-dihydro analog served as an alternate substrate for ring (C-3 and C-6) hydroxylation, but not side chain (C-7) hydroxylation. In addition to the regiospecificity of the allylic hydroxylation, these enzymes are also readily distinguishable based on differential inhibition by substituted imidazoles.