Inhibition of the microsomal metabolism of 1,8-cineole in the common brushtail possum (Trichosurus vulpecula) by terpenes and other chemicals

<p>1. Brushtail possums (<i>Trichosurus vulpecula</i>) ingest large amounts of terpenes in their diet of <i>Eucalyptus</i> leaf. Previously, we showed that dietary terpenes induce the cytochrome P450 enzymes (CYPs) responsible for their metabolism. The present study examined the effects of various CYP inhibitors on the metabolism of 1,8-cineole, the major dietary terpene, by liver microsomes from the possum and rat. </p> <p>2. Ketoconazole inhibited the major reactions of terpene-induced microsomes in both species: 9-hydroxylation in the possum and 2-hydroxylation in the rat. This suggests the involvement of CYP3A enzymes, although in the possum there was a lack of the expected inhibition by troleandomycin or activation by <i>α</i>-naphthoflavone, highlighting the differences between species in CYP forms. Diethyldithiocarbamate also inhibited 9-hydroxylation in the possum, indicating that a CYP2E1-like enzyme contributes to this reaction. </p> <p>3. Three other dietary terpenes were potent competitive inhibitors of 9-hydroxylation in the possum. <i>K</i>_i (µM) (mean ± SE, <i>n</i> = 4) were: <i>α</i>-pinene, 4.4 ± 1.1; limonene, 7.8 ± 2.1; <i>p</i>-cymene, 44.3 ± 11.2; cuminyl alcohol (a <i>p</i>-cymene metabolite), 6.0 ± 0.8. It appears likely that <i>p</i>-cymene acts via its metabolite to inhibit 1,8-cineole metabolism. </p> <p>4. Inhibitory interactions between dietary terpenes, as well as other plant secondary compounds, may impose a significant constraint on foliage consumption in the common brushtail possum, therefore explaining the obligatory generalist nature of this browsing marsupial and other generalist herbivores.</p>