Mammalian herbivores, particularly dietary specialists must have an efficient means to metabolize the high doses of plant secondary compounds they consume. We found previously that Neotoma stephensi, a juniper specialist, upregulated catechol-O-methyl transferase (COMT) mRNA almost seven fold in response to an ecologically relevant diet (70% juniper). To further investigate the relevance of this enzyme with respect to juniper metabolism, we compared the protein expression, activity and kinetics of the two forms of COMT, soluble (S-COMT) and membrane bound (MB-COMT), in the blood, kidneys and liver of N. stephensi on its natural juniper diet to that of N. stephensi fed an experimental diet of 70% juniper as well as a non-toxic control diet under laboratory conditions. In addition, we compared these results to that of Neotoma albigula, a generalist species, which consumes a diet of 25% juniper in the wild. The specialist consuming juniper under both field and laboratory conditions had increased S-COMT expression and activity in their livers and kidneys, and increased S-COMT activity in their blood compared to the specialist and generalist fed the control diet. The specialist showed expression and activity of S-COMT in their kidneys that was as high as or higher than that in their livers. The generalist had an elevated V(max) for MB-COMT compared to the specialist that resulted in higher activity for MB-COMT than the specialist despite lower expression of MB-COMT in the generalist's livers and kidneys. This high activity MB-COMT may be in part responsible for differences in the behaviors of the generalist compared to the specialist. We conclude that S-COMT is important in the specialist's ability to consume high levels of juniper.
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