Inhibition of Snowshoe Hare Succinate Dehydrogenase Activity as a Mechanism of Deterrence for Papyriferic Acid in Birch
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The plant secondary metabolite papyriferic acid (PA) deters browsing by snowshoe hares (Lepus americanus) on the juvenile developmental stage of the Alaska paper birch (Betula neoalaskana). However, the physiological mechanism that reduces browsing remains unknown. We used pharmacological assays and molecular modeling to test the hypothesis that inhibition of succinate dehydrogenase (SDH) is a mode of action (MOA) of toxicity of PA in snowshoe hares. We tested this hypothesis by measuring the effect of PA on the activity of SDH in liver mitochondria isolated from wild hares. In addition, we used molecular modeling to determine the specific binding site of PA on SDH. We found that PA inhibits SDH from hares by an uncompetitive mechanism in a dose-dependent manner. Molecular modeling suggests that inhibition of SDH is a result of binding of PA at the ubiquinone binding sites in complex II. Our results provide a MOA for toxicity that may be responsible for the concentration-dependent anti-feedant effects of PA. We propose that snowshoe hares reduce the dose-dependent toxic consequences of PA by relying on efflux transporters and metabolizing enzymes that lower systemic exposure to dietary PA.
Key WordsChemical defense Enzyme inhibition Mode of action Papyriferic acid Succinate dehydrogenase Snowshoe hare
We thank Thomas Allen and Dr. Ethan Ellsworth for assistance with the collection of hares and tissues and the field. We thank Dr. Thomas Clausen, Department of Chemistry and Biochemistry, University of Alaska, Fairbanks, for his donation of papyriferic acid. We are grateful to Dr. Stuart McLean for initiating ideas for this study, his assistance in developing assays, and critique of the manuscript. We thank Dr. Rajesh Nagarajan, Department of Chemistry, Boise State University, for assistance with interpretation of enzyme kinetic data. We appreciate the comments from three anonymous reviewers for improving the manuscript. Research was supported by Boise State University, the Bonanza Creek LTER program (NSF-DEB-0423442), and the US Forest Service (PNW01-JV11226952-231).
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