Papyriferic acid (PA) is a
triterpene that is secreted by glands on twigs of the juvenile ontogenetic phase of resin producing tree birches (e.g., Betula neoalaskana, B. pendula) and that deters browsing by mammals such as the snowshoe hare (Lepus americanus). We investigated the pharmacology of PA as a first step in understanding its antifeedant effect. After
oral administration to rats, PA and several metabolites were found in feces but not urine, indicating that little was absorbed systemically. Metabolism involved various combinations of hydrolysis of its acetyl and malonyl
ester groups, and hydroxylation of the
terpene moiety. The presence of a malonyl group suggested a possible interaction with
succinate dehydrogenase (SDH), a mitochondrial
enzyme known to be competitively inhibited by
malonic acid. The effect of PA on the oxidation of
succinate by SDH was examined in mitochondrial preparations from livers of ox, rabbit, and rat. In all three species, PA was a potent inhibitor of SDH. Kinetic analysis indicated that, unlike
malonate, PA acted by an uncompetitive mechanism, meaning that it binds to the
enzyme-substrate complex. The hydrolysis product of PA, betulafolienetriol
oxide, was inactive on SDH. Overall, the evidence suggests that PA acts as the intact molecule and interacts at a site other than the
succinate binding site, possibly binding to the
ubiquinone sites on complex II.
Papyriferic acid was potent (K(iEIS) ranged from 25 to 45 microM in the three species) and selective, as
malate dehydrogenase was unaffected. Although rigorous proof will require further experiments, we have a plausible mechanism for the antifeedant effect of PA: inhibition of SDH in gastrointestinal cells decreases mitochondrial energy production resulting in a noxious stimulus,
5-HT release, and sensations of
nausea and discomfort. There is evidence that the co-evolution of birches and hares over a large and geographically-diverse area in Northern Europe and America has produced marked differences in the formation of PA by birches, and the tolerance of hares to dietary PA. The present findings on the metabolic fate and biochemical effects of PA provide a rational basis for investigating the mechanisms underlying differences among populations of hares in their tolerance of a PA-rich diet.