SKOPEC, MM; KARASOV, WH; University of Wisconsin, Madison: Do salivary proline-rich proteins decrease the metabolic cost of tannin consumption?

Tannins are polyphenolic compounds in plants that readily bind protein and can reduce palatability, digestibility and cause toxicity. Many mammals that come in contact with tannins in their diets have developed the ability to secrete proline-rich proteins (PRPs) in their saliva that preferentially bind to tannins. It is thought that the PRPs bind to tannins forming indigestible complexes that escape both enzymatic and microbial degradation and are excreted in the feces; thus protecting the animal from digestibility reduction and metabolic costs associated with the detoxification of absorbed tannin components. However, no one has shown quantitatively that PRPs increase fecal tannin excretion while decreasing detoxification products in urine. Thus we tested whether the presence of salivary PRPs would decrease the metabolic cost of pentagaloyl glucose (PGG – a hydrolysable tannin) consumption in laboratory rats. We used radiolabeled pentagaloyl [C14] glucose to determine if rats secreting PRPs excreted more radiolabeled tannin in their feces than rats not secreting PRPs. We also collected urine from the rats to determine if the rats secreting PRPs excreted less glucuronic acid (a measure of detoxification) in their urine than rats not secreting PRPs. We found that rats which were secreting PRPs excreted four times more of the ingested radiolabel in their feces than rats that were not secreting PRPs (20.5+/-5.1% vs. 5.0+/-3.3%). We also found that rats which were secreting PRPs excreted less than half as much glucuronic acid in their urine as rats that were not secreting PRPs (11.9+/-5.5 vs. 28.4+/-19.0 mg glucuronic acid/g PGG consumed). Thus we conclude that the presence of salivary PRPs does decrease the metabolic costs of PGG consumption in rats. Supported by USDA (Hatch) WISO4322 NSF IBN-9318675 IBN-9723793