Salivary proteins as a defense against dietary tannins

Tannins, a diverse group of water-soluble phenolics with high affinity to proteins, are widely distributed in various parts of plants, and have negative effects in herbivores after ingestion. Some mammalian species are thought to counteract tannins by secreting tannin-binding salivary proteins (TBSPs). Several types of TBSPs are found in the saliva of laboratory animals, livestock, and wildlife. Among them, proline-rich proteins (PRPs) and histatins are effective precipitators of tannins. It is widely accepted that, at the least, PRPs act as a first line of defense against tannins. Many observations support this idea: in vitro affinity of PRPs to tannins is far higher than that of other proteins such as bovine serum albumin; complexes formed between PRPs and tannins are stable even under the conditions in the stomach and intestine; and PRP production is induced by ingesting tannins. It is believed that species that usually ingest tannins as part of their natural diets produce high levels of PRPs, whereas species not exposed to tannins produce little or no PRPs. This hypothesis is generally supported, although studies on TBSPs in wildlife are limited. This work stresses the importance of gathering basic information on such items as the characteristics of unidentified TBSPs, and seasonal and geographical variations in PRP production.