SALIVARY PROLINE-RICH PROTEINS IN MAMMALS - ROLES IN ORAL HOMEOSTASISAND COUNTERACTING DIETARY TANNIN

We review information on the structure of proline-rich proteins (PRPs) , their various functions related to oral homeostasis and dietary tann in, and the structural basis of these functions. Considerations of the multifunctional nature of these salivary proteins helps explain both the subtle and large variations found in structure and secretion rates both within individuals and between species. We propose that the ance stral function of PRPs is in maintaining oral homeostasis and that cou nteracting dietary tannins by binding with them is a derived function. PRPs are effective in oral homeostasis at low secretion levels, where as counteracting tannin depends on high secretion levels. In the dieta ry habits ranging from carnivores through omnivores to exclusively pla nt-eaters, the dietary nitrogen level is progressively reduced, and pl ant allelo-chemical intake, including tannins, increases. We suggest t hat during this evolution from meat-eater to plant-eater, there was so me point in omnivory at which selective pressure from nitrogen limitat ions, arising from a low nitrogen/high tannin diet, became sufficientl y great for the evolution of increased secretion level and diversifica tion of PRPs for dealing with tannin. If this hypothesis is correct, c arnivorous mammals should secrete low levels of PRPs for oral homeosta sis, but should never secrete high levels, unless they are secondarily carnivorous. Omnivores consuming a diet of very little animal tissue but higher levels of tannin-containing foliage or fruit should general ly have the capacity to produce high levels of salivary PRPs. Browsers and frugivores should also produce high levels of PRPs, but grazers m ay have reduced secretion rates depending on the antiquity of the diet ary habit. This hypothesis is consistent with the limited information available on the abundance, type, and distribution of PRPs in mammals. Studies are suggested which would test the functional and evolutionar y arguments presented.