Abstract
Primates are known to be basically herbivorous, eating leaves and buds of trees, fruits, etc. However, the diversified food habits are appreciable in several primates such as humans, chimpanzees, and common marmosets who actively eat animal flesh. Not only morphological adaptations but also molecular adaptations of digestive enzymes might be necessary to adapt new food habits. We discuss the molecular adaptations of pepsinogen, the gastric digestive proteinase, during primate evolution. Pepsinogen levels in primate stomachs are found to be the highest among all mammals examined so far. This high level might be due to the tact that primates are basically herbivorous, and need to digest plant foods efficiently. Although the occurrence of type-A and C pepsinogens is known in mammals, the multiplicity of type-A pepsinogen is found to be extreme in primates. The exception to this being, New World monkeys, for which the multiplicity of pepsinogen A and C is not found. Instead, New World monkeys express prochymosin at adult stage, a known neonate-specific pepsinogen in other mammals. Three types of pepsinogen work for digestion in New World monkeys. Old World monkeys have 3-4 types of pepsinogen A. Gene duplication generates the multiple forms bearing different enzymatic functions in protein digestion. In hominoids, the occurrence of 7-15 pepsinogen A isozymogens is remarkable. They can be classified into different groups acconling to their amino acid composition and enzymatic properties. The presence of multiple pepsinogens in Old World monkeys and hominoids might be advantageous in the efficient digestion of a variety of foods and may be correlated with the development of the central nervous system of these primates.
