Feeding and Digestion 165 



in tissues of the animals in which the trypsin is produced have not been very 

 successful. The pH optimum for trypsin is not always 7.5 to 8.0. A protease 

 acting at about pH 6.0 and activated by enterokinase has been reported for 

 Sepia; ^'*^ this needs confirmation. In two crickets trypsin has a broad optimum 

 with a peak at 6.2. In the silkworm the optimum is pH 9.5,^^^ and in wax 

 moth and clothes moth larvae the optimum is above 9.0. ^^ A similar high 

 optimum is found for Liinidus trypsin. In the clothes moth the gut fluid has 

 a very low oxidation-reduction potential and contains a strong reducing sub- 

 stance without which the trypsin cannot attack the disulfide bonds of kera- 

 tin. ^^'^ Trypsin reported from pyloric caeca of fish probably comes from 

 attached pancreatic tissue. 



The hydrogen ion concentration at which proteolytic enzymes function in 

 an animal is not necessarily identical with the pH optimum of the enzymes. 

 In Paranieciinn, for example, proteolytic digestion occurs in the alkaline or 

 near neutral phase of the gastric vacuole. ^^^' ^-^ Yet the cytoplasm contains 

 a protease with maximum activity at pH 4.6. ^^^ In snails the pH optimum 

 of the trypsin is much above the pH of the mid-gut. '^- Similar diflferences 

 are found in Astacus, Liinidus, and Polypus. In the fish Esox the pH of the 

 stomach contents varies with the food; it averages 5.2, whereas the optimum 

 pH for fish pepsin is 2.2. ^^^ Vonk suggests, however, that at the surface of 

 food particles where the enzyme is acting, the pH may be lower than the mean 

 pH of the whole food mass. 



Pepsin is characteristic of vertebrates only. Reports of proteolytic action in 

 very acid solutions by tissues of coelenterates -*'• ^' and Limnaea ■^- are not 

 convincing and probably do not indicate a true pepsin but rather a cathepsin. 

 This also seems likely for proteases acting at pH 3 to 4 as in Peripatopsis^^ 

 and in a copepod. -" The prochordates and microphagous cyclostomes lack a 

 stomach and have no peptic digestion. ^^ Beginning with the jawed carnivor- 

 ous fish, elasmobranchs, and bony fish, peptic digestion occurs in the stomach. 

 The pepsin of all vertebrates appears to be similar, ^"' and crystalline trypsin 

 from the tuna ^'-^^ resembles that from mammals. In several groups of teleosts 

 the stomach is lacking (Fig. 29, D), and no pepsin is found, for example, in 

 Fundidiis. "^ Acid and pepsinogen are secreted by different cells, and the 

 parietal cells (acid-secreting) may be located in different regions from the 

 chief cells (pepsin-secreting). In the frog pepsin is secreted by the esophagus 

 and upper stomach; hydrochloric acid is secreted by the lower (pyloric) stom- 

 ach. In birds such as the chicken the glandular stomach or proventriculus 

 lies between the crop and the grinding gizzard and secretes HCl and pepsin- 

 ogen. The crop is somewhat acid (pH 5.0), but most peptic digestion occurs 

 in the muscular gizzard. ^^ The gizzard contents are less acid in an adult 

 (pH 3.1) than in a young chicken (pH 2.7 in 23 day chicken). ^■^- 



It is difficult to compare the amount of proteinase from different animals. In 

 general, proteinase is more active in carnivorous than in herbivorous animals. 

 Proteinases are very weak in lamellibranch and herbivorous gastropod molluscs 

 and in tunicates,^''" but are powerful in coelenterates, echinoderms, and 

 cephalopods. In carnivorous insects such as cockroaches proteinases are strong, 

 whereas they are weak in herbivorous insects. ^"^ 



Distribution of Peptidases. Exopeptidases are widely distributed (Table 

 27). They have a broad pH optimum in the low alkaline range. In mammals 



