Feeding and Digestion 



163 



TABLE 26 (continued) 

 CHARACTERISTICS OF PROTEINASES 



The proteolytic enzymes of tissues (cathepsins) have not been characterized 

 except tor the mammalian liver and spleen. In fact little separation of pro- 

 teases has been made by substrate. Hence for practical purposes we can dis- 

 tinguish proteases with low pH optimum (pH 1.5-3) as pepsins, those working 

 in an alkaline medium (pH 7-9) as trypsins, and those with a weakly acid 

 optimum (pH 4.5-6.5) as cathepsins. This implies that there are both intra- 

 cellular and extracellular cathepsins; those known from vertebrates are intra- 

 cellular. The pH optimum has little meaning without a statement of the 

 substrate, since with certain substrates the optimum is lower than with 

 others. '•^" 



Rennin is an enzyme which clots the milk protein casein (paracasein). Some 

 proteolytic enzymes have a rennin action. However, there are probably specific 

 rennins, as in the stomach of infants, distinct from pepsin. 



Distribution of Proteinases. Proteolytic enzymes have been detected in 

 tissue extracts and digestive fluids of many animals. Selected data are sum- 

 marized in Table 26. Several generalizations are evident from this table. 



An enzyme extracted from glandular tissues is essentially the same as the 

 enzyme from the lumen of a digestive organ into which the glands secrete. 

 This has been shown for the hepatopancreas and stomach fluid of Limulus,^*^ 

 Astacus,'^^ and Maja,^^^ and is well substantiated for pepsin and trypsin in 

 mammals. 



Cathepsin is an enzyme hydrolyzing proteins in the range of pH 4.5 to 6.5 

 and activated by HoS, HCN, or cysteine. It occurs in digestive fluids and 

 digestive glands of many invertebrate animals. This type of protease has been 

 demonstrated in Paramecium, '^•'* several snails, ^"^' the cephalopod Polypus 

 (Fig. 31), ^^^ and in several echinoderms, especially Distolasterias (Fig. 

 30). ^^'^ Often two pH optima occur, one in weak acid for a cathepsin and 

 the other in the alkaline range for a trypsin (Figs. 30 and 31). Catheptic 



