Feeding and Digestion 1 59 



(4) Cathepsin II. Cathepsin II is similar in its substrate requirements to 

 trypsin but acts in a weakly acid medium (pH 4.9). It is activated by HCN, 

 HoS, cysteine, and glutathione. 



IMIDO-PROTEINASE. 



(1) Chymotrypsin. Chymotrypsin is a separate pancreatic enzyme from 

 trypsin. It attacks a peptide link with an aromatic amino acid on the carboxyl 

 side of the link. 



COOH .OH 



I / 



(CHO. ,^^ 



CoHsCHsCONH-CH-CONH-CH-CO-HN-CH.CO-NH, 



carbobenzoxyl glutamyl tyrosyl | glycine amide 



chymotrypsin break 



Chrymotrypsin acts in an alkaline medium, pH 7 to 9, and is activated by 

 trypsin or by enterokinase in the presence of trypsin. 



Exo peptidases. 1 hese enzymes attack terminal peptide links and act largelv 

 on products of proteinase action, particularly polypeptides. Their pi 1 require- 

 ments are not strict; they usually show a broad pH optimum in the neutral or 

 weakly alkaline range. 



CARBOXYPEPTiDASES. A carboxypcptidasc removes a terminal amino acid 

 with a free carboxyl group. It is inhibited by a free amino group nearby. A 

 favorite substrate used to test for carboxypcptidasc is chloracetyltyrosine. The 

 enzyme is formed in the pancreas of mammals. 



AMiNOPEPTiDASES. An aminopcptidasc acts on a peptide bond next to a 

 terminal amino acid with a free amino group. It is inhibited by a free carboxyl 

 nearby. Leucyldiglycine is often used as substrate to test for aminopeptidase. 

 The difference in specificity of carboxypeptidases and aminopcptidases is 

 shown in the following tripeptide substrate: 



CH. CH3 



\ / aOH 



CH /\ 



f ! IS/ 



H.N-CH-CO ~ HN-CH=-CO-HN — CH-COOH 



leucyl I glycyl , tyrosine 



amino- carboxy- 



pcptidasc peptidase 



break break 



The substrate requirement of cathepsin III is similar to that ot aminopepti- 

 dase. 



DiPEPTiDASES. Thcrc may be numerous dipeptidases. Like all other pro- 

 teolytic enzymes, they act on compounds of the L-series Qaevo configuration). 

 Dipeptidases break dipeptides into their component amino acids. Glycylglycin 

 and leucylglycin are commonly used substrates. Some dipeptidases are activ- 

 ated by magnesium and inhibited by HCN. 



