158 Cmnparative Animal Physiology 



the older schemes and of the modern classification of Bergman "=* are com- 

 bined in a general plan based on that of Baldwin. '' Proteases differ according 

 to the substrates attacked, the optimum pH, and the effects of activating and 

 inhibiting agents. 



Endo peptidases. These enzymes attack central peptide bonds, and may also 

 act on smaller molecules than proteins, for example peptones. 



CARBONYL-PROTEINASES. 



(1) Pepsin. Pepsin attacks a peptide linkage between a dicarboxylic and 

 an aromatic amino acid if the second carbonyl of the dicarboxylic acid is free 

 and if there is no free amino group near the peptide link. That is, pepsin 

 requires amino acids such as phenylalanine or tyrosine in the backbone of the 

 molecule, attached as in the following example: 



COOH (freecarboxyl) /vOH 



CeHsCH^O-C-HN-CH-CO -\- HN-CH-CO-HN-CH.-CO-NH2 



I 

 Pepsin 

 break 



carbobenzoxy glutamyl tyrosyl glycine amide 



(dicarboxylic) (aromatic) 



Pepsin is usually characterized physiologically by working in an acid medium 

 and by being inactivated in an alkaline one. According to Northrup, pepsin 

 acts on positively charged proteins. Pepsin is secreted as pepsinogen which is 

 activated autocatalytically in an acid medium (HCl). 



(2) Cathepsin I. Cathepsin I, isolated from mammalian liver and spleen, 

 resembles pepsin in the specificity of its substrate requirements. However, it 

 acts in a medium which is neutral or slightly acid and is acti\'ated by reducing 

 agents such as MCN, H^S, cysteine, and glutathione. 



(3) Trypsin. Trypsin acts at a peptide link adjacent to arginine or lysine. 

 It may attack some other peptide links as well but is identified by use of a 

 substrate such as the following: 



C=NH 



I 



(CHOa 



NH, 



CH2 



! 



(CHO. 



I -HN-CH-CO — HN-C- 



-HN-CH-CO ^ HN-C- ! I 



arginine trypsin or lysine trypsin 



break break 



Trypsin is characterized physiologically by acting in an alkaline medium (pH 

 7 to 9), in which most proteins are negati\'ely charged. It is secreted as tryp- 

 sinogen, which is activated by some specific enzyme (zookinase), such as the 

 intestinal cnicrokinasc; once started, tr\psin actixation ma\ occur autocata- 

 Ivticallv. 



