DIGESTION 



267 



the ends until dipeptides are formed; and the latter are split into their 

 amino-acid components by dipeptidases. In some instances the secreted 

 enzymes carry hydrolysis only as far as polypeptides, and further degrada- 

 tion takes place intracellularly (7). 



Carbohydrases. Amylases capable of hydrolysing starch and glycogen 

 probably occur in most animals, either extracellularly or intracellularly. 

 Starches are broken down to dextrins and finally to maltose. Disaccharides, 



50 



60 



20 30 40 



Digestion Time (minutes') 



Fig. 6.7. Digestion of 2% Hb at pH 2-0 by Crystalline 

 Salmon pepsin at various temperatures 



Ordinates represent the tyrosine equivalent liberated in 6 ml of digest. (From N orris 

 and Elam (55).) 



especially maltose and sucrose, are broken down by sucrases (maltase and 

 sucrase or invertase), which have been identified in many species. 



Of special interest are those enzymes attacking other complex carbo- 

 hydrates, notably cellulose and chitin. These are both chain compounds, 

 cellulose consisting of /S-glucoside (cellobiose) units, and chitin of acetyl- 

 glucosamine units. Cellulases capable of digesting cellulose have a very 

 restricted distribution among animals, some notable examples being 

 herbivorous pulmonates and wood-boring insects. Among marine animals 

 again we find a similar distribution, cellulase occurring in herbivorous 

 molluscs (lamellibranchs and gastropods) and wood-boring species 

 {Teredo, Limnoria). In Teredo and its relative Bankia, wood particles are 

 passed to a specialized region of the gut diverticula, where they are attacked 

 by an intracellular cellulase. Similarly, the digestive diverticula of Limnoria 

 produce a cellulase capable of hydrolysing cellulose in the wood eaten 

 by this animal. Two prosobranch gastropods Pterocera and Strombus 



