DIGESTION 



271 



Temperature. Like most chemical reactions the enzymatic hydrolysis of 

 foodstuffs is influenced by temperature, the reaction rate increasing with 

 rise in temperature and decreasing with fall in temperature. But enzymes, 

 being proteins, are subject to thermal inactivation, and the higher the 

 temperature at which digestion is occurring the more rapidly the enzyme is 

 destroyed. At low temperatures, therefore, hydrolysis proceeds more 

 slowly but the enzyme lasts longer; at high temperatures hydrolysis is 

 rapid but the enzyme is more rapidly inactivated. For any given set of 

 experimental conditions it is possible to determine the optimum tempera- 

 ture at which digestion proceeds most rapidly. 



Much attention has been devoted to determining optimal temperatures 

 for digestive enzymes of invertebrates. The curve in Fig. 6.9, for example, 



Fig. 6.10. Digestion of Hb by Crystalline Pepsin. Dependence 

 of Initial Rate of Digestion on pH 



Curve A, salmon pepsin. Curve B, Northrop's swine pepsin. (From Norris and 

 Elam (55).) 



shows that the optimal temperature for style amylase of Ensis is about 

 35 C C for a digestion time of 5 hours. Style amylase, however, is rapidly 

 destroyed at temperatures above 38 C C and the optimum temperature de- 

 pends on the incubation time. If the incubation time is short at high 

 temperatures, little enzyme is destroyed and the reaction rate is high; if 

 the incubation time is long, much more enzyme is inactivated and the 

 reaction rate is reduced. The optimal temperature for an enzyme, conse- 

 quently, is high over short periods, and falls as digest time of the experi- 

 ment is prolonged. This effect is brought out nicely in a study of the effect 

 of temperature on the digestive proteinase of the sea-squirt Tethyum 

 (Fig. 6.11), in which it has been shown that the optimum temperature of 

 the enzyme is about 50 C C over a period of 2 hours but only about 20 C 

 over 55 hours. In cold-blooded marine animals environmental temperatures 



