216 THE HYDROLYZING ENZYMES 



exceed the "temperature-optimum/' otherwise the secondary inactiva- 

 tion of the enzyme introduces a disturbing factor. 



It is more simple, however, although less accurate, to estimate the 

 effect of temperature by the change in velocity produced by a rise of 

 10 C. It is found, as a very general rule, excepting in the case of 

 photochemical reactions, that the value of ^ for chemical transforma- 

 tions is of such a magnitude (10,000 or over) that a rise of 10 at ordi- 

 nary room- or incubator-temperatures doubles or more than doubles 

 the velocity of transformation. 



The "Temperature-coefficient," or ratio: 



Velocity at T + 10 

 Velocity at T 



for chemical reactions is therefore 2 or over, while for purely physical 

 processes, such as changes in viscosity or capillarity or for photo- 

 chemical reactions the value of the coefficient generally only slightly 

 exceeds unity or, in the case of capillary phenomena, may be less than 

 unity. 



The following are illustrative values of "/*" for various hydrolyses 

 brought about by enzymes. For comparison the value of n for the 

 hydrolysis of cane-sugar by acids is included. 



Process. At 



Hydrolysis of cane-sugar by acids 25,600 



Hydrolysis of cane-sugar by invertase 11,000 



Hydrolysis of starch by amylase 12,300 



Hydrolysis of triacetin by lipase 16,700 



Hydrolysis of egg-albumin by pepsin . . . . . . . . 15,570 



Hydrolysis of casein by trypsin 37,500 



Inactivation of rennet 90,000 



Inactivation of pepsin 75,000 



Inactivation of invertase 72,COO 



Inactivation of trypsin 62, 000 



On comparing these various figures it will be seen that the effect of 

 temperature upon enzymatic hydrolyses is of the same general order 

 as its effect upon other chemical reactions. The Inactivation of an 

 enzyme by heat, however, is exceptionally accelerated by rise of 

 temperature, the coefficients for all inactivations being very much 

 higher than those for the hydrolyses which the enzymes accelerate. 

 This accounts for the relative "steepness" with which the curve of 

 enzymatic activity falls off after the temperature has passed the opti- 

 mum; at this point the inactivation of the enzyme is very much 

 accelerated by a rise in temperature sufficient only to produce a slight 

 modification of the velocity of the hydrolysis which the enzyme is 

 accomplishing. 



Enzymes are also inactivated by exposure of their solutions to Light, 

 and especially to the Ultra-violet Rays. The inactivation by ultra- 

 violet light occurs in the absence of oxygen, but the visible rays of 

 light, especially in the presence of fluorescent dyes such as Eosin, are 



