CONDITIONING FACTORS 467 



enzymes, some of which are mentioned at the end of this 

 section, 



I. TEMPERATURE. 



I. In general terms, the velocity of a reaction, enzymic or 

 otherwise, increases with a rise of temperature in accordance 

 with van't Hoff's law. Enzymes are thermolabile ; they are 

 destroyed at 100° C, and in the majority of cases cannot be 

 heated with safety above 60° C. This sensitivity to heat may 

 in part be explained by attributing it to the colloidal nature 

 of the enzyme and the consequent tendency to coagulation by 

 heat. Even at low temperatures enzymes become inactive 

 at varying rates ; an increase in temperature hastens this 

 inactivation which obtains at various degrees and marks the 

 thermal death-point. This point is difficult to determine 

 since it depends on various factors such as the reaction of the 

 medium ; the degree of purity, which may be indeterminate ; 

 the presence of various substances in the substrate, protein, 

 for example, which may act as a protective colloid ; the pres- 

 ence of various salts, particularly phosphates and chlorides, 

 which may enter into combination with the enzyme ; together 

 with other factors. For these reasons it is essential to have an 

 arbitrary definition of the thermal inactivation point of en- 

 zymes : Euler * defines it as that temperature at which the 

 activity of the enzyme is halved when heated for one hour in 

 an aqueous solution free from the appropriate substrate and 

 having a definite hydrogen ion concentration. Enzymes in- 

 activated by heat may in some cases recover their activity 

 by suitable treatment ; thus Falk f observed the partial 

 recovery of lipase on adding manganese to the solution, 

 similarly Biedermann J found that the diastatic activity of 

 saliva, after heating to 100° C, could be restored by vigorous 

 shaking with air. Gallagher,§ moreover, found that the per- 

 oxidase of the mangold could be temporarily inactivated by 

 heating for a little less than two minutes at 100°, but recovered 



* Euler : " Chemie der Enzyme," Miinchen, 1925. 

 t Falk : " J. Amer. Cham. Soc," 1913, 35, 601. 

 I Biedermann : " Biochem. Zeit.," 1922, 129, 582. 

 § Gallagher ; " Biochem. Journ.," 1924, 18, 39. 



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