214 THE HYDROLYZING ENZYMES 



comparable in experiments with different enzymes, and we are there- 

 fore left frequently in uncertainty whether the observed differences in 

 temperature-optimum are in reality due to specific differences between 

 the enzymes investigated or are not simply attributable to the circum- 

 stances attending their action. In the very great majority of cases, 

 however, it is found that the temperature-optimum from hydrolyzing 

 enzymes lies very slightly above the body-temperature of the warm- 

 blooded animals, namely between 40 and 45 C., the normal tempera- 

 ture of man being 37.8 C. and the temperature of birds about 41 C. 



This remarkable correspondence is certainly not accidental, and we 

 may infer that the processes which have brought about the evolution 

 of the warm-blooded animals from cold-blooded ancestors has consisted 

 essentially in an improvement of the adaptation of the more recent 

 forms to the properties of their enzymes, whereby swifter transforma- 

 tions and exchanges of material are rendered possible without at the 

 same time incurring the wasteful expenditure of catalysts which would 

 be involved by still higher bodily temperatures. The factor which 

 determines the bodily temperature of the warm-blooded animals is 

 certainly not the coagulation-temperature of their tissue-proteins, for 

 that lies very considerably above the maximum body-temperature 

 which is observed in any species. The cold-blooded animals and 

 plants, therefore, are handicapped by a disharmony between the 

 properties of their enzymes and the temperature of their tissues. 

 Whether or not this is in some instances compensated for by greater 

 specific activity of their enzymes or by the production of enzymes in 

 greater quantity is a question which the data at present in our posses- 

 sion do not enable us to answer. 



In a few exceptional cases the temperature-optimum lies far above 

 the usual level. We have seen that some enzymes, especially certain 

 oxidizing enzymes and the proteolytic enzymes derived from certain 

 bacteria (Bacillus prodigiosus, for example) will withstand the tem- 

 perature of boiling water. Two vegetable proteolytic enzymes, 

 namely the Papain from the pawpaw, or fruit of Carica papaya and the 

 Bromelin in pineapples act best at about 60 C. Generally speaking, 

 the more nearly neutral the solution of the enzyme and the higher the 

 concentration of substrate it contains, the higher is the optimal tem- 

 perature. It would therefore appear that acids and alkalies accelerate 

 the inactivation of enzymes and that their substrates protect them, 

 probably owing to the fact that they combine with them. 



Exposure of an enzyme to moderately high temperatures, for example 

 60 C. for some hours generally results, not only in loss of hydrolyzing 

 power, but in the acquirement of a power to inhibit the very hydrolysis 

 which the active enzyme normally accelerates. This phenomenon 

 has been attributed by Bayliss to the formation of "Zymoids" which, 

 he believes, combine with the enzymes from which they are derived 

 to form an inactive compound. In some instances, however, it 

 appears that the retarding influence of heated enzymes arises not so 



