836 NUTRITION AND HEAT REGULATION. 



that is, they induce splitting or decomposition of the material by 

 the introduction of water. The supposition has naturally been 

 made that probably the oxidations of the body are effected also by 

 enzymes which in some way activate the oxygen. Enz} r mes of this 

 character have been found; they are designated in general as oxi- 

 dases or as oxidases and peroxidases, the former term referring to 

 those enzymes that effect oxidations in the presence of oxygen, 

 while the latter is applied to certain enzymes supposed to act 

 only in the presence of hydrogen peroxid. The most conspicuous of 

 the oxidases found in the animal body is the one capable of oxidiz- 

 ing aldehydes (salicylic or benzoic aldehyde) into the corresponding 

 acids, and hence designated specifically as aldehydase. This enzyme 

 has been extracted especially from the liver, lung, and spleen. 

 An oxidase known as tyrosinase, first found in plant juice, has also 

 been isolated from animal tissues. It oxidizes ty rosin with the 

 formation of homogentisinic acid. So also oxidases are described 

 capable of oxidizing xanthin to hypoxanthin or to uric acid.* The 

 process of destructive metabolism of sugar in the body, glycolysis, 

 may be effected, it will be remembered, by the tissue juices squeezed 

 from the organs, or even by extracts of the tissues of muscle, 

 liver, etc. It may well be believed, therefore, that the oxidation 

 of this most important food material is accomplished by the action 

 of one or more enzymes. Such facts as these lend great probability 

 to the belief that eventually it will be shown that the oxidations in 

 the body are effected by the influence of oxidases or peroxidases act- 

 ing singly or in combination or in sequence with the hydrolytic 

 enzymes. As a matter of fact, however, such an hypothesis is 

 not by any means demonstrated. Oxidases capable of destroying 

 proteid, fat, and carbohydrate material have not been actually iso- 

 lated, but the idea seems to be a fruitful one. 



While it is perfectly obvious that more facts are needed before 

 positive statements are warranted regarding the chemistry of the 

 oxidations in the body, the view entertained regarding the general 

 process in the body is that the material proteid, fat, or carbohy- 

 drate is first split into simpler products by the action of a hy- 

 drolytic enzyme, or a series of hydrolytic enzymes, formed in the 

 cells. These reactions are not attended by any marked formation 

 of heat. The split products thus produced are then acted upon by 

 oxidases with the formation of carbon dioxid, water, etc., and the 

 liberation of heat. A specific instance of this serial action has been 

 given in reference to the oxidation of sugar (p. 794) : According to 

 Stoklasa, the sugar is first split into lactic acid, and this into carbon 

 dioxid and alcohol; the alcohol then by the action of a series of 



* For further details see Oppenheimer, " Die Fermente und ihre Wir- 

 kungen," second edition, 1903. 



