PHYSIOLOGICAL OXIDATIONS. 963 



number of reactions, chiefly color reactions, such as the bluing of a 

 tincture of guaiacum in the presence of a peroxide or the conversion 

 of a colorless or leucobase to a colored oxidation product. Some of 

 these oxidases or peroxidases have been given specific names in 

 accordance with the particular compounds whose oxidation they 

 effect. For example, xanthinoxidase, which effects the oxidation 

 of hypoxanthin and xanthin to uric acid; the glycolytic oxidase or 

 oxidases, which effect the oxidation of the intermediary products 

 of sugar metabolism in the tissues; tyrosinase, which effects the 

 oxidation of tyrosin, and in this way is supposed by many observers 

 to give rise to various animal pigments, such as melanin; the 

 aldehydases, which effect the oxidation of aldehydes to their cor- 

 responding acids salicylic aldehyd, for instance, to salicylic acids. 

 This list might be greatly extended, particularly if those that occur 

 in the plants were also considered, but as it is, it suffices perhaps 

 to illustrate the general belief regarding the wide-spread occurrence 

 and the specific properties of these important substances. Whereas, 

 formerly, the general belief among physiologists was that physio- 

 logical or vital oxidations are effected as part of the metabolism 

 of the living substance, the tendency at present is to assume that 

 these oxidations are not effected directly by changes in the living 

 substance, but indirectly, in that the latter forms these organic 

 peroxides and peroxidases through whose interaction the oxygen 

 is liberated in active form. The oxidations effected by this 

 means are the principal source of the development of heat in the 

 body they are especially exothermic reactions. Many other of 

 the chemical changes of metabolism, such as the hydrolytic cleav- 

 ages, liberate but little heat, and others still, such as the syntheses 

 of one kind or another in which there is a union of compounds to 

 form more complex substances, may even be attended by an 

 absorption of heat, that is, a conversion of heat energy to the energy 

 of chemical affinity. The oxidizing reactions constitute, therefore, 

 a large and very characteristic feature of the metabolism of the 

 warm-blooded animals. The heat thus produced by the oxidation 

 of our food material serves to maintain the body temperature at 

 its normal high level, and establishes a temperature environment 

 at which the enzymatic and chemical metabolism of the tissues 

 is at its optimum. In addition some physiologists believe that 

 a portion of this heat is used in the work of the body, that is to say, 

 they regard the body as a sort of thermodynamical engine in which 

 the energy of the food is obtained first as heat, and the heat is then 

 utilized in part for the other energy needs of the body. Others, 

 however, are unwilling to accept this view of the body mechanism, 

 and prefer to believe that the chemical energy of the food can be 

 utilized directly for the various energy needs of the body without 

 passing through the form of heat. 



