COMPOSITION OF FOOD AND ACTION OF ENZYMES. 757 



Oxidases. Lungs, liver, mus- Cause oxidation of organ- 



cle, etc. ic substances, as in the 



conversion of hypoxan- 



thin to xanthin and of 



xanthin to uric acid. 



Reductases. Tissues generally. Cause reduction or deoxi- 



dation as, for example, 

 the reduction of oxy- 

 hemoglobin to hemo- 

 globin. 



Catalase. Many tissues. Decomposes hydrogen 



Seroxid. 

 ts arginin with pro- 

 duction of urea and 

 ornithin (diamino-vale- 

 rianic acid). 



Chemical Composition of the Enzymes. It was formerly 

 believed that the enzymes belong to the group of proteins. They 

 are formed from living matter, and their solutions as usually 

 prepared give protein reactions. Increased study, however, has 

 made this belief uncertain. The enzymes cling to the proteins 

 when precipitated, and it seems possible that the protein reac- 

 tions of their solutions may be due, therefore, to an incomplete 

 purification. In fact, it is stated that solutions of some of the 

 enzymes may be prepared which show ferment activity, but 

 give no protein reactions. In this group may be included the 

 lipase, diastase, invertase, pepsin, oxidase, and catalase. Appar- 

 ently, however, all enzymes contain nitrogen and most of them 

 also sulphur. They probably also contain some ash, especially 

 calcium. Much of the older work upon the composition of 

 supposedly purified preparations of enzymes is not accepted 

 to-day, on the ground that the evidence for the purity of the 

 preparations is insufficient. In spite, however, of the very great 

 amount of attention that has been paid to these substances in 

 recent years, there is at present no agreement as to their chemical 

 structure. The statement made above that they are organic 

 substances, derived from proteins and of a colloidal nature, is 

 perhaps as much as can be said positively in regard to their 

 chemical structure. As a rule, they are destroyed by moderately 

 high temperatures (80 C. or below), they are not easily diffusible 

 through parchment membranes, and, like the proteins, are " salted 

 out " by certain concentrations of neutral salts. 



