GENERAL METHODS HISTORY OF PROTEIN FOOD. 817 



p. 777) and are closely related chemically. By hydrolytic processes 

 outside the body one may obtain their purin bases from nucleins 

 or nuclein acids, and inside the body the metabolic processes give 

 similar products. The histoiy of the intermediary metabolism 

 may be outlined as follows: When nucleoprotein is eaten the 

 nuclein is split off apparently by the action of the pepsin, the 

 protein portion then undergoing the digestive and metabolic changes 

 already described. The nuclein is not acted upon further by the 

 pepsin or trypsin, except that under the influence of the latter it is 

 rendered soluble possibly by an act of hydration.* Once within 

 the body the nuclein is submitted to the action of a series of enzymes 

 as follows: Under the influence of nuclease it is split with the 

 production of some of the purin bases, adenin, guanin. The amino- 

 purins so far as they are formed are converted to the corresponding 

 oxypurins by the action of a deamidizing enzyme. According 

 to Jones two such enzymes may occur in the body, namely, adenase 

 and guanase. Their action takes place as follows: 



C 6 H 5 N 5 + H,0 = C 5 H 4 N 4 + NH 3 



Adenin. Hypoxanthin. 



C 6 H 5 N 5 + H 2 = QH 4 NA {- NH, 



Guanin. Xanthin. 



The xanthin and hypoxanthin in turn are converted to uric acid 

 by the action of an oxidase, known as xanthinoxydase, and finally 

 the uric acid formed may in turn be partly split under the influence 

 of a special uricolytic enzyme so that part of its nitrogen is elimi- 

 nated as urea. Eventually, therefore, we may believe that the 

 nitrogen of the nuclein is excreted largely as uric acid and urea, 

 and to a smaller amount as xanthin, hypoxanthin, adenin, or guanin. 

 The proportion of the uric acid which is further split to form urea 

 varies in different animals (see p. 779). In man the proportion is 

 estimated at about one-half. Burian has given reasons for be- 

 lieving (see p. 779) that most of the purin nitrogen excreted arises 

 in the metabolism of the muscle, and represents presumably the 

 break down of organized structure. The action of the series of 

 enzymes just described seems adapted simply to remove the waste 

 nuclein with little corresponding profit to the body from the stand- 

 point of liberated heat energy. It is to be borne in mind, however, 

 that the activity of some of these enzymes, the nuclease, for example, 

 may be concerned in the synthesis of nucleins and nucleoproteins 

 within the body, a process about which at present little or nothing 

 is known. 



It is usually believed that the creatinin of the urine is formed 

 from the creatin found in muscular tissue and that the latter is 



*See Abderhalden and Schittenhelm, "Zeit. f. physiol. Chem.," 1906, 

 xlvii., 452. 

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