588 METABOLISM, NUTRITION AND DIETETICS 



chiefly, the nitrogen of the surplus amino-acids which are not utilized 

 in the building of new or the repair of old tissue elements. The 

 fact that the amount of kreatinin excreted by different persons 

 seems to be related to the weight of active tissue in the body, exclud- 

 ing fat, is in favour of this suggestion, and there is other evidence 

 pointing in the same direction; for example, in ordinary circum- 

 stances kreatin is either absent from the urine or present in very 

 small amount, except in young children. When, however, the de- 

 composition of tissue-protein is abnormally increased, as in starva- 

 tion, in fevers, in women after delivery, while involution of the 

 uterus and the associated destruction of a considerable mass of 

 smooth muscle is taking place, kreatin appears in larger quantities 

 in the urine,* per haps because it can no longer be all converted into 

 kreatinin. Now, the increased excretion of kreatin in starvation 

 can be prevented by giving carbo-hydrate food, which is known 

 (p. 595) to lead to sparing of tissue-protein (Mendel and Rose). 



The statement that the content of the urine in kreatinin is in- 

 creased by muscular work may indicate that the muscular machine 

 wears out faster during activity than during rest, or perhaps only 

 that already-formed kreatin leaves the muscles in greater amount 

 when the blood-flow is increased; but recent observations tend to 

 show that this statement may require revision. 



As to the manner in which kreatin is changed into kreatinin in 

 the body, a highly suggestive fact is the presence of ferments in 

 various organs which possess this power. Ferments also exist 

 which can decompose both kreatin and kreatinin. The existence 

 of such enzymes is presumptive evidence that the changes which 

 they are capable of producing actually occur in the organism ; but 

 the seat of the changes if they do take place, and their metabolic 

 significance, are unknown. Kreatin when given by the mouth or 

 injected into the blood does not cause any increase in the urinary 

 kreatinin, nor when administered in moderate quantities does it 

 seem to be excreted as kreatin. Kreatinin, on the other hand, 

 when added to the food, causes an increase in the kreatinin of the 

 urine. 



Intracellular Ferments Autolysis. As to the agencies by which 

 the decomposition of the proteins is carried out in the cells, we have 

 already spoken of the oxidizing cell ferments, or oxydases (p. 267). Re- 

 ducing ferments, or reductases, are also known, and can be extracted 

 from most organs, if not all. Like oxydases, they act in a weakly 

 alkaline medium, causing in the presence of hydrogen such reduc- 

 tions as the formation of nitrites from nitrates. There is some 

 evidence that one and the same ferment may act as an oxydase or 

 a reductase according to the conditions. Recent researches have 

 brought to light in addition hydrolytic intracellular ferments, which 

 split up proteins very much in the same way as the proteolytic 

 ferments of the digestive juices. 



