METABOLISM, NUTRITION AND DIETETICS 509 



proteins (and gelatin) . It is also a constituent of glycocholic acid, 

 and may be derived in part from the bile which is reabsorbed. 



Kreatinin can be so readily obtained from kreatin outside the 

 body that it is tempting to suppose that the portion of the 

 kreatinin of the urine which is not formed from the kreatin in 

 the food is derived from the kreatin of the muscles and other 

 tissues. The constancy of the kreatinin elimination on a meat- 

 free diet, and its complete independence of the changes in the 

 total nitrogen excretion, show that it has a different significance 

 in protein metabolism from the urea. There is some reason to 

 suspect that the kreatinin may represent nitrogen given off in 

 the constant wear and tear of the bodily machinery. The fact 

 that the amount of kreatinin excreted by different persons seems 

 to be related to the weight of active tissue in the body, excluding 

 fat, is in favour of this suggestion. The statement that the 

 content of the urine in kreatinin is increased 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. 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. 



Formation of Carbon Dioxide from Proteins. We cannot 

 say whether any carbon dioxide is normally produced at the 

 moment when the nitrogenous portion of the protein molecule 

 splits off, or whether a carbonaceous residue may not always hang 

 together for a time and pass through further stages before 

 the carbon is fully oxidized. We shall see that under certain 

 conditions some of the carbon of proteins may be retained in 

 the body as glycogen or fat ; and this suggests that in all cases 

 it may run through intermediate products as yet unknown 

 before being finally excreted as carbon dioxide. 



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. 264). Reducing 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 reductions 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. Not only do unicellular organisms, like leucocytes, 



