152 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



herbivorous animal like a rabbit, the urinary ammonia is but little 

 increased. It reacts with sodium carbonate in the tissues, forming 

 ammonium carbonate (which is excreted as urea) and sodium chloride. 

 Herbivora also suffer much more from, and are more easily killed by, 

 acids than carnivora, their organisation not permitting a ready supply 

 of ammonia to neutralise excess of acids. 



CREATININE 



Creatinine is one of the substances in the urine which represents 

 the end-stage of the tissue or endogenous katabolism of protein. It 

 is the most abundant of such katabolites, and the one concerning 

 which we know most. 



The most abundant of our active tissues is muscle, and the 

 amount of nitrogenous waste in this tissue which leaves it as urea 

 is insignificant. The place of urea is taken by another substance 

 called creatine. As already explained on p. 32, creatine is a com-' 

 pound of a urea radical with another group, and when it is boiled 

 with baryta water it takes up water and splits into two substances 

 namely, urea and sarcosine or methyl-glycine ; this is shown in 

 the following equation : 



H 2 N\ (T) 

 - N(CH 3 )CH 2 .COOH + H 2 O=H 2 N/ L 



[creatiuo] [water] [urea] 



+ NH.CH 3 .CH 2 .COOH 



[sarcosine] 



It is, however, extremely doubtful whether this decomposition occurs 

 in the body, and therefore whether creatine is to any important degree 

 a precursor of urea. For, when creatine is introduced directly into 

 the blood-stream, the amount of urea is not increased in the urine. 

 What is increased is another substance called creatinine, which is 

 creatine minus water, as shown in the following equation : 

 C 4 H 9 N 3 2 =C 4 H 7 N 3 +H 2 



[creatine] [cmitinine] [water] 



Creatinine, therefore, comes from nitrogenous katabolism in the 

 tissues, especially muscular tissues, and creatine is an intermediate 

 stage in its formation. Some of the creatinine, however, has a 

 different origin namely, an exogenous one ; that is, from the food 

 instead of from tissue metabolism ; and the substance in the food 

 which gives rise to it is the creatine contained in the flesh eaten. 



The best method for estimating creatinine and creatine is given in 

 Lesson XXV. 



