acid (Dessaignes). When creatin is oxidized with hydrogen peroxid in 

 the presence of ferrous sulphate, glyoxylic acid is formed (Dakin (c) ). Ee- 

 cently a new substance, methylguanidoglyoxylic acid, was obtained upon 

 oxidizing creatin with mercuric acetate in watery solution (Bauman and 

 Ingvaldsen). The successive steps in the oxidation of creatin may be 

 formulated as follows: 



1. NH 2 C(:NH)N(CH 3 )CH 2 COOH1 + O = NH 2 C(:NH)N 

 (CII 3 )CHOHCOOH. 



2. NH 2 C(:NH)N(CH 3 )CHOHCOOH + O = NH 2 C(:NH)N 

 (CH 3 )COCOOH. 



3. NH 2 C(:NH)N(CH 3 )COCOOH + H 2 O = NH 2 C(:NH3)N 

 (CH 3 )H + COOHCOOH. 



The ease with which creatin is oxidized by metallic salts is noteworthy. 

 The alleged occurrence of methylguanidin in the blood, muscle and urine 

 may in reality be the result of oxidation of creatin by the mercuric or 

 argentic salts which are ordinarily used for the purpose of isolation. 



When picric acid is added to urine a characteristic potassium creatinin 

 picrate is precipitated (Jaffe (e), 1886) ; this compound may be readily 

 converted into the time-honored zinc chlorid salt according to the method of 

 Benedict (a) (1914). In this manner relatively large quantities of 

 creatinin (and creatin) may be prepared so that it has become readily 

 accessible to most laboratories and is now used to prepare standard solu- 

 tions for its quantitative colorimetric determination. 



Jaffe (e) (1886) first noted that an alkaline solution of creatinin re- 

 duces picric acid to a reddish compound (probably picramic acid). Folin 

 (a) (1904) proved that the intensity of the color was directly proportional 

 to the amount of creatinin and therefore that this reaction was well adapted 

 for its quantitative colorimetric determination. The publication of this 

 method proved to be an incentive for numerous investigations of the 

 physiological behavior of creatin and creatinin, since the former may 

 readily be converted into the latter by relatively simple means. 



The Creatin Content of Muscle and other Tissues 



Creatin is a characteristic constituent of the muscle tissue of all 

 vertebrates. In the skeletal muscle of the horse, for example, it forms 

 approximately one-third of the total extractive nitrogen, the remainder 

 being formed by carnosin and other compounds (Von Fuerth and 

 Schwartz). Creatin is most abundant in voluntary muscle; there is less 

 in heart muscle, and least in involuntary muscle. The following table 

 gives the average percentage of creatin in the moist tissues of various 

 animals: 



