236 LECTURE XI. 



We have already indicated, while discussing arginine and arginase, that 

 urea can be formed from guanidine. Creatine, therefore, may also be 

 looked upon as an antecedent of urea, although we do not possess any 

 confirmatory proof of this. The position of creatine in the general 

 metabolism is, in fact, extremely uncertain. We do not even know its 

 source. It is generally considered as being directly related to the proteins. 

 Its formation from albumin has, as yet, never been proven. It appears 

 far more probable that it is more directly related to the nourishment; at 

 any rate, the elimination of creatine depends upon the introduction of 

 food. It is principally found in the muscles, although it has also been 

 isolated from the blood, brain, transudates, and in the amniotic fluid. 

 It is stated that the amount of creatine is increased by muscular work. 

 This has given rise to the assumption that its formation is related to 

 muscular contraction. It is, however, also possible that its elimination 

 may be due to an increased circulation of the blood and the changes which 

 occur in a working muscle. 



Uric acid must also be looked upon as a source of urea. It has been 

 shown that a part, at least, of this substance when administered to the 

 organism of mammals, is changed into urea. 1 This form of urea produc- 

 tion plays only an insignificant part in mammals. It was long thought 

 that uric acid was first formed from albumin, and that this was then con- 

 verted into urea. It was believed to have been observed that a dimin- 

 ished oxidation in the tissues was responsible for an increase in the 

 amount of uric acid at the expense of urea. Such an assumption was 

 supported by the observation that those animals, namely the reptiles, 

 whose metabolism is very slow, showed only uric acid in their excreta. 

 Opposed to this assumption was the fact that birds, whose metabolism 

 we generally consider to be a very rapid one, likewise excreted the larger 

 part of the nitrogen from their food in the form of uric acid. To-day, 

 the question of uric acid production in the organism of mammals, and 

 also the other vertebrates, has been more satisfactorily explained. It is 

 quite independent from that of urea. We shall see later, that the uric 

 acid of mammals, in contradistinction to that of birds and reptiles, has 

 no relation whatever to albumin-metabolism, but is derived from the 

 nucleins. We shall consider this in connection with the constitution of 

 uric acid and related compounds, and shall deal with the subject here only 

 in so far as is related to the metabolism of albumin. 



There are, undoubtedly, direct relations between the decomposition of 

 albumin and the elimination of uric acid in those animals in which the 

 larger part of the nitrogen administered reappears in the form of uric 

 acid. This is especially noticeable with reptiles. Here the elimination of 

 uric acid stands in the same relation to the albumin taken into the 



1 Wohler and Frerichs: Ann. 65, 335 (1848); Neubauer: Ann. 99, 206 (1856). 



