756 SYNTHESIS OF UREA. [BOOK n. 



of the nitrogenous metabolism of the body, and that the leucin 

 thus produced is in a normal state of things converted into urea 

 by the liver. And in this connection it may be remarked that 

 not only is leucin found in nearly all the tissues after death, 

 especially in the glandular tissues, but also appears with striking 

 readiness in almost all decompositions of proteids, and is moreover 

 a product of decomposition of gelatiniferous substances. Without 

 going however so far as to conclude that leucin is the chief 

 antecedent of urea, we may take the above observation as indicating 

 that the normal liver has, in some way or other, the power of 

 converting leucin into urea. If this be so then we may also 

 venture to suppose that when such bodies as leucin, glycin, &c., 

 introduced into the alimentary canal appear in the urine as urea 

 the transformation has taken place in the liver. The body tyrosin 

 which so often accompanies leucin, belonging as it does to the 

 aromatic series, stands on a different footing from leucin and the 

 like. 



489. The transformation however of leucin into urea raises a 

 new point of view. Leucin, as we know, is amido-caproic acid; 

 and, with our present chemical knowledge, we can conceive of no 

 other way in which leucin can be converted into urea than by the 

 complete reduction of the former to the ammonia condition (the 

 caproic acid residue being either elaborated into a fat or oxidized 

 into carbonic acid) and by a reconstruction of the latter out of the 

 ammonia so formed. We have a somewhat parallel case in glycin, 

 which is amido-acetic acid ; here too a reconstruction of urea out 

 of an ammonia phase must take place. Moreover when ammonium 

 chloride is given to a dog a very large portion reappears as urea, 

 i.e. there is an increase in the urea of the urine corresponding to a 

 large portion of the nitrogen contained in the ammonium chloride. 

 And in the case of other animals also, indeed of man himself, there 

 is evidence that somewhere in the body ammonia may be con- 

 verted into urea. Hence in all these cases where ammonia or 

 ammonia compounds are changed into urea the last step at all 

 events is one of synthesis ; and this suggests the possibility that in 

 the ordinary proteid metabolism also, the downward katabolic 

 series of changes may finish off with a synthetic effort, the last 

 stage of the former being the appearance of an ammonia compound 

 which is subsequently reconstructed into urea. 



This synthesis, like the transformation of leucin and other 

 bodies, probably takes place in the liver ; and in support of this 

 view we have a certain amount of experimental evidence. Birds 

 may be kept alive after total extirpation of the liver for a longer 

 time than can mammals ; and when in geese the liver is removed 

 the uric acid (representing in these animals the urea of the 

 mammal) is largely decreased, while the ammonia of the urine is 

 largely increased. After the removal of the liver also, leucin, 

 glycin, and other amides or amido-acids administered by the 



