462 THE METABOLIC PROCESSES OF THE BODY. 



403. 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 leu- 

 ciu 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 gly- 

 cin, 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 converted 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 ordi- 

 nary 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, prob- 

 ably takes place in the liver; and in support of this view we have a certain 

 amount of experimental evidence. Birds maybe kept alive after total extir- 

 pation 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 alimentary canal no longer 

 increase the uric acid of the urine, as they do in the intact animal. In these 

 animals, the synthesis of ammonia compounds into uric acid, which is parallel 

 to the synthesis into urea occurring in the mammal, seems to take place in 

 the liver, and we may infer is in some way or other effected by the hepatic 

 cells. 



As to the exact way in which ammonia, either as such or in form of an 

 amide or amido-acid changes into urea, we have no certain knowledge. 

 Ammonium carbonate, we know, is readily formed out of urea by simple 

 hydration, and we may imagine that the living organism can carry out the 

 reverse process and dehydrate ammonium carbonate into urea. There is, 

 however, a certain amount of evidence that not ammonium carbonate but 

 ammonium carbamate is the immediate antecedent of urea ; and, indeed, out 

 of the body, by electrolyzing a solution of ammonium carbamate with alter- 

 nating currents, a certain amount of urea may be artificially produced. But 

 this is a matter too obscure to be discussed here. 



404. Uric acid. This, like urea, is a normal constituent of human 

 urine, and, like urea, has been found in the blood, in the liver, and in the 

 spleen ; it is a conspicuous constituent of an extract of the latter organ. 

 In some animals, such as birds and most reptiles, it takes the place of urea. 

 In various diseases the quantity in the urine is increased ; and at times, as 

 in gout, uric acid accumulates in the blood, and a deposit of urates takes 

 place in the tissues. Since by oxidation a molecule of uric acid can be 

 split up into two molecules of urea, and a molecule of some carbon acid, 

 uric acid is commonly spoken of as a less oxidized product of proteid metab- 

 olism than urea. But there is no evidence whatever to show that the 

 former is a necessary antecedent of the latter ; on the contrary, all the facts 

 known go to show that the appearance of uric acid is the result of a metab- 



