86 THE NITROGENOUS DERIVATIVES OF THE ALBUMINS. 



The most important members of the group which will here be 

 considered are glycocoll, leucin, asparaginic acid, glutaminic acid, 

 and tyrosin. They are represented by the formula? : 



CH2.(NH 2 ).COOH, glycocoll. 

 (CH 3 ) 2 .CH 2 .CH(NH 2 ).COOH, leucin. 

 CH 2 .CH(NH 2 ).(COOH) 2 , asparaginic acid. 

 (CH 2 ) 2 .CH(NH 2 ).(COOH) 2 , glutaminic acid. 

 C 6 H 4 (OH).CH 2 .CH(NH 2 ).COOH, tyrosin. 



It will thus be seen that glycocoll and leucin are amido-deriva- 

 tives of the mono-basic acids of the formic series, viz., amido-acetic 

 acid and a-amido-capronic acid, while asparaginic acid and glutam- 

 inic acid are dibasic acids of the oxalic series, viz., amido-succinic 

 and amido-glutaric acids. Tyrosin, on the other hand, is an amido- 

 derivative of the aromatic series, viz., para-oxy-a-amido-propionic 

 acid. 



They are all derivatives of the albumins, and, as has been indi- 

 cated, integral constituents of the albuminous molecule. Their 

 quantitative relations, however, are subject to considerable variation, 

 and from some albumins indeed not all can be obtained. Tyrosin, 

 for example, is lacking in glutin and collagen, and glycocoll is 

 similarly not found in casein. In some leucin predominates, while 

 in others tyrosin stands in the foreground. These variations will 

 be considered in greater detail when we shall deal with the various 

 digestive products of the albumins. 



The amido-acids of the fatty series are of special interest to the 

 physiological chemist, owing to the fact that they are apparently 

 intimately concerned in the production of urea. Von Schroder, 

 Nencki, and others have thus shown that in the liver the ammonium 

 salt of carbamic acid, viz., amido-formic acid, is transformed into 

 urea, and we also know that in the mammalian organism leucin, 

 glycocoll, and asparaginic acid are likewise transformed into urea 

 and eliminated as such. That a portion of the urea, indeed, origin- 

 ates in this manner can scarcely be doubted. As regards the 

 nature of the chemical changes which take place during the trans- 

 formation of the amido-acids into urea, our knowledge is not 

 complete. It was formerly supposed that uric acid represented the 

 immediate antecedent of urea and was transformed into this by 

 oxidation. We find, as a matter of fact, that in birds and reptiles 

 uric acid constitutes the final decomposition-product of the nitro- 

 genous metabolism, and is thus analogous to the urea of mammals. 

 I have also pointed out that as a ureid, uric acid on oxidation 

 can yield urea. But as far as is known, the uric acid of mammals is 

 exclusively derived from the nucleinic bases, and is thus scarcely 

 found in sufficient quantity to give rise to the large amount of urea 

 which is daily eliminated in the urine. That a small fraction of 



