82 THE CLEAVAGE-PRODUCTS OF THE ALBUMINS. 



Tryptophan is a skatol-arnino-acetic acid and has the formula : 

 C. ^CH 2 .CH(NH 2 )COOH 



NH/CH 

 or 



C.CH(NH 2 )CH 2 COOH. 



All the amino-acids mentioned are albuminous derivatives and 

 represent integral constituents of the albuminous molecule. Their 

 quantitative relations, as has been indicated, are not constant, how- 

 ever, and upon these variations no doubt the characteristics of the 

 individual members of the group are in great part dependent. 



The amino-acids of the fatty series are of special interest, as they 

 are intimately concerned in the production of urea. Von Schroder, 

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

 of carbonic acid, viz., amino-formic acid, is transformed into urea, 

 and we also know that in the mammalian organism the nitrogen of 

 leucin, glycocoll, and aspartic acid is eliminated in the form of urea. 

 As regards the nature of the chemical changes which take place 

 during the transformation of the amino-nitrogen into urea, our 

 knowledge is not complete. It was formerly supposed that uric 

 acid represented the immediate antecedent of urea and was trans- 

 formed into this by oxidation. We find, as a matter of fact, that 

 in birds and reptiles uric acid is the final decomposition-product of 

 the nitrogenous 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 normally exclusively derived from the nucleinic bases, 

 and is thus not formed in sufficient quantity to give rise to the 

 large amount of urea which is daily eliminated in the urine. That 

 a small fraction of the urea may result from uric acid by simple 

 oxidation is possible, and indeed probable, but the greater portion 

 must of necessity originate in a different manner. 



In birds, on the other hand, some of the uric acid apparently 

 results from glycocoll, and we thus see that in both classes of animals 

 the amino-acids may be the antecedents of the final products of 

 nitrogenous metabolism. 



It has been suggested that in mammals cyanic acid may be pro- 

 duced as an intermediary product, and that urea results through a 

 condensation of two molecules of this substance in statu nascendi, 

 according to the equation : 



/NH, 

 CONH + CONH = CO< + CO,. 



Then again we may imagine that a transformation of the amino- 



