THE METABOLISM OF PROTEIN 601 



called peptides. This linking occurs between the amino group of one 

 amino acid and the carboxyl group of the other. When alanine and glyco- 

 coll, with which we are familiar, are thus linked together, the reaction 

 takes place according to the equation: 



H CH 3 H 



CH 3 | 



/ ! H + HO : OC - C - NH 2 = HOOC -C-NH-CO-C- NH 2 + H 2 O 

 HOOC-C-N | III 



h X H H H H 



(alanine) (glycocoll) (alanyl - glycocoll) 



In this manner, then, a so-called dipeptide is formed, in which it will 

 be observed there still remain free carboxyl and amino groups, thus per- 

 mitting the linking on of other amino-acid groups to form tripeptides or 

 tetrapeptides or other polypeptides. Indeed, this process of condensa- 

 tion may go on practically indefinitely, a polypeptide containing eighteen 

 amino-acid groups namely, three leucine and fifteen glycocoll groups hav- 

 ing actually been synthesized. The resulting polypeptides have the proper- 

 ties of derived proteins like the proteoses; thus, they give the biuret 

 and other reactions characteristic of proteins and are precipitated by 

 such reagents as mercuric chloride and phosphotungstic acid. Some are 

 also digested by trypsin and erepsin. They have the same optical activities 

 as proteins. One of them has been prepared which produces a typical 

 anaphylactic reaction. So far a polypeptide that can be coagulated by 

 heat or is in other ways identical with the naturally occurring proteins, 

 has not been synthesized; but there is no doubt that it is only a matter 

 of time before this will be accomplished. 



Eighteen distinctly different amino acids have been isolated from pro- 

 tein, and it may assist in the conception of our problem if we place these 

 amino acids side by side and link them together in the manner described 

 above. This is done in the accompanying chart compiled by D. D. Van 

 Slyke, in which also various other important facts concerning the chem- 

 istry of the amino acids are incidentally added. 



At the lower part of each formula will be seen the characteristic car- 

 boxyl and amino groups of neighboring acids linking together the ter- 

 minal carbon atoms. The upper vertical bond of this carbon atom is con- 

 nected with the characteristic group of the amino acid, which may be very 

 simple and represented only by hydrogen, as in glycocoll, or highly com- 

 plex and including a ring formation, as in tryptophane. It will further 

 be observed that there may be other amino groups connected in various 

 positions in this radicle. This is particularly the case in the first three of 

 the amino acids in the table namely, the basic amino acids. In lysine 

 the extra amino group reacts with nitrous acid, liberating free nitrogen 



