638 METABOLISM 



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 synthetized; 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 

 by the Van Slyke method; but in other cases, as in arginine, it fails to 

 give this and the other characteristic reactions of the amino group. 



It will further be observed that the amino acids are arranged in three 

 main groups: one basic, another neutral, and the third acid. The basic 

 amino acids are three in number and have an alkalinity similar to 

 that of ammonia. They have been called the hexone 'bases, because each 

 contains six carbon atoms. They are alone present in certain forms of pro- 

 tein called prot amines. The neutral amino acids contain one amino group 

 and one carboxyl group, which exactly neutralize each other. This is 

 the largest group of amino acids, and is further subdivided into three: 

 one containing aromatic or benzene rings and including the very im- 

 portant amino acids, tyrosine and tryptophane; another containing the 

 so-called pyrrolidine ring; and the third, the largest of all, containing 

 the so-called aliphatic chains; that is, the chains characteristic of the 

 fatty acids and which may be either straight or branched. When the chains 

 are branched, the substance is called an isosubstance, as in isoleucine. 

 The acid amino acids, including glutamic acid and aspartic acid, are 



