THE CHEMICAL BASIS OF THE BODY 753 



A third series of mon-amino acids is the aromatic. In these 

 the acid is united to the benzene ring, giving rise to the forma- 

 tion of — Aromatic amino acids : Phenyl-alanine, Tyrosine, and 

 Tryptophane. It is from tryptophane that the substances which 

 give rise to the offensive odour of faeces, indole and skatole, are 

 derived. 



In the group of di-amino acids, in which two amidogen groups 

 (NH 2 ) replace two hydrogen atoms in the fatty acid series, greater 

 complexity occurs, for the substances may act the part of either 

 acids or bases. In this way are formed — Lysine, Arginine, Histi- 

 dine, Ornithine, Creatine, and Cystine. The three first contain 

 six atoms of carbon, and are in consequence spoken of as the hexone 

 bases. 



In addition to the amino acids, the protein molecule yields Pyrroli- 

 dine derivatives, Pyramidine bases, and Ammonia. 



All the above groups of bodies are potentially present in the protein 

 molecule — not necessarily in every variety of protein, for in some, as 

 will presently be shown, certain cleavage products, as they are termed, 

 are characteristically absent. Within the body the cleavage of the 

 protein molecule is brought about by enzymes and putrefaction; 

 in the laboratory, enzymes, putrefaction and acids at high tem- 

 perature are employed, and in this way, piece by piece, the complex 

 structure is taken apart. Within the body the reconstitution of 

 the cleavage products into protein is rapidly effected ; but in the 

 laboratory attempts to reconstruct it have not yet been successful, 

 though the question has engaged the attention of chemists for many 

 years. Some portion of the structure has been erected, and sub- 

 stances giving protein reactions have been constructed by synthesis ; 

 but the chemist has not yet succeeded in making a typical protein. 



The number of possible groupings of these end-products must 

 be enormous. Even the simpler sugars containing six atoms 

 of carbon present thirty-six possible combinations. The protein 

 molecule contains not less than 700 atoms of carbon, and its variety 

 of groupings is almost infinite. When the protein molecule is 

 pulled to pieces, either in the laboratory or in the body, the process 

 is effected by hydrolytic cleavage — i.e., the protein takes up the 

 elements of water and yields simpler substances. In the recon- 

 struction of protein the chemical process is reversed, and dehydra- 

 tion employed. When proteins are pulled to pieces either within 

 the body or in the laboratory, they yield proteoses (see p. 189) as 

 the first cleavage product ; these, in turn, furnish peptones. In the 

 stage below peptones are groups of two, three, or more amino acids, 

 termed peptids or polypeptids, and from these, as their connecting- 

 links are broken, the individual amino acids are set free. Poly- 

 peptids are obviously much simpler chemically than protein, yet 

 some notion of their complexity may be formed from the fact that 

 a polypeptid has been prepared by synthesis with a molecular 

 weight of 1 2 13, and containing eighteen mon-amino acids, fifteen 

 molecules of glycine, and three of leucine. The polypeptids in 

 digestion are not numerous; the largest number are formed by 

 synthesis. 



Some proteins yield all the amino acids ; in others certain of 

 the acids are characteristically absent. For instance, gelatin does 

 not contain either the tyrosine or tryptophane nucleus ; serum- 

 albumin, egg-albumin, the caseinogen of cow's milk, the protein 

 of maize (zein), do not yield glycine, and the protein of wheat 



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