136 THE PROTEINS AND THE AMINO-ACIDS 



In addition to these acids two hydroxy-diamino-acids have been iso- 

 lated from among the cleavage-products of one protein, namely, casein. 

 The very great variety of proteins and protein derivatives which exist 

 in nature are therefore constructed out of a relatively small and limited 

 number of amino-acid building-stones, differing proportions and 

 arrangements of these components being responsible for the wide 

 variety of characteristics displayed by the native and derived proteins. 



In many instances a definite parallelism can be traced between the 

 chemical and physical behavior of the proteins and their amino-acid 

 content. Thus, the Albumins, which are soluble in distilled water and 

 are not coagulated by half-saturation of their solutions with am- 

 monium sulphate, contain no glycine, while the Globulins, which (when 

 uncombined "with acids or bases) are insoluble in distilled water and 

 are coagulated by half-saturation of their solutions with ammonium 

 sulphate, do contain this amino-acid. The alleged transformation of 

 serum albumin into globulin by warming in alkaline solutions observed 

 by Moll, and not infrequently quoted, is therefore, an impossibility, 

 since it would involve the synthesis of amino-acetic acid and its union 

 with the albumin molecule which could not be brought about by any 

 such simple procedure. The product actually obtained by Moll was 

 an infraprotein, alkali-albuminate, which mimics globulin in being 

 insoluble in neutral water, but differs from it in fundamental consti- 

 tution. 



The alcohol-soluble vegetable proteins (Prolamines) contain a trace 

 (probably attributable to associated impurities) of glycine, and some of 

 them contain no glycine, their content of diamino-acids is very small, 

 while their content of glutamic acid and of proline is very high. The 

 phosphoproteins, Casein and Vitellin, are also rather high in glutamic- 

 acid content. Gelatin is characterized by its high glycine content and 

 Keratin by its high content of cystine. The Histones, which are pre- 

 dominantly basic substances, contain about thirty per cent, of diamino- 

 acids, while the Protamines, which are still more predominantly basic, 

 contain only small amounts of monoamino-acids, Salmine (from 

 salmon sperm) containing over eighty per cent, of arginine, while 

 Sturine (from sturgeon sperm) contains sixty-seven per cent, of its 

 nitrogen as arginine, ten per cent, in the form of histidine, and from 

 six to seven per cent, in the form of lysine. 



/" The amino-acids are white, crystalline, readily diffusible substances 

 / and the crystal form is characteristic for each amino-acid. The 

 \ crystal forms of glycine, leucine and histidine are shown in the 

 V accompanying figures (3-5). 



The amino-acids are usually readily soluble in water, cystine and 

 tyrosine affording exceptions to this rule. They are, with the exceptions 

 of proline and oxyproline, insoluble in alcohol and ether. They have 

 high melting-points and melt with decomposition, splitting off carbon 

 dioxide. With the exception of glycine the amino-acids are optically 

 active, some of them being dextrorotatory and others levorotatory. 



