2 A MANUAL OF PHYSIOLOGY 



no glycin or glycocoll (amino-acetic acid, CH 2 .NH 2 .COOH), while 

 glycin is constantly found among the cleavage products of serum- 

 globulin. And while leucin (a-aminoisobutylacetic acid) is present 

 to the extent of about 20-5 per cent, in the cleavage products of 

 (horse's) serum-albumin, (hen's) egg-albumin yields only y i per cent. 



On the other hand, egg-albumin yields 8'i per cent, of alanin 

 (amino-propionic acid, C 2 H 4 .NH 2 .COOH), while serum-albumin 

 yields only 2^7 per cent. Of the aromatic amino-acids that is, 

 amino-acids united to the benzene ring phenyl-alanin (amino- 

 propionic acid in which one atom of H is replaced by phenyl, Q.H-) 

 is obtained to the extent of 4-4 per cent, from egg-albumin, and a 

 little over 3 per cent, from serum-albumin. Tyrosin or oxyphenyl- 

 alanin (amino-propionic acid in which a H atom is replaced by 

 oxyphenyl, C 6 H 4 .OH) appears to the amount of 1:5 per cent, among 

 the cleavage products of egg-albumin, and to the amount of 2'i per 

 cent, among those of serum-albumin. It is an interesting point in 

 this connection that gelatin, which yields 16-5 per cent, of glycin, 

 yields no tyrosin at all; tryptophane, an aromatic amino-acid 

 still more complex than tyrosin, is also absent. These facts afford 

 an explanation of certain colour reactions of proteins long known 

 empirically, but only recently understood (p. 7) . The process by which 

 the protein molecule is thus decomposed is called hydrolysis that 

 is, the molecule takes up water, and then splits into smaller mole- 

 cules. The hydrolysis occurs in various stages, bodies like acid- or 

 alkali-albumin (meta- or infra-proteins) being first formed, then 

 proteoses, then peptones. The peptones are further split into bodies 

 containing a relatively small number of amino-acids linked together. 

 These bodies are called peptides or polypeptides, which finally are 

 decomposed so as to yield the individual amino-acids. The inverse 

 process can also be carried on to a certain extent, and Fischer has 

 taken an important step towards the eventual synthesis of proteins 

 by showing how polypeptides of increasing complexity can be built 

 up by linking amino-acids together. Bodies may thus be formed in the 

 laboratory which give some of the characteristic reactions of peptones. 



The numerous substances included in the group of proteins may 

 be classified as follows, beginning with the simplest : 



1. Protamins, such as the bodies called salmin and sturin 

 present in fish-sperm. 



2. Histones, bodies separated from blood-corpuscles. Globin, the 

 protein constituent of haemoglobin, is one of them. Unlike the other 

 groups of proteins, they are precipitated by ammonia. 



3. Albumins. 



4. Globulins. 



5. Sclero-proteins or albuminoids, such as gelatin and keratin. 



6. Phospho-proteins, including such substances as vitellin, a body 

 obtainable from egg-yolk, and caseinogen, the chief protein of milk. 

 They are rich in phosphorus, but are to be distinguished from nucleo- 

 proteins, which also contain a relatively large amount of phosphorus, 

 by the fact that they do not yield the purin bases, the characteristic 

 products of the decomposition of nucleo-proteins. 



7. Conjugated proteins, substances in which the protein molecule 

 is united to another constituent, usually spoken of as a ' prosthetic ' 

 group. Thus the nucleo-proteins consist of protein united with 

 nucleic acid, the chromo-proteins (e.g., haemoglobin) of protein united 

 with a pigment, and the gluco-proteins (e.g., mucin) of protein united 

 with a carbo-hydrate group. 



