2 INTRODUCTION 



isobutylacetic 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 7' 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, C 6 H 6 ) 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 i' 5 per cent, among the cleavage products of egg-albumin, and to 

 the amount of 2*1 per cent, among those of serum-albumin. It is an 

 interesting point in this connection that gelatin, which yields i6'5 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. 8). 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 molecules. 

 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 polypeptides, which finally are decomposed so as to yield the 

 individual amino-acids, also called in this connection the peptides or 

 monopeptides, the " building-stones " out of which the protein molecule 

 is constructed. 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 com- 

 plexity can be built up by linking amino-acids together. When two 

 amino-acids are so united, the resulting compound is called a dipeptide ; 

 with three amino-acids we get tripeptides, etc. Still more complicated 

 polypeptides 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 pig- 

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

 'carbo-hydrate group. 



