CH. XXYIII.] PROTEIN-HYDROLYSIS 433 



the protein characters. The products may be classified in order of 

 formation as follows: 



1. Meta-proteins. 



2. Proteoses. 



3. Peptones. 



4. Polypeptides. 



5. Amino-acids. 



The polypeptides are linkages of two or more ammo-acids, as 

 already explained. Although most of the polypeptides at present 

 known are products of laboratory synthesis, many have been 

 definitely separated from the digestion products of proteins. The pro- 

 teoses, peptones, and some of the longer polypeptides give the biuret 

 reaction ; the peptones and polypeptides, however, cannot be salted out 

 of solution as the proteoses can : their molecules are smaller than those 

 of the proteoses. We shall study them more fully under digestion. 



It is, however, convenient to add here a brief description of the 

 meta-proteins. They are obtained as the first stage of hydrolysis, and 

 also by the action of dilute acids or alkalis on either albumins or 

 globulins. The general properties of the acid meta-protein and 

 alkali meta-protein (formerly called acid-albumin or syntonin and 

 alkali-albumin), which are thereby respectively formed, are as 

 follows : They are insoluble in pure water, but are soluble in either 

 acid or alkali, and are precipitated by neutralisation unless certain 

 disturbing influences like sodium phosphate are present. They are 

 precipitated as globulins are by saturation with such neutral salts as 

 sodium chloride or magnesium sulphate. They are not coagulated by 

 heat if in solution. 



The word albuminate is used for compounds of protein with mineral 

 substances. Thus if a solution of copper sulphate is added to a solution of 

 albumin, a precipitate of copper albuminate is formed. Similarly, by the addition 

 of other salts of the heavy metals, other metallic albuminates are obtainable. The 

 halogens (chlorine, bromine, iodine) also form albuminates in this sense, and may 

 be used for the precipitation of proteins. 



It should be noted in conclusion that the foregoing classification of proteins is 

 mainly applicable to those of animal origin. 



There are certain vegetable proteins, such as gliadin from the gluten of 

 wheat, hordein from barley, and zein from maize, which stand apart from all other 

 members of the group in being soluble in alcohol. 



The vegetable proteins which have been mainly studied are those contained in 

 the seeds of plants. They may provisionally be grouped into four main classes : 



1. Albumins, such as leucosin in wheat. 



2. Globulins, such as edestin of hemp and other seeds ; most of these are readily 

 crystallisable. 



3. Glutelins. These are insoluble in water and saline solutions, and are soluble 

 only in dilute alkali. They are probably not very strongly marked off from the 

 globulins, since it has been shown that the solubility of globulins in dilute saline 

 solutions is also due to a trace of alkali. The best example of this third class is the 

 glutenin of wheat gluten. 



2 E 



