48 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



proteins, and so they must appear in our classification. The proteoses 

 and peptones give the biuret reaction ; the peptones, however, cannot 

 be salted out of solution like the proteoses ; their molecules are 

 smaller than those of the proteoses. We shall study them more 

 fully under digestion. It is, however, necessary to add here a brief 

 description of the infra-proteins, since some of the practical exercises 

 at the head of this lesson deal. with them. 



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-albumin or syntonin and the 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. In alkali-albumin some of the sulphur in the original 

 protein is removed. 



The name albuminate used to be applied to these substances ; but this is 

 an objectionable term, for these first degradation products of protein hydro- 

 lysis are not salts, as the termination -ate would imply. Moreover, they are 

 obtainable from both albumins and globulins. The prefix ' infra- ' (or pos- 

 sibly ' meta-, which some prefer) may be taken as an indication of compara- 

 tively slight chemical alteration. 



A variety of alkali- albumin (probably a compound containing a large 

 quantity of alkali) may be formed by adding strong potash to undiluted 

 white of egg. The resulting jelly is called Lieberkuhn's jelly. A similar 

 jelly is obtainable by adding strong acetic acid to undiluted egg-white. 



The word ' albuminate ' is also 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. The vegetable pro- 

 teins may roughly be arranged under the same main headings, although it is 

 doubtful if a real and complete analogy exists in all cases. The cleavage 

 products of the vegetable proteins are in the main the same as those of the 

 animal proteins, but the quantity of each yielded is usually different. 

 Vegetable proteins, for instance, as a rule give a very much higher yield of 

 glutamic acid than do those of animal origin. 



Further, there are certain vegetable proteins which have hitherto been 

 regarded as peptones, but which do not give the biuret reaction. It seems 

 impossible at present to bring exceptional substances of this kind into any 

 general classification, and the same is true for those curious vegetable pro- 

 teins, such as gliadin from the gluten of wheat, and zein from maize, which 

 stand apart from all other members of the group in being soluble in alcohol, 



