124 THE PROTEINS AND PROTEASES [CH. 



(l) Precipitation by Esbacli's solution^. Add a little Esbach's solution: the pro- 

 tein is precipitated. 



{m) Precipitation hy phosphotungstic acid. Add a little 2 "/o solution of phos- 

 photungstic acid in 5 % sulphuric acid : the protein is precipitated. 



The substances used in the tests {k)-{m) are termed " alkaloidal re- 

 agents" because they also cause precipitation of alkaloids (see Chap. x). 



We are now in a position to deal with the different groups of pro- 

 teins in detail : 



Albumins. Very few vegetable albumins have been investigated. 

 They can be best defined as proteins which are soluble in water and are 

 coagulated by heat. Animal albumins are distinguished by the fact 

 that they are not precipitated by saturating their neutral solutions with 

 sodium chloride or magnesium sulphate ; nor are they precipitated by 

 half-saturation with ammonium sulphate. This distinction cannot be 

 applied to vegetable proteins, since some are precipitated by the above 

 treatment. It is often not easy to determine whether a plant protein is 

 an albumin, on account of the difficulty of removing traces of salts, acids 

 or bases which cause solubility, and also of separating the albumins from 

 the globulins with which they occur. Albumins are however probably 

 Avidely distributed in plant tissues. 



The best-known albumins are : 



Leucosin, which occurs in the seeds of Wheat {Triticum vulgare), 

 Rye {Secale cereale) and Barley (Hordeum vulgare). 



Legumelin, which occurs in seeds of the Pea {Fisum sativum), Broad 

 Bean {Vicia Faha), Vetch [Vicia sativa), Lentil (Ervum Lens) and some 

 other Leguminous seeds. 



Phaselin, which occurs in the Kidney-bean (Fhaseolus vulgaris). 



Ricin, which occurs in the Castor-oil Bean {Ricinus communis). 



Expt. 122. Demonstration of the presence of an albumin {leucosin) in wheat or 

 barley flour {see also Expts. 128 and 130). Weigh out 10 gms. of wheat or barley flour, 

 add 100 c.c. of distilled water and allow to stand, with occasional stirring, for 

 2-6 hrs. Then filter off the solution. Slowly heat the solution to boiling, and note 

 that a precipitate of coagulated protein is formed. 



Globulins. These may be defined as the proteins which are in- 

 soluble in water but soluble in dilute salt solutions, the concentration- of 

 the salt solution necessary for complete solution (see p. 125) varying 

 with the salt or protein under consideration. It should be noted that, 

 in making wa^er-extracts of plant tissues, it may happen that globulins 



1 Esbach's solution is prepared by dissolving 10 gms. of picric acid and 10 gms. of citric 

 acid in water and making the solution up to a litre. 



