IX] PROTEINS AND AMINO-ACIDS 139 



in making z(;a^er-extracts of plant tissues, it may happen that globulins 

 pass into solution to some extent owing to the presence of inorganic 

 salts in the tissues themselves. This has also already been illustrated 

 in Expt. 127 in which an extract of the globulin of the Pea was obtained 

 by treating ground Pea seeds with distilled water only. 



It is characteristic of animal globulins that they are precipitated by 

 saturation of their solutions with magnesium sulphate. Many of the 

 vegetable globulins cannot be precipitated by the above means, though 

 they are all, as far as tested, precipitated by sodium sulphate at 33" C. 

 Many also (like animal globulins) are precipitated by half-saturation 

 with ammonium sulphate, though others are not precipitated until their 

 solutions are nearly saturated with this salt [see Expt. 127 (j)]. 



Unlike animal globulins, vegetable globulins are, as a rule, only 

 imperfectly coagulated by heat, even on boiling. 



Bxpt. 129. Demonstration of the coagulation of globulin. Heat a few c.c. of the 

 solution of dialyzed Pea globulin (from Expt. 127) in a test-tube. Note that the 

 protein is largely precipitated, but the solution does not become quite clear. 



One very important characteristic of the vegetable globulins is the 

 ease with which a number of them can be obtain ed in crystalline form. 

 This result may be achieved by dialyzing a salt solution of the globulin. 

 The salt passes out through the membrane, and the protein is deposited 

 in the form of crystals. An alternative method is to dilute the saline 

 solution of globulin with water at 50 — 80° C. until a slight turbidity 

 appears. Then warm further until this goes into solution, and cool 

 gradually, when the protein will separate in crystals. The globulin, 

 edestin, from seeds of the Hemp {Cannabis sativa) crystallizes very 

 readily (see Expt. 139) and crystals can also be obtained of the globulins 

 from the seeds of the Brazil nut {Bertholletia excelsa), the Flax or Linseed 

 (Linum usitatissimum), the Oat {Avena sativa) and the Castor-oil plant 

 (Ricinus communis); other globulins separate out on dialysis as spheroids, 

 sometimes mixed with crystals. 



The solubilities of plant globulins are further complicated by the 

 fact that some of these substances form acid salts which have different 

 solubilities from the proteins themselves. Thus edestin is insoluble in 

 water, but soluble in either dilute salt solution or acid. In the presence 

 of acid it forms salts which are insoluble in dilute salt solutions. Thus 

 edestin in dilute acid solution is precipitated by a trace of salt, or in 

 dilute salt solution by a trace of acid (see Expt. 130). Legumin, on the 

 other hand, from the Pea and other Leguminosae is soluble in water in 



