26 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



If we use the term ' proteid ' in the widest sense, the first main sub- 

 division of these substances is into 



A. The simple proteids. 



B. The compound or conjugated proteids. 

 c. The albuminoids. 



D. The protamines. These substances are regarded by Kossel as 

 the simplest proteids, and though it is doubtful whether this view 

 will be ultimately accepted, it will be convenient to consider them 

 here. 



We may now take these four classes one by one. 



A. The Simple Proteids 



Class I. Albumins. — These are soluble in water, in dilute saline 

 solutions, and in saturated solutions of sodium chloride and mag- 

 nesium sulphate. They are, however, precipitated by saturating 

 their solutions with ammonium sulphate. Their solutions are 

 coagulated by heat, usually at 70-73° C. The following are 

 instances : — 



(a) Serum albumin, {b) Egg albumin, (c) Lact-albumin (see 

 Milk). 



Class II. Globulins. — These are insoluble in water, soluble in 

 dilute saline solutions, and insoluble in concentrated solutions of 

 neutral salts like sodium chloride, magnesium sulphate, and am- 

 monium sulphate. A globulin dissolved in a dilute saline solution 

 may therefore be precipitated — 



1. By removing the salt — by dialysis (see p. 23). 



2. By increasing the amount of salt. The best salts to employ 

 are ammonium sulphate (half saturation) or magnesium sulphate 

 (complete saturation). 



The globulins are coagulated by heat ; the temperature of heat 

 coagulation varies considerably. The following are instances : — 



(a) Fibrinogen ] . , , ^ , 



W\ a T u ^• f 1 1, T \ 1 111 blood plasma, 



(o) Serum globulm (paraglobulm) J ^ 



(c) Egg globulin in white of egg. 



(d) Myosinogen in muscle. 



(e) Crystallin in the crystalline lens. 



If we compare together these two important classes of proteids, 

 we find that they all give the same general tests, that all are coagu- 

 lated by heat, but that they differ in solubilities. This difference in 

 solubility may be stated in tabular form as follows : — 



