10 ESSENTIALS OF PHYSIOLOGY. 



(3) albumins, (4) globulins, (5) phosphoproteins, (6) scleroproteins, and 

 (7) conjugated proteins. 



(1) Protamines are basic in character and only occur in combination. 

 They are chiefly found, combined with nucleic acid, in the spermatozoa 

 of certain fishes. 



(2) Histories are also basic in character, and occur in the combined 

 form. An example is globin, the protein constituent of haemoglobin. 



(3 and 4) Albumin and globulin occur in all cells and in many of 

 the body fluids, and are distinguished from each other by their 

 solubilities. Albumin is soluble in water or weak salt solution, and its 

 molecules are aggregated to form a precipitate in a saturated solution 

 of ammonium sulphate. Globulin is insoluble in water, soluble in 

 weak salt solution, and is precipitated in a half-saturated solution of 

 ammonium sulphate. Albumin or globulin in solution, on being 

 heated, undergoes first of all a change which is probably chemical 

 in nature, and is known as denaturation. A physical change follows, 

 and consists in the aggregation .of the molecules to form a coagu- 

 lum. The presence of inorganic salts is favourable to aggregation, 

 but is unfavourable to denaturation. Acids and alkalies, on the 

 other hand, favour denaturation but hinder aggregation. Thus, if 

 the protein is heated with more than the merest trace of acid or 

 alkali, an acid or alkaline solution of metaprotein is obtained which 

 will not coagulate on heating, but yields a precipitate of metaprotein 

 on neutralisation. A trace of acid favours coagulation of an albumin 

 or globulin solution, because the acid combines with the protein to 

 form a salt. 



The effect of salts in favouring the coagulation or precipitation of 

 proteins is due to the ionisation of the salt. A solution of protein is 

 really a suspension, and the suspended particles carry an electric 

 charge, which is positive in an acid solution, negative in an alkaline 

 solution. As all the particles carry a similar charge, they will tend to 

 repel one another, but if the charge be reduced, precipitation or co- 

 agulation will take place. Hence, in the case of an ionised salt, the 

 effective ion which brings about coagulation is that which carries a 

 charge opposite in sign to that of the protein particles. It is found 

 that the effectiveness of an ion is determined by its valency. In the 

 case of acid solutions the trivalent anion of potassium citrate has a 

 greater coagulating power than the divalent S0 4 , and the latter again is 

 more effective than the monovalent Cl. So also in alkaline solutions, 

 barium chloride with a divalent kation is more effective than sodium 

 chloride with a monovalent Na ion. 



(5) Caseinogen, the chief protein of milk, is a phosphoprotein, that 



