THE ACTION OF ENZYMES 83 



by heat, and are unaltered by pepsin, although they 

 break down under the influence of trypsin. Protamines 

 contain neither phosphorus nor sulphur, but have a high 

 nitrogen content (20 to 25 per cent.) 



(2) Histones are also basic substances, not coagulated 

 by heat, very soluble in acids, but precipitated by ammonia. 

 They are mostly obtained from spermatozoa of fish, the 

 thymus glands of some animals, and from red blood-cells 

 of birds. 



(3) Albumins. 



(4) Globulins. These two groups include the com- 

 monest and most representative proteins met with in 

 animal and vegetable tissues. Both are coagulated by 

 heat, and contain sulphur but little phosphorus. Albu- 

 mins present in egg-white, blood serum and milk are 

 soluble in water, and can be salted out from their solu- 

 tions by saturating the latter with ammonium sulphate. 

 Globulins are more numerous than the albumins, and 

 are present in eggs, blood, milk, most animal and vege- 

 table tissues, and in many seeds. They are insoluble in 

 distilled water and weak acids, and are readily precipi- 

 tated from solutions by half saturation with ammonia 

 sulphate ; magnesium sulphate, which does not throw 

 down albumins, precipitates globulins. They are feebly 

 acid substances, and dissolve easily in alkaline solutions. 



(5) Sclero-proteins. This ill-defined class includes 

 gelatine, the most abundant insoluble protein in bone and 

 cartilage and keratin, the chief constituent of wool, hair 

 and hoofs of animals. To these substances the term 

 " albuminoid " is sometimes restricted by physiologists. 



(6) Phospho-proteins. The chief representatives of this 

 class are caseinogen of milk, and vitellin, a constituent of 

 the yolk of eggs. They contain from J to I \ per cent, of 



