PROTEINS. 631 



(d) Haemoglobins. Compounds of the protein molecule, with 

 hseinatin or some similar substance. Haemoglobins form the coloring- 

 matters of the blood. On hydrolysis they yield a simple protein and 

 a substance called haemochromogen, which contains iron and is read- 

 ily oxidized to hsematin. (Further discussion under Blood.) 



(e) Lecithoproteins. Compounds of the protein molecule with 

 lecithins, which will be considered later. (See Index for lecithins.) 



III. Derived proteins. 



These substances are derivatives of proteins, and are obtained from 

 them by hydrolytic changes of various kinds, e. g., through the action 

 of acids, alkalies, heat, or enzymes. 



1. Primary protein derivatives. 



Derivatives of the protein molecule apparently formed through 

 hydrolytic changes which involve only slight alterations of the pro- 

 tein molecule. 



(a) Proteans. Insoluble products which apparently result from 

 the incipient action of water, very dilute acids, or enzymes on pro- 

 teins originally soluble. 



(6) Metaproteins. Products of the further action of acids and 

 alkalies, whereby the molecule is so far altered as to form products 

 soluble in very weak acids and alkalies, but insoluble in neutral 

 fluids. The two principal metaproteins are the alkali metaprotein or 

 alkali albuminate and the acid metaprotein or acid albuminate. 



Alkali metaprotein is formed when native proteins are acted upon 

 by alkalies to such an extent that part of the nitrogen, and occasion- 

 ally sulphur also, is eliminated from the molecule. The change 

 takes place slowly at the ordinary temperature, more rapidly on 

 heating. 



Acid metaprotein is obtained by digesting a native protein with 

 dilute acid. 



Alkali and acid metaproteins (album inates) have certain properties 

 in common. Both are insoluble in water or neutral salt solution, but 

 easily soluble in the presence of a small amount of either an acid or 

 an alkali. The solution does not coagulate on boiling, but is com- 

 pletely precipitated when neutralized. A solution in dilute acid is 

 also precipitated by saturation with magnesium sulphate, ammonium 

 sulphate, or sodium chloride, while a solution in alkali is not precipi- 

 tated by similar treatment. Although agreeing in many reactions, 

 alkali and acid metaproteins are essentially different. Thus the 



