88 THE CHEMICAL COMPOSITION OF THE BODY 



whereby the molecule is so far altered as to form products soluble in very 

 weak acids and alkalies, or insoluble in neutral solutions; e.g., acid meta- 

 protein, acid albuminate, alkali metaprotein or alkali albuminate. 



h. Coagulation Proteins. Insoluble products resulting from i, the action 

 of heat on protein in solution or, 2, the action of alcohol on the protein. 



Class 2. Secondary Protein Derivatives. Products of more extensive 

 hydrolytic cleavage of the protein molecule than that in the preceding class. 



i. Proteases. Soluble in water, non-coagulable by heat, and precipitated 

 by saturating their solutions with ammonium or zinc sulphate. 



/. Peptones. Soluble in water, non-coagulable by heat, and not pre- 

 cipitated by saturating their solutions with ammonium sulphate. 



k. Peptides. Definitely characterized combinations of two or more 

 amino acids, the carboxyl group of one being united with the amino group 

 of the other with the elimination of a molecule of water. 



CHARACTERISTICS OF THE VARIOUS CLASSES OF PROTEINS. 



Albumins. Albumins constitute the first class of simple proteins. 

 They may be defined as simple proteins which are coagulable by heat and 

 are soluble in pure (salt-free) water. As a rule, they are not precipitated on 

 saturating their solutions with sodium chloride or magnesium sulphate,, 

 unless the solution be then acidified with dilute acid. They do not coagu- 

 late out on heating their solution unless a trace of a salt is present. Some 

 albumins have been prepared in crystalline form; e.g. ovalbumin, serum- 

 albumin, and lactalbumin. Crystallization is obtained on adding ammonium 

 sulphate to the protein solution until slight turbidity results, then clearing 

 the solution by adding a little water and acidifying slightly with acetic acid. 

 The albumins, as a rule, are precipitated on saturating with neutral ammo- 

 nium sulphate. Zinc sulphate may be employed when it is desired not to 

 introduce ammonium salts into the precipitation. The proteins are pre- 

 cipitated and subsequently coagulated by the addition of an excess of alco- 

 hol. They remain in solution on dialysis. 



On heating a solution of an albumin (or a globulin) the turbidity and 

 flocking out of the coagulun occur at a temperature which is more or less 

 characteristic for the individual protein. However, the coagulation tem- 

 perature can be varied according to the concentration of the protein solu- 

 tion, the presence of inorganic salts, and by the reaction of the solution. The 

 coagulation temperatures, then, cannot be given as definite characters for 

 individual proteins unless the conditions under which the figures were ob- 

 tained are comparable. 



The albumins differ among themselves in the cleavage products they 

 yield on hydrolysis, in their elementary composition, and in the specific 

 rotation and coagulation temperatures. The serum albumin and lact- 



