74 PHYSIOLOGICAL CHEMISTRY 



out of chaos until more is known of the structural differences 

 which differentiate the various members of the group. Another 

 classification, in use by English biochemists, differs only slight- 

 ly from that given above. Hemoglobins are called chromopro- 

 teins ; albuminoids, scleroproteins ; and the third division is sub- 

 divided somewhat differently in the English classification. 



Preparation of Proteins from Materials in Which They Oc- 

 cur. Most proteins occur naturally mixed with a large number 

 of other materials. Only a few are found in a fairly pure 

 state in nature. For purposes of study it is desirable to get 

 them as free as possible from other compounds. Various meth- 

 ods are useful, according to the properties of the protein to be 

 isolated and those of the other substances present in the mixture 

 or tissue. But even at best the purification of a protein is a 

 difficult task and when finished it is often uncertain whether 

 or not the substance is contaminated by some other protein of 

 very similar composition and properties or by other compounds. 

 Plant proteins often may be dissolved out with 10% sodium 

 chloride or with alcohol and separated from one another by 

 dialyzing the salt, or by fractional precipitation. In preparing 

 proteins from animal tissues, the process is made somewhat 

 more difficult by the fact that such tissues contain larger 

 amounts of autolytic enzymes. These have the property of 

 breaking down the cell constituents or altering them if the tis- 

 sues are allowed to stand for some time before the proteins are 

 extracted. Also, it is difficult to free the proteins from the 

 fatty or phosphatid constituents of the cell without altering the 

 nature of the protein itself. Various solvents may be used, such 

 as salt solutions, dilute acids or alkalies. A method which is 

 very satisfactory consists in freezing the tissue quickly, reduc- 

 ing it to a fine powder and drying while still frozen. This ma- 

 terial then may be extracted with petroleum ether to remove 

 fats, phosphatids, cholesterol, etc. The residue is extracted with 

 10% sodium chloride solution, and the various proteins sepa- 

 rated by fractional precipitation. 



Some of the proteins may be purified by re-crystallization. 



