86 THE CHEMICAL COMPOSITION OF THE BODY 



diffusible. Their osmotic pressure is very slight. Some of the simpler 

 derived proteins, however, are diffusible. 



Proteins in aqueous solution rotate polarized light to the left. The 

 specific rotation of the individual proteins varies. The compound proteins, 

 hemoglobin and nucleoproteins are dextrorotatory. 



Proteins chemically are rather unstable bodies and are easily hydrolyzed, 

 through heating and standing in alcohol, into modifications which differ only 

 slightly from the original substances. This change is ordinarily termed 

 coagulation. 



Proteins may be precipitated from their solutions by the addition of the 

 neutral inorganic salts in high concentration or to saturation. This pre- 

 cipitation is essentially a physical one, the protein remaining unchanged. 

 Salting out is, then, a most valuable method for the separation and purifica- 

 tion of the protein substances. The proteins form insoluble combinations 

 with the so-called alkaloidal reagents; phosphotungstic acid, phospho- 

 molybdic acid, picric acid, trichloracetic acid, potassium mercuric iodide, 

 and tannic acid. The protein also forms insoluble albuminates with the 

 salts of the heavy metals. 



The color reactions for the proteins, which are given in the laboratory 

 experiments at the end of this chapter are due to a reaction between some 

 one or more of the constituent radicals of the complex protein molecule and 

 the chemical reagent or reagents used. Thus certain color tests are due 

 to the presence of individual amino acids in the protein molecule, and the 

 intensity of the reaction obtained will vary with the amount of the amino 

 acids present. The negative results for a certain test will indicate the ab- 

 sence of the particular amino acid in the molecular complex. The color 

 tests, then, are important because they throw light on the chemical consti- 

 tution of the protein under observation. 



CLASSIFICATION OF THE PROTEINS. 



The following classification is that adopted by the American Physio- 

 logical Society and the American Society of Biological Chemists: 



I. Simple Proteins. Protein substances which yield only a-amino acids 

 or other derivatives on hydrolysis. 



a. Albumins. Soluble in pure water; e.g., ovalbumin, seralbumin, and 

 the vegetable albumins. 



b. Globulins. Insoluble in pure water, but soluble in neutral solutions 

 of strong bases with strong acids; e.g., ovoglobulin, edestin, and other vege- 

 table globulins. 



c. Glutelins. Simple proteins insoluble in neutral solvents, but readily 

 soluble in very dilute acids and alkalies. These substances occur abundantly 

 in the seeds of cereals. 



