AMINO-ACIDS, PROTEINS, UREA AND URIC ACID 89 



ing points, and their chief differences from each other lie in 

 their slightly different solubility. The classification of proteins 

 has thus been based largely upon these slight differences in 

 solubility and upon certain general properties. The differences 

 in solubility are also not sharp, as in the case of many crystal- 

 line, gaseous and liquid compounds, so that their sharp separa- 

 tion is an impossibility. Differences in solubility such as have 

 been used to distinguish proteins from each other have given 

 us certain groups such as the following : 



Albumins, soluble in water, 

 Globulins, soluble in dilute salt solutions, 

 Prolamines, soluble in dilute alcohol, 

 Albuminoids, insoluble in all neutral solvents, 

 etc. 



The solubility in water may be easily tested with such pro- 

 teins as egg albumin, wheat gluten and milk casein, as in Experi- 

 ment Study XVIII, I. This is also shown in Experiment Study 

 XXIX, by the separation of the proteins in milk where we ob- 

 tain milk albumin which is soluble in water, and milk casein, 

 a phospho-protein which is insoluble in water. 



A few proteins are more complex than the ones given above, 

 classified by dift'erences of solubility, and these are known as 

 double or conjugated proteins. They contain two distinct 

 parts and are distinguished by the parts shown to be present 

 as follows : 



Glyco-proteins, protein and a carbohydrate (mucin in saliva), 

 Nucleo-proteins, protein and nucleic acid, 

 Phospho-proteins, protein and a phosphorus compound 



(casein). 

 Haemoglobins, protein and an iron-containing compound 



(blood hsemoglobin). 



We thus see how difficult a matter it is to arrive at any con- 

 clusion as to the true chemical nature of proteins from a study 

 of their physical properties because they each lack the sharply 

 distinctive properties of ordinary chemical compounds. 



