THE NITROGEN COMPOUNDS 59 



which arise from the cleavage of nucleo-proteins are 

 regarded as the progenitors of uric acid, persons with 

 uric-acid tendency have been advised to avoid eating 

 certain animal foods such as beef and liver, or any others 

 known to contain these compounds abundantly. Experi- 

 ments show that the feeding of certain tissues rich in 

 nucleo-proteins increases the output of uric acid, while 

 adding to the diet a large amount of purin-free proteins 

 such as albumin does not have this effect. 



75. Glyco-proteins. These are bodies that upon cleav- 

 age are decomposed into a protein and a carbohydrate. 

 The best-known glyco-proteins are the mucins that are 

 secreted by the mucous membranes of the air passages 

 and of the alimentary canal and by certain glands such as 

 the salivary. Certain of these compounds contain phos- 

 phorus and others do not. 



76. Phospho-proteins. Like the nucleo-proteins, 

 these compounds contain phosphorus, Jbut on cleavage 

 do not yield the purin bases. The best-known phospho- 

 protein is the casein of milk, a compound exceedingly 

 important in human nutrition, especially with the 

 young. 



This compound is a secretion of the mammary gland 

 of many species of animals, and doubtless originates in 

 the gland cells. The casein from different species of 

 mammals differs somewhat in chemical and physical 

 properties. Casein is insoluble in water, but exists in 

 milk in suspension. It is not coagulated by heat but 

 curdles when a weak acid is added to milk, as, for instance, 

 vinegar. The same result is produced by a generous quan- 

 tity of common salt. When milk is received into the 

 human stomach, the casein coagulates (the milk curdles) 

 through the action of a ferment in the gastric juice and 



