1 8 TEXT-BOOK OF PHYSIOLOGY 



its viscid, tenacious character. It is also a constituent of the inter- 

 cellular substance of the connective tissues. It is readily precipitated 

 by acetic acid. When heated with dilute acids, mucin undergoes a 

 cleavage into a simpler protein and a carbohydrate termed mucose, 

 , which is capable of reducing Fehling's solution. 



(b) Mucoids. The mucoids resemble the mucins though differing from 

 them in solubility and in not being precipitable from alkaline solutions 

 by acetic acid. They are found in the vitreous humor, white of egg, 

 cartilage, tendon, bone, and in other situations. Chondromucoid 

 differs from other connective-tissue mucoids in the large amount of 

 chondroitin-sulphuric acid obtained upon decomposition. When 

 decomposed through the action of acid, the chondroitin-sulphuric 

 acid yields sulphuric acid and a nitrogenous body chondroitin, which 

 in turn yields acetic acid and a new nitrogenous substance chon- 

 drosin, which has a more strongly reducing action on Fehling's solution 

 than dextrose. They differ slightly one from the other in proper- 

 ties and chemic composition. They yield on decomposition a 

 carbohydrate. 



NUCLEO-PROTEINS. 



The nucleo-proteins are obtained from the nuclei and cell-substance 

 of tissue-cells. Chemically they are characterized by the presence of 

 phosphorus in relatively large amounts. When hydrolyzed, they 

 separate into a protein and a nuclein. The nucleins derived from 

 cell nuclei can be still further separated into a simpler protein and 

 nucleic acid, which latter in turn yields phosphoric acid, carbohy- 

 drates mostly pentoses, pyrimidine bases thymine, cytosine, and 

 uracyl, and the so-called purin bases, xanthin, hypoxanthin, adenin, 

 and guanin. All nucleins which yield the purin bases are termed 

 true nucleins. 



DERIVATIVES OF PROTEIN 



The protein derivatives include a variety of substances which arise 

 through a process of hydrolysis of simple proteins under the action of enzymes 

 and in the presence of acids and alkalies. The number of derivatives 

 obtained between the first cleavage of the protein molecule and its final 

 cleavage to amino-acids is large and will be presented at length in the para- 

 graph relating to protein digestion. The chief derivatives are as follows: 



INFRA-PROTEINS. 



(a) Acid-albumin. This is formed when a native albumin is digested 

 with dilute hydrochloric acid (0.2 per cent.) or dilute sulphuric acid 

 for some minutes. It is precipitated by neutralization with sodium 

 hydroxid (o.i per cent, solution). After the precipitate is washed, 

 it is found to be insoluble in distilled water and in neutral saline solu- 

 tions. In acid solutions it is not coagulated by heat. 



(b) Alkali-albumin. This is formed when a native albumin is treated 

 with a dilute alkali e.g., o.i per cent, of sodium hydroxid for five 

 or ten minutes. On careful neutralization with dilute hydrochloric 

 acid, it is precipitated. It is also insoluble in distilled water and in 

 saline solutions; it is not coagulable by heat. 



