CHEMIC COMPOSITION OF THE HUMAN BODY 17 



less, undergo a change, for if the solution be treated as above and then 

 boiled to destroy the rennin, it will curdle upon the addition of calcium 

 salts. 



(b) Vitellin. Vitellin is a constituent of the vitellis or yolk of eggs. 

 It differs from other proteins in the fact that it is semicrystalline 

 in character. Though usually regarded as a nucleo-protein it is 

 not definitely known whether or not it contains phosphorus in its 

 composition. 



CONJUGATED OR COMBINED PROTEINS 



The conjugated proteins are compounds in which the protein molecule is 

 combined with some other molecule or molecules, the chemic nature of 

 which varies considerably in the different members of the group, e.g., 

 coloring matter, carbohydrates and nuclein. The chemic character of 

 the non- protein substance furnishes the basis for the following classification: 



CHROMO-PROTEINS. 



(a) Hemoglobin. Hemoglobin is the coloring matter of the red cor- 

 puscles, of which it constitutes about 30 per cent, of the total weight. 

 It possesses the power of absorbing oxygen as it passes through the 

 lung capillaries and of yielding it up to the tissues as it passes through 

 the tissue capillaries. In the arterial blood it is known as oxy- 

 hemoglobin, and in the venous blood as deoxy- or reduced-hemoglobin. 

 When hydrolyzed by acids or alkalies, hemoglobin undergoes a 

 cleavage into a protein, globin, and a coloring matter, hemochromogen, 

 containing iron, which is easily oxidized to hematin. 

 (6) Myohematin. Myohematin is a protein supposed to be present in 

 muscle. It has never been isolated, hence its chemic features are 

 unknown. Spectroscopic examination indicates that it is capable of 

 absorbing and again yielding up oxygen. For this reason it is believed 

 to be a derivative of hemoglobin. 



GLUCO-PROTEINS. 



(a) Mucin. Mucin is the protein which gives the mucus, secreted by 

 the epithelial cells of the mucous membranes and related glands, 

 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 chpn- 

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



