CHEMIC COMPOSITION OF THE HUMAN BODY. 17 



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 hydrolysed by acids or alkalies, hemoglobin undergoes a 

 cleavage into a protein, globin, and a pigment hematin. 



(b) 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, and in other situations. They differ slightly one from the 

 other in properties and chemic composition. They yield on decom- 

 position 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 hydrolysed, 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 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 



