CHEMIC COMPOSITION OF THE HUMAN BODY 17 



(a) Caseinogen. This is the principal protein of milk, in which it 

 exists in association with calcium in a form known as calcium- 

 caseinogenate. It is precipitated by acetic acid and by magnesium 

 sulphate. It is coagulated by rennin, though the nature of the process 

 is not very clear. It was formerly taught that under the action of 

 rennin, an enzyme of the gastric mucous membrane, caseinogen was 

 separated into a solid portion, casein or tyrein, and a soluble portion. 

 The cleavage action of rennin thus indicated has not been verified by 

 subsequent investigations. It is more in accordance with the facts to 

 assume that the process is a double one and that the action of rennin is 

 to change the caseinogen to a soluble form, termed paracasein, after 

 which the lime salts present react with the paracasein in such a 

 manner as to cause it to assume the solid condition. Calcium 

 phosphate seems to be the natural alkali necessary to this process, for 

 if it be removed by dialysis, or precipitated by the addition of potassium 

 oxalate, the casein will not be curdled by rennin, but it will neverthe- 

 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. 



(6) 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. 



(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, 



