DERIVED PROTEINS.
Primary Protein Derivatives.— Those formed from the protein molecule apparently through hydrolytic changes, with but slight al teration in the protein molecule: embracing (1) Proteins—insoluble products of the action of water, very dilute acids or enzymes; (2) Metaproteins — products of such action of acids and alkalis as alters the molecule, form ing substances soluble in very weak acids and alkalis, but not soluble in neutral solvents (including the so-called acid-albuminates and alkali-albuminates) ; (3) Coagulated Proteins— insoluble products resulting from the application of heat to their solutions, or to the action of the alcohols on proteins.
Secondary Protein Products of a more extended cleavage of the protein molecule: embracing (1) the Proteoses— those which are soluble in water; not coagulated by heat ; and which are precipitated by saturating their solutions with ammonium sulphate or zinc sulphate; (2) the Peptones—those soluble in water; not coagulated by heat; but not precipi tated by saturating their solutions with am monium sulphate; (3) Peptids— those com pounds of definite structure in which two or more amino-acids are combined through the union of the carboxyl group of one with the amino group of the other and the loss of a molecule of water.
Albumins.— Serum-albumins have been sep arated from the lymph and blood-serum of vertebrate animals, from milk and from cer tain seeds. Lact-albumin is found in milk though in the small proportion of less than one half of 1 per cent. The principal vegetable albumins are leucosin — found in wheat, rye and barley; and legumelin —of peas, beans and lentils.
Globulins are found in blood-serum and in certain body tissues and organs and in eggs, milk, seeds, etc. A typical globulin is fibrin ogen, found in the blood of vertebrate animals. Upon being separated from the living animal the fibrinogen is almost immediately changed into fibrin or blood-clot by the action of an attendant enzyme. Musculin and myogen are
globulins found in the tissue of what are known as a striped muscle. It is the coagulation of musculin after death which produces the stif fening of a corpse, known as rigor mortis.
Glutelins and Prolamine are found in the grains, a familiar example being the gluten of wheat, which is a mixture of glutenin, a glute lin ; and gliadin, a prolamin. Zein, of Indian corn, is another example of the same group.
Albuminoids.—In this group are found the protein substances which in large measure give stability to the structure of the animal body. Collagen is a typical example. It is the founda tion of cartilage and connective tissue and en ters largely into the bony skeleton. ' When boiled in water, collagen yields gelatin. When treated with tannic acid, as in the making of leather, it becomes tough and highly resistant to decay. To this group belong also keratin, the principal substance of hair, horns, hoofs and feathers; and elastin, the largest constituent of the ligaments of the animal body.
The most familiar example of this division of the proteins is globin, the pro tein constituent of the red 'coloring matter of the blood, hemoglobin.
Nucleoproteins are comparatively ous, forming the chief constituents of the nuclei of all nucleated cells and hence found in every part of the body and its organs. They are found also in yeast and in certain plants.
common example of this group is mucin, a constituent of the saliva. As previously noted, egg-albumen is probably to be classed in this group; and here belong also the mucoids of egg-albumen and blood-serum.
The most familiar ex ample is, the casein of milk, regarded as a cal cium salt in combination with calcium phosphate. When casein is coagulated with rennet, the prod uct is paracasein, differing from casein chiefly in that its calcium salt is insoluble. Vitellin, found in the yelks of hen eggs, is another ex ample of the phosphoproteins.