[336] Vid. op. cit. 7, p. 515.
PART FIFTH.
SEPARATION AND ESTIMATION OF
BODIES CONTAINING NITROGEN.
370. Nature of Nitrogenous Bodies.—The nitrogenous bodies, valuable as foods, belong to the general class of proteids and albuminoids. They are composed chiefly of carbon, hydrogen, oxygen, sulfur and nitrogen. Some of them, as lecithin and nuclein, contain phosphorus instead of sulfur, but these resemble the fats rather than the proteids.
Nitrogenous organic bodies of the class mentioned above are designated by the general name proteids. The term albumin is restricted in a physiological sense to a certain class of proteids. The term albuminoid is often used synonymously, as above, for proteids, but, more strictly speaking, it should be reserved for that class of bodies such as gelatin, mucin, keratin and the like, not really proteids, but, nevertheless, closely resembling them.[337] In chemical composition the proteids are characterized by the relative constancy of their nitrogen content, the mean percentage of this element being about sixteen, but varying in some instances more than two units from that number.
371. Classification of Proteids.—Many classifications of the proteids have been given based on physical, chemical and physiological characteristics. In respect of origin, they are divided into two great classes, viz., vegetable and animal. In respect of their physical and chemical properties the following classification of the proteids may be made.[338]
Albumins.—These are proteids soluble in water and not precipitated from their aqueous solutions by sodium chlorid or magnesium sulfate. They are easily coagulated by heat and are represented by three great classes, viz., egg-, serum-, and lactalbumin.
Egg albumin occurs in the white of egg; serum albumin is found in the serum of the blood. Vegetable albumins have been prepared from wheat, rye, potatoes, and papaws. (Carica Papaya). These vegetable albumins are coagulated by heat at about 70° and are not precipitated by the salt solutions named above, nor by acetic acid. The myrosin of mustard seeds also resembles vegetable albumin.
Globulins.—These bodies are insoluble in water, soluble in dilute solutions of neutral salts, but precipitated therefrom by saturation with sodium chlorid or magnesium sulfate. They are coagulated by heat. Among others belonging to this group are serum globulin, fibrinogen, myosin, crystalin, and globin.
Serum globulin is found in the serum of blood; cell globulin is found in lymph cells; fibrinogen occurs in the blood plasma; plasmin, in blood plasma; myosin, in dead muscles; vitellin, in the yolk of eggs; crystalin, in the lens of the eye; haemoglobin, in the red pigment of the blood; haemocyanin, in the blood of certain low grade animals.