Proteids may first be divided into three main groups as follows:
I. Simple Proteids.—Composed of carbon, hydrogen, nitrogen, sulphur, and oxygen, and yielding by decomposition aromatic bodies such as tyrosin, phenol, indol, etc.
II. Compound Proteids.—Composed of a simple proteid united to some non-proteid body.
III. Albuminoids.—A class of nitrogenous bodies related to and derived from proteids, but differing especially from the latter by great resistance to the ordinary solvents of true proteids.
The individual members of these three groups may be arranged as follows on the basis of solubility, coagulability, etc.:
I. Simple Proteids.—A. Soluble in water.—a. Coagulable by heat, and by long contact with alcohol. Albumins: serum-albumin, egg-albumin, lacto-albumin, myo-albumin, vegetable albumins. b. Non-coagulable by heat and by long contact with alcohol. Proteoses:[51] protoproteoses, deuteroproteoses. Peptones:[51] amphopeptones, antipeptones, hemipeptones.
B. Insoluble in water, but soluble in salt solutions.—a. More or less coagulable by heat. Globulins. 1. Soluble in dilute and saturated NaCl solutions. Vitellins. 2. Soluble in dilute NaCl solutions, but precipitated by saturation with NaCl. Myosins, paraglobulin[52] or serum-globulin, fibrinogen, myo-globulin, paramyosinogen, cell-globulins. b. Non-coagulable by heat, soluble in dilute NaCl solution and precipitated by saturation with NaCl. Heteroproteoses.
C. Insoluble in water and salt solutions, soluble in dilute alcohol—Zein, gliadins.
D. Insoluble in water, salt solutions and alcohol; soluble in dilute acids or alkalies.—a. Coagulable by heat when suspended in a neutral fluid. Acid-albumins, alkali-albumins or albuminates. b. Non-coagulable by heat when suspended in a neutral fluid. Antialbumids, dysproteoses, glutenins.