Many of the proteins that are soluble are precipitated from their solutions in a perma nently insoluble condition by the action of heat, the substance which is thus thrown down being known as ((coagulated protein.° The tempera ture at which heat-coagulation takes place varies with the nature of theprotein, and (to some slight extent) with the degree of acidity or alkalinity of the solvent from which they are thrown down. The fibrinogen of the blood, for example, coagulates at about 133° F.•, while serum-albumin and egg-albumen coagulate at 162° F., and crystallin at 199° F. When a pro tein is precipitated from solution by heat, the liquid from which it was thrown down almost invariably has its alkalinity increased, or its acidity diminished. Dry proteins may be brought into the "coagulated° condition by raising their temperature to 230° F. Raised to higher temperatures they carbonize, and evolve gases.
Coagulated proteins are exceedingly insol uble, and in fact cannot be brought into solu tion at all except by the action of some agent which changes their chemical nature pro foundly. Pepsin (in an acid medium) or pail creatin (in an alkaline medium) convert them into soluble peptones, when the experiment is performed at a temperature not very close to that of the human body. A protein which is precipitated from its solution by alcohol is not at first thrown into the "coagulated° condition, but passes into it gradually upon prolonged con tact with the alcohol. When a solution of pro teins is saturated with ammonium sulphate, all of its proteins are precipitated except the pep tones; but the precipitate is not in the coagu lated form, for it is capable of dissolving again, either in pure water or in a dilute saline solu tion. This reagent is used, in the laboratory, for estimating the amount of protein in a given solution; the precipitate that it gives being re moved by filtration; washed with a saturated solution of ammonium sulphate; heated to 230° to produce coagulation; and finally weighted, after the ammonium sulphate has been removed by thorough washing with pure water, and the precipitate has been dried. Many of the salts of the heavy metals (basic acetate of lead, for example), precipitate proteins from solution by forming, with them, an insoluble proteo-metallic compound. From this compound the metal
may be subsequently removed by the action of sulphuretted hydrogen gas:the protein then being set free in its original, soluble state.
Of the chemical reactions manifested by the proteins as a class, the following may be espe cially mentioned: (1) The xanthoproteic reac tion. When strong nitric acid is added to a solution containing a protein the protein is often (but not invariably) precipitated; and both the precipitate and the solvent turn yellow from the formation of xanthoproteic acid, the precipitate dissolving upon prolonged boiling, if the nitric acid is present in excess. The yellow color changes to an orange upon the addition of an excess of sodium hydroxide. (2) Milton's reaction. °Millon's reagent,° which is one of the best tests for detecting the presence of a protein, is prepared by dissolving mercury, by the aid of gentle heat, in an equal weight of nitric acid of specific gravity 1.4. When the metal has entirely dissolved, the solution is diluted with twice its bulk of water, and the abundant precipitate which comes down is al lowed to subside. The clear liquid remaining is the reagent desired. Millon's reagent throws the proteins down in the form of a white pre cipitate; and both the precipitate and the solu tion become colored, ranging from pink in the cold to brick red upon boiling, though time is sometimes required before the coloration is complete. Solid proteins give this same reac tion, when boiled with the test solution. Tyro sine and silk respond to it also. (3) Sulphur reaction. Warmed with a solution containing sodium hydroxide and a lead salt, a black pre cipitate or brownish color is produced in a solu tion of a protein containing sulphur, due to the formation of sulphide of lead.
The classification of the proteins has been essayed by several groups of students both in Europe and America, and the work, extremely difficult in itself, has been complicated by the use of terms to which different meanings have been attached by various writers. The Amer ican Society of Biological Chemists and the American Physiological Society have agreed on the following classification, which is regarded as standard by American students.