PROTEIN, (from WpWTE&O, I am first,) is the name given by its discoverer, Mulder, to a chemical substance of the highest interest and importance ; since it appears to form the basis of by far the greater portion of the bodies of all animals.
When pure fibrin, of which animal flesh or muscle chiefly consists, is analysed, it is found to be composed of Q„,„ H3, N3 0, 2 and a small quantity of sulphur and phosphorus. Albumen, whether obtained from the serum of the blood, white of egg, or any of the albumi nous tissues of the body, is found also to con sist of C40 N, 0, 2 and a little sulphur and phosphorus. Casein, too, or the curd of milk, yields on analysis C40 H3 N, 0, and a little sulphur, differing from the others in not containing, any phosphorus. Hence it appears thatfibrin, albumen, and casein, are, chemically speaking, almost identically the same ; and that if we were enabled to separate from each the minute portion of sulphur and phos phorus, we should obtain a compound in every case the same. Such a substance is protein ; so called fi.om its being the initial letter, as it were, of all this class of organic principles.
I shall first describe it as obtained artifi cially, together with the changes produced upon it by reagents, and afterwards speak of its more common natural modifications, which play so important a part in building up the fabric of organic beings.
Protein is most readily obtained from the white of egg, which, as is well known, consists of a solution of nearly pure albumen, contained in a delicate network of cellular membrane. This substance should be well beaten up, in order to break the minute cells in which the albumen is lodged, mixed with about an equal bulk of water, and filtered through a linen cloth to separate the cellular matter, which is insoluble in water ; or it may be allowed to stand until this has subsided to the bottom of the vessel, when the clear liquid may be poured off, or removed by means of' a syphon. The solution should then be evaporated to dryness on a water bath, the residue pounded in a mortar, and washed successively with alcohol, ether, and dilute hydrochloric acid, by which means it is purified from extractive matters, fat, phosphate of lime, and the other salts with which it is associated. The pure albumen thus obtained is digested for several hours in a dilute solution of caustic potash, at a temperature of from 120° to 130° ; it readily dissolves in the alkaline solution, and the sulphur and phos phorus are gradually separated, forming sul phuret of potassium, and phosphate of potash.
Acetic acid is now added in very slight excess, when the protein separates in the form of a white flocculent precipitate, which, when washed with water until all soluble matter is removed, and dried at 212°, is pure protein. In order to asertain, however, whether the whole of the sulphur is removed, a small quan tity should be dissolved in potash, and some of the solution boiled in two test tubes, to one of which a drop of solution of acetate of lead is added. They will both become rather brown, owing to the decomposition of the protein; but if any sulphur is present, the portion to which the lead had been added will become, after boiling for a few minutes, much darker in colour than the other, owing to the formation of sulphuret of lead.
Protein, when dry, is a hard, semitransparent brownish yellow substance, having a good deal the appearance of amber. It is without taste or smell, and when exposed to damp air rapidly absorbs moisture, which may be expelled by heating it to about 220°. When further heated it melts, and almost immediately afterwards begins to decompose, leaving a residue of char coal, which, if ignited for some little time in the air, burns completely away, leaving scarcely a trace of incombustible ash. Protein is in soluble in water, alcohol, and ether ; it appears to combine with most of the mineral acids, forming compounds which may be considered neutral, some of which are soluble in water, though insoluble in an excess of the acid. Tribasic phosphoric, and acetic acids, how ever, do not reprecipitate it when added in excess. It combines also with the alkalies, giving rise to soluble compounds, from which the protein may be again separated by the ad dition of an acid. It may be thrown down in an nsoluble form from any of its acid solutions by.the ferrocyanide and ferridcyanide of potas sium, which are among the most delicate tests for it ; also, by absolute alcohol, tannin, many of the metallic salts, and by carefully neutral izing with an alkali.