Peptones.—These bodies are very soluble in water but are not thrown out by heat, by saturation with neutral salts, nor by nitric acid. They are completely precipitated by tannin and by strong alcohol.
The peptones are the only soluble proteids which are not precipitated by saturation with ammonium sulfate. The principal animal varieties are hemi- and anti-peptones. These forms of proteids do not appear to exist as such in vegetable products but are produced in large quantities by treating other proteids with pepsin or pancreatin. In sprouting plants, there appears to be a widely diffused ferment capable of converting the proteids of the cotyledons into peptonoid bodies and thus fitting them for entering the tissues of the new plant.
Insoluble Proteids.—This class includes a miscellaneous collection of nitrogenous bodies not belonging to any of the definite groups already mentioned. Fibrin and gluten are types of these insoluble bodies. Fibrin is formed from the fibrinogen of fresh blood and causes coagulation. When washed free of red blood corpuscles it is a white elastic solid. It is insoluble in water and is converted into albumoses and peptones by trypsin and pepsin. It swells up when treated with a very weak one-tenth per cent solution of hydrochloric acid and dissolves to acid albumin when heated therewith.
Gluten is the most important of the insoluble vegetable proteids and forms the chief part of the nitrogenous constituents of wheat. It is readily prepared by washing wheat flour in cold water, as will be described further on. It is probably a composite body formed by the process of extraction from at least two proteid bodies existing in wheat. When dried it forms a horny elastic mass of a yellow-gray color. Gluten is composed of two bodies, one soluble the other insoluble in alcohol. The part insoluble in alcohol has been called vegetable fibrin, and the soluble part is subdivided into two portions, one unicedin or vegetable unicin, and the other glutin (gliadin) or vegetable gelatin. Gluten, according to some authorities, does not properly exist in wheat flour, but is formed therein by the action of water and certain ferments from free existing proteids. A better explanation of the composition of gluten is that of Osborne, which will be given further on.
372. Albuminoids.—In this paragraph the term albuminoids is not employed as synonymous with proteids but as characteristic of a class of bodies nearly resembling them, but, nevertheless, differing from them in many important particulars. Following is an abstract of their classification as given in Watt’s dictionary.[340]
Collagen.—The nitrogenous portions of connective tissues are largely composed of collagen. By boiling water it is converted into gelatin. It may be prepared from tendons as follows: The tendinous tissues are shredded as finely as possible and extracted with cold water to remove the soluble proteids. Thereafter they are subjected for several days to the action of lime water, which dissolves the cement holding the fibers together. The residual insoluble matter is washed with water, weak acetic acid, and again with water. The residue is chiefly collagen, mixed, however, with some elastin and nuclein. With dilute acids and alkalies collagen swells up after the manner of fibrin. The organic nitrogenous matter of bone consists largely of collagen, which is sometimes called ossein.
Gelatin.—When the white fibers of collagen, obtained as above, are subjected to the action of boiling water or of steam under pressure they dissolve and form gelatin. Isinglass is a gelatin made from the swimming bladder of the sturgeon or other fish. Glue is an impure gelatin obtained from hides and bones. Pure gelatin may be prepared from the commercial article by removing all soluble salts therefrom by treatment with cold water, dissolving in hot water and filtering into ninety per cent alcohol. The gelatin separates in the form of white filaments and these are removed and dried. Gelatin is insoluble in cold but soluble in hot water. It is insoluble in alcohol, ether and chloroform. Its hot aqueous solutions deflect the plane of polarized light to the left. Its gyrodynat varies with temperature and degree of dilution and is also influenced by acids and alkalies. At 30° it is [α]D³⁰° = -130.
Gelatin is not precipitated by acetic acid nor lead acetate solution, in which respect it differs from chondrin.
If boiled for a day, or in a short time if heated to 140° in a sealed tube, gelatin loses its power of setting and is split up into two peptonoid bodies, semi-glutin and hemi-collin. Gelatin is easily digested but cannot take the place of other proteids in nutrition.
Mucin.—This albuminoid, together with globulin, forms the principal part of connective tissue. It is also present in large quantities in mucus and is the chief lubricant of mucous membranes. It is extremely difficult to prepare mucin in a state of purity, and it is not certain that it has ever been accomplished. It is precipitated but not rendered subsequently insoluble by sodium chlorid, magnesium sulfate and alcohol. When boiled with sulfuric acid it yields leucin and tyrosin and, with caustic soda, pyrocatechin.