Met- and Paralbumin.—Metalbumin is a form of mucin and differs from paralbumin by giving no precipitate when boiled. Both bodies yield reducing sugars when boiled with dilute sulfuric acid.
Nuclein.—The nitrogenous matters which form the nuclei of the ultimate cells are called nuclein. Nuclein resembles mucin in many physical properties but contains phosphorus. It is also, like mucin, resistant to pepsin digestion. The nuclein of eggs and milk probably contains iron. Nuclein is found also in cells of vegetable origin and in yeast and mildew.
Nucleoproteids.—These are bodies which yield both nuclein and albumin when boiled with water or treated with dilute acids or alkalies. Many nucleoproteids have the physical properties of mucus and the sliminess of the bile and of the synovial liquid is due to them. They are the chief nitrogenous constituent of all protoplasm.
Chondrin.—Chondrin is obtained from cartilage by boiling with water. The solutions of chondrin set on cooling in the manner of gelatin. They are precipitated by the same reagents used for throwing out gelatin and mucin. Chondrin is also levorotatory. By some authorities chondrin is regarded as a mixture of gelatin and mucin.
Elastin.—The elastic fibers of connective tissue are composed of this material. It can be prepared from the neck muscles by boiling with ether and alcohol to remove fats and then for a day and a half with water to extract the collagens. The residue is boiled with strong acetic acid and thereafter with strong soda until the fibers begin to smell. It is then treated with weak acetic acid and for a day with dilute hydrochloric acid. The acid is removed by washing with water and the residue is elastin. There is no solvent which acts on elastin without decomposing it. It is digested by both pepsin and trypsin with the formation of peptones.
Keratin.—This nitrogenous substance is found chiefly in hairs, nails, and horns. It is essentially an alteration proteid product due to peripheral exposure. It is prepared by digesting the fine ground material successively with ether, alcohol, water and dilute acids. The residue is keratin. An imperfect aqueous solution may be secured by heating for a long time under pressure to 200°. It is also dissolved by boiling the materials mentioned above with alkalies, and when the solution thus obtained is treated with water, hydrogen sulfid is evolved, showing that the sulfur of the molecule is loosely combined.
Horn swells up when treated with dilute acetic acid and dissolves in the boiling glacial acid. When treated with hot dilute sulfuric acid it yields aspartic and volatile fat acids, leucin and tyrosin. Keratin, when burning, gives off a characteristic odor as is perceived in burning hair.
Other Albuminoids.—Among the albuminoids of less importance may be mentioned neurokeratin found in the medullary sheath of nerve fibers; chitin occurring in the tissues of certain invertebrates; conchiolin, found in the shells of mussels and snails; spongin, occurring in sponges; fibroin forming silk and spiders webs; and hyalin or hyalogen found in edible birds’ nests.
The nitrogenous bases in flesh which are soluble in cold water, viz., kreatin, kreatinin, carnin, sarkin and xanthin are not classed among the albuminoid bodies, since they have a much higher percentage of nitrogen than is found in true proteid bodies, and are further differentiated from them by the absence of sulfur.
373. Other Forms of Nitrogen.—In addition to the proteids and albuminoids mentioned above, agricultural products may contain nitrogen in the form of ammonia, amid nitrogen and nitric acid. The quantities of nitrogen thus combined are not large but often of sufficient magnitude to demand special study. In general, these bodies belong to transition products, representing stages in the transfer of nitrogen from the simple to complex forms of combination, or the reverse.