Glycoproteins

GLYCOPROTEINS, conjugated proteins, compounds of a protein with a carbohydrate or its derivative. They contain less carbon and nitrogen and more oxygen than simple proteins. Phosphorus-free glycoproteins and phosphoglycoproteins are distinguished. The former are divided into: 1) mucinous substances (mucins, mucoids), containing the aminosugar glucosamine (10–35%) in the form of paired mucoitinsulfuric acid; 2) chondroproteids, which contain chondrosamine in the form of chondroitinsulfuric acid; and 3) hyalogens, which upon the action of alkali turn into hyalines and yield carbohydrates upon further decomposition.—M u c i n s have acid properties. They are insoluble in water, but dissolve in weak alkalis. Their solutions are viscid and stringy, and do not coagulate upon boiling. Acetic acid precipitates them, and the precipitate is almost insoluble in an excess of precipitant at ordinary temperature; K₄FeCy₆ gives no precipitate; they are digested by gastric and pancreatic juices; indole and skatole are absent in the products of putrefaction. Mucins are secreted by certain salivary glands, are found on mucous membranes, in the skin secretions of snails, and in the membranes of frog and fish eggs; along with mucoid, they are found in Wharton's jelly of the umbilical cord. By making tissues slippery and elastic, they protect them from external harmful influences.—M u c o i d s have been found in ovarian cysts (pseudomucin, paramucin), in the cornea (corneamucoid), in the vitreous body of the eye (hyalomucoid), in egg white (ovomucoid), and in certain ascitic fluids.—To the hyalogens, which are generally poorly studied, belong: neossin in the edible nests of Chinese swiftlets, the membranine of Descemet's membrane and the lens capsule of the eye, the hyalin of echinococcal cysts, chondrosin of slime sponges, etc.—To the chondroproteids belong: chondromucoid of cartilage, mucoid of tendons, osseomucoid of bone, mucoids of the aorta, skin, and the sclera of the eye. Chondromucoid reacts acid, is insoluble in water, dissolves in alkali, splitting off chondroitinsulfuric acid and forming sulfurous alkali. Solutions of chondromucoid in a very small amount of alkali are precipitated by acids. K₄FeCy₆ in the presence of NaCl and HCl gives no precipitate; it is not precipitated by tannic acid, and the latter does not yield a precipitate with gelatin in its presence.—Phosphoglycoproteins upon hydrolysis split off a reducing substance and phosphoric acid without yielding purine bases. Known examples are: ichthytulin from carp eggs and helicoproteid from the albumin gland of Helix pomatia. A. Barkhash. In microscopic technique, the determination of glycoproteins is carried out as follows: the pieces under investigation are fixed with 96–100% alcohol or Carnoy's fluid (alcohol + chloroform + acetic acid), embedded in paraffin, and stained according to Fischer. For this, sections are treated for 5–10 minutes in a 10% aqueous solution of tannin, rinsed with a 1% solution of K₂Cr₂O₇, immersed for 5–10 minutes in a 10% solution of K₂Cr₂O₇, rinsed with distilled water, and stained for 10 minutes in a saturated solution of safranin in aniline water, differentiated in a clear alcoholic solution of Lichtgrün, quickly passed through alcohols, and mounted in Canada balsam in the usual way. Results: glycoprotein granules stain bright red. In some cases, the staining of the granules is enhanced by adding 3 drops of formic acid per 5 cubic centimeters to the 10% K₂Cr₂O₇ solution. To avoid dissolving the glycoproteins, it is necessary to shorten the stay of sections in aqueous solutions as much as possible. The method is applicable not only to tissue sections, but also to whole mounts on smears, for example, in the study of protozoa.