Studies on the structure of the vitreous body. V. Soluble protein content.

Since iLlijrner (l), the proteins of the vitreous body have been generally classified as residual and soluble proteins. The former includes those elements of the gel which can be collected as a residue after filtration or as a sediment after high speed centrifugation (more than 50,000 X g). The latter refers to the proteins in the liquid vitreous, the viscous fluid remaining after removal of the structural elements of the gel, viz. fibers, filaments, and cells. The soluble protein content of the cattle vitreous body has been studied by several authors and was reported to be between 0.02 and 0.2% (2). The albumin-globulin ratio, based on ammonium sulfate precipitation, was found to be much lower than that of blood serum (2). Ultraviolet absorption studies on the proteins of the vitreous body also indicate a low albumin-globulin ratio (3). Electrophoretic studies suggest the presence of two protein components with mobilities similar to those of serum albumin and y-globulin (4, 5). However, it was found that y-globulin added to the soluble proteins forms a separate boundary in the course of electrophoresis (6) and ultracentrifugation (5). All available data indicate that the protein composition of the vitreous body is different from that of plasma. Since this is the only connective tissue in the mammalian body in which the composition of intercellular substance, uncontaminated by blood, lymph, or cells, can be studied, a closer investigation of proteins, with special regard to glycoproteins, appeared to be of great interest. Previous investigations have shown that certain macromolecular components, such as hyaluronic acid and collagen, are unevenly distributed in the vitreous bodies of various animal species (7, S).l Therefore, the distribution of soluble proteins and glycoproteins was studied in the gel. The investigations reported herein are based on sialic acid, hexosamine and nitrogen analyses, on the separation of the perchloric acid-soluble protein fraction and on electrophoretic analyses. A preliminary report has been given (9). While these studies were in progress a brief report was given by Dische et al. (10) on the isolation of a glycoprotein fraction rom the bovine vitreous body representing 15% of the total solu-