Isolation and preliminary characterization of a structural glycoprotein complex from bovine corneal stroma

After extraction of bovine corneal stroma with 1M CaC1(2) and subsequent digestion of the insoluble residue with purified bacterial collagenase, two crude structural glycoprotein (SPG) fractions were obtained; one which precipitated upon dialysis against water of the collagenase solubilized material and the other which was extracted by 8M urea from the collagenase insoluble material. Amino acid analyses of these crude SGP fractions indicated that they were primarily non-collagenous but that very small amounts of collagen-derived amino acids were present. Upon gel filtration of these SGP fractions on Sepharose 4B-CL, void volume fractions were isolated from each of the crude fractions which were enriched, relative to the original crude fractions, in the collagen-derived amino acids. Carbohydrate analysis indicated that the void volume fractions had the properties of glycoproteins rather than proteoglycans. Upon disulfide reduction and SDS-PAGE, each of these fractions was resolved into five major protein bands with molecular weights of 155,000, 137,000, 117,000 82,000 and 34,000. Only the three largest bands contained the collagen derived amino acids. These data are consistent with the presence within bovine corneal stroma of a large structural glycoprotein complex comprised of at least five protein components associated through disulfide bonds. Collagen apparently is associated with three of these, either through covalent crosslinkage, or as part of the primary structure.