Studies on the cornea. X. Incorporation of 14C-glucosamine into glycosaminoglycans and glycoproteins of elasmobranch corneas, extractability of these components and morphological effects of such extractions.

Incorporation of 14C-glucosamine into dogfish (Squalus acanthias) corneas occurs primarily in the glycosaminoglycans (GAG) and glycoproteins (GP) extractable with elasmobranch Ringer's solution but the rate at which this component can be extracted varies inversely with the age (size) of the fish. Whereas this fraction represents only one-fourth of the total hexosamine (Hex-N) it contains nearly three-fourths of the newly incorporated 14C-glucosamine. The CaCl2-extractable HexN contains only 13% of the incorporated radioactivity. Collagen-associated GAG in both soluble and insoluble collagen fractions contain keratan sulfate, dermatan sulfate and chondroitin-4-sulfate, with the GAG associated with the salt-insoluble collagen having only half the radioactivity of those associated with the soluble fraction. However, the specific activities of both the salt-insoluble and the salt-soluble fractions were very low, suggesting that the GAG-GP which are tightly bound to collagen may have a high degree of stability. Partially de-epithelialized corneas incorporated 14C-glucosamine at rates ranging from 22% to 58% of the efficiency of intact corneas, suggesting a possibly significant role of the epithelium in the biosynthesis of GAG-GP in the adult dogfish. Morphologic results exactly paralleled those previously reported by us for the bovine cornea with the exception that the effects of CaCl2 extraction of GAG-GP were not evident on the collagen fibers until the second to fifth sequential CaCl2 extraction. The morphologic findings, therefore, exactly parallel the analytic findings which show that a significant portion of the collagen and collagen-associated GAG-GP of the dogfish cornea remains after the double extraction. How much of this remaining material represents sutural fibers and their associated GAG-GP is yet to be determined.