Antifreeze glycoproteins from Arctic fish.

Four species of fish in the Barents Sea of the Arctic, north of Norway and Russia, were caught and examined for the presence of antifreeze proteins in their blood sera. Only one species of Arctic fish, the polar cod (Boreogadus saida), was found to contain an antifreeze protein. Antifreeze glycoproteins were isolated and characterized from the blood serum of the Arctic fish, and their structure and function were compared with the antifreeze glycoprotein from the Antarctic fish, Trematomas borchgreuinki. The composition and sequence of the active fraction were similar, if not identical, in both species, although there were differences in the number of multiple molecular forms present. The active glycoproteins and smaller related glycopeptides contained alanine, threonine, and disaccharide. Although the distributions of different lengths of the peptides varied between the species, the active antifreeeze glycoproteins from both polar regions had the identical fundamental structure consisting of repeating units of the glycotripeptide, Ala-Ala-Thr, with all the threonine residues glycosidically a-linked to a /3,1 --f 3 galactosyl-IV-acetylgalactosamine. The smaller glycopeptides also contained proline residues following some of the threonine residues, but the positions were different in the Arctic and Antarctic materials. Also similar were the NHZand COOH-terminal residues, the inhibition of hemagglutination by osage orange seed lectin, and the inhibition of antifreeze activity by borate.