Glycopeptide 8 is the smallest component among the antifreeze glycoproteins from polar fish bloods. In a study of the contribution to its functional properties of each part of its structure, glycopeptide 8 from the Antarctic fish Trematomus borchgrevinki was subjected to Edman degradation, p elimination and periodate oxidation of its disaccharide side chains, and a number of chemical modifications of its lone NHz-terminal and COOH-terminal groups. Also degraded in the Edman experiment was glycopeptide 8 from Boreogadus saida, the Arctic polar cod. This contains the same number of proline residues (two) as the T. borchgreuinki glycopeptide, but in different positions in the 14-residue chain. The activity of the modified and degraded glycopeptides 8 was compared with that of native glycopeptide 8 in a series of tests. It was shown that the structural requirements for activity of the glycopeptide with respect to the size and structure of its peptide chain are subject to variation with the conditions of freezing, but that the carbohydrate portions of the molecule are indispensable for activity under all conditions. Structural requirements for the weak activity of the prolinecontaining glycopeptides tested alone are much more stringent than those for the activity of the same glycopeptides tested in mixtures with the larger antifreeze glycoproteins under conditions where their special cooperative function (potentiation) occurs.

The structure and general properties of the antifreeze glycoproteins have been described in many places (1-5), most recently in the preceding paper (6). The present work is concerned with the smallest component, antifreeze glycopeptide 8. Glycopeptide 8 has a molecular weight of-2650 and a peptide chain of 14 residues, including some prolines and four threonines. Each threonine carries in a-glycosidic linkage a disaccharide, galactosyl-N-acetylgalactosamine. Morris et al.(7), following the partial proof of a sequence by Lin et al.(a), established the amino acid sequence of antifreeze glycopeptide 8 from the Antarctic fish Trematomus borchgreuinki. They showed that it consists of a mixture of three closely related glycopeptides and that these occur in the relative proportions of 7: 2: 1 as follows.