Measurements of ion concentrations, freezing points and melting points of intestinal fluid were made for several Antarctic fishes and two North Atlantic species. These measurements indicated that plasma and intestinal fluid are nearly isosmotic. Freezing points of intestinal fluid were approximately 0·9 °C below the melting points, suggesting the presence of glycoprotein antifreeze within the intestinal fluid of the Antarctic fishes. Polyacrylamide gel electrophoresis and specific

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... ation with glycoprotein antifreeze antibody confirmed the presence of appreciable quantities of antifreeze and showed that the major antifreeze fractions present in the intestinal fluid are low molecular weight glycopeptides.