Biological antifreeze agents in coldwater fishes

1. 1. Biological antifreezes are present in coldwater fishes and lower their freezing point below the freezing point of seawater (- 1.9°C), without substantially increasing the osmotic content of their body fluids. 2. 2. The antifreeze agents present in the Antarctic notothenioid and northern gadid fishes are glycopeptides composed of alanine, threonine, galactose and N-acetylgalactosamine. In other northern fishes they are peptides which differ substantially in composition between species, but are similar in that two thirds of their residues are alanine and they are rich in the polar residues aspartate, glutamate, threonine and serine. 3. 3. Although the average mol. wt of the antifreezes is between 5000 and 10,000. they have a very large effect, on the freezing point of water which has been termed an antifreeze effect. The depression of the freezing point occurs via the non-colligative mechanism, adsorption-inhibition. Adsorption to ice prevents water from joining the ice lattice by increasing the radius of curvature of the fronts on the growth steps of the crystals thereby increasing their surface free energy. 4. 4. The antifreezes are a permanent feature of the Antarctic fishes, but in many northern fishes they are present only during the winter and the antifreeze cycle appears to be an endogenous one. 5. 5. Antifreeze concentrations in the blood are between 3 and 4%, but they are not present in the urine. In the Antarctic and many northern species, the antifreezes are conserved in the circulation because the kidneys are aglomerular or functionally aglomerular. In the glomerular winter flounder, the acidic peptide antifreeze is conserved in the circulation because it is not filtered. Retention of the peptide occurs because it is repelled from the negatively charged basement membrane of the capillary wall. Small amounts of low mol. wt antifreeze are lost via the bile and digestive system where their role appears to be prevention of freezing of the dilute intestinal fluid.