r macromolecules, that can depress freezing points is required.
The usual strategy is to produce large, colloidal macromolecules, glycopeptides and peptides (4). These molecules have been termed anti-freeze polypeptides (AFPs) and anti-freeze glycoproteins (AFGPs) (2). Produced primarily in liver cells, they are subsequently liberated into plasma and extracellular fluids. Of molecular weights averaging between 2,400 and 34,000 Daltons, such macromolecules can effectively depress the freezing point of fish plasma 200-300x more than would be expected on the basis of strict colligative relationships (i.e., not related to a dependency on the sheer number of particles added to solution).
The concentration of solutes in seawater is about 3x that seen in plasma. Because of this hypertonicity, the freezing temperature of the sea can lower to -1.9¦ C -- a full degree below the freezing point of unprotected plasma. The effective role of AFPs and AFGPs is to lower the freezing point of an organism's blood plasma without affecting or lowering the melting point, and to do so without causing osmotic imbalance in the various many, sensitive, physiological systems.
Evidence suggests that fish plasma anti-freezes exert their influence by altering the structure of water around them as the temperature lowers (4) and binding specificity occurs along specific axes of the anti-freeze protein in relation to the ice planes (16).
In addition to the danger of actual freezing, low temperatures can limit the efficiency of many cellular reactions, among them those that control oxidative phosphorylation. Nathanailides (11) examined the significance of cold-related changes in enzyme activities in fish muscle and found that fish attempt to compensate for these effects by increasing the activity of aerobic enzymes. When fish were cold-acclimated, aerobic enzymes increased in content and capillarization was seen, making oxygen and metabolites more ...