Blood of Fishes: Anti-Freeze Proteins

The colligative effects of solutes in seawater (around 450 mM) depresses sea water freezing point to –1.9°C , while ordinary teleost blood (much more dilute) freezes at –0.4°C. So to avoid the blood and all tissues freezing solid, marine coldwater teleosts produce small anti-freeze proteins in the scales, skin, fins, and gills, and in the liver. There are five kinds of anti-freeze molecules, four proteins and one glycoprotein, and they are as chemically diverse as they are in shape.

The structure of a single unit of an anti-freeze glycoprotein, with a flat surface of hydrogen bonding residues attached to the surface of an ice crystal. The anti-freeze proteins are produced in the liver and in the skin, while the glycoproteins come from the exocrine pancreas and enter the gut via the pancreatic duct to prevent the gut contents freezing (Cheng et al., 2006). As so often happens, these substances were first discovered in fish, and later found in insects and plants. They recognize ice crystal surfaces and bind to them, seemingly modifying the ice surface, or inserting themselves along steps in the lattice, but it is still not entirely clear how these interesting substances work (Ewart et al., 1999; Fletcher et al., 2001).

The latter authors suggest that the diversity of anti-freeze types and their curious phylogenetic distribution indicate that they evolved recently, in response to sea level glaciation some 1–2 mya in the N hemisphere and 10–30 mya around Antarctica. The rate of loss of these small molecules in the kidney in the Antarctic fish is minimized because the kidneys are aglomerular (Eastman, 1993).

The little rainbow smelt (Osmerus mordax) living in the sea under the winter ice around Newfoundland at –1.8°C, show no evidence of death by freezing (as Driedzic and Ewart (2004) remark, perhaps thinking of rigor mortis). This fish uses an anti-freeze protein, but also uses the same cryoprotectant that various invertebrates use glycerol. As any stock-farming reader will know, we use glycerol ourselves in the cryopreservation of sperm straws. Naturally, glycerol diffuses out of the fish by the gills and so in winter has to be produced continually.