Fish Antifreeze Proteins

Abstract

Fish antifreeze proteins (APs) including both glycoproteins and small proteins are associated with freezing avoidance in most polar fishes. There are two major types of APs, the antifreeze glycoproteins (AFGPs) found in Antarctic notothenioid fishes and northern cod fishes (gadids) and three other structurally unique small antifreeze proteins (AFPs) in unrelated taxa. Although the APs differ in composition, size, and structure, on a molar basis almost all depress the freezing point of water to the same extent by adsorbing to and inhibiting the growth of ice by the Kelvin effect. Liver is the site of synthesis and source of the blood APs except in the Antarctic notothenioids. Pancreatic tissues also synthesize APs that enter the anterior intestine where they prevent freezing of the hyposmotic intestinal fluid. Occasionally ice crystals most likely enter at the gills or intestinal mucosa and APs bind to them and inhibit their growth. Systemic macrophages recognize the AP decorated crystals, phagocytose them, and migrate to the spleen. Seasonal exposure to warm water less than −1 °C melts the ice in some fish, but this is not an option for those inhabiting year round freezing seawater and the fate of their splenic ice is unknown. Aglomerular kidneys conserve the APs in the circulation in many fishes and if glomeruli are present, they are generally non-functional. Some body fluids lack APs such as the urine and ocular fluids and the stable undercooled state is maintained because the surrounding tissues are fortified with APs. There is a correlation between severity of the environment and levels of blood APs with the caveat that in some northern fishes there is a seasonal presence of APs in some cods, eel pouts, sculpins, and flounder.