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Freeze Resistance in Rainbow Smelt (Osmerus mordax): Seasonal Pattern of Glycerol and Antifreeze Protein Levels and Liver Enzyme Activity Associated with Glycerol Production

Rainbow smelt (Osmerus mordax) inhabit inshore waters along the North American Atlantic coast. During the winter, these waters are frequently ice covered and can reach temperatures as low as −1.9°C. To prevent freezing, smelt accumulate high levels of glycerol, which lower the freezing point via colligative means, and antifreeze proteins (AFP). The up‐regulation of the antifreeze response (both glycerol and AFP) occurs in early fall, when water temperatures are 5°–6°C. The accumulation of glycerol appears to be the main mechanism of freeze resistance in smelt because it contributes more to the lowering of the body’s freezing point than the activity of the AFP (0.5°C vs. 0.25°C for glycerol and AFP, respectively) at a water temperature of −1.5°C. Moreover, AFP in smelt appears to be a safeguard mechanism to prevent freezing when glycerol levels are low. Significant increases in activities of the liver enzymes glycerol 3‐phosphate dehydrogenase (GPDH), alanine aminotransferase (AlaAT), and phosphoenolpyruvate carboxykinase (PEPCK) during the initiation of glycerol production and significant correlations between enzyme activities and plasma glycerol levels suggest that these enzymes are closely associated with the synthesis and maintenance of elevated glycerol levels for use as an antifreeze. These findings add further support to the concept that carbon for glycerol is derived from amino acids.