Rainbow smelt: the unusual case of cryoprotection by sustained glycerol production in an aquatic animal
Rainbow smelt flourish at −1.8 °C, the freezing point of sea water. An antifreeze protein contributes to freeze point depression but, more importantly, cryoprotection is due to an elevation in osmotic pressure, by the accumulation of glycerol. The lower the water temperature, the higher the plasma glycerol with levels recorded as high as 400 mmol l−1. Glycerol freely diffuses out in direct relation to the glycerol concentration and fish may lose as much as 15 % of their glycerol reserve per day. Glycerol levels decrease from a maximum in February/March while water temperature is still sub-zero. The decrease in glycerol may respond to a photoperiod signal as opposed to initiation which is triggered by low temperature. The initial increase in glycerol level is supported by liver glycogen but high sustained glycerol level is dependent upon dietary carbohydrate and protein. The metabolic pathways leading to glycerol involve flux from glycogen/glucose to the level of dihydroxyacetone phosphate (DHAP) via the initial part of glycolysis and from amino acids via a truncated gluconeogenesis again to the level of DHAP. DHAP in turn is converted to glycerol 3-phosphate (G3P) and then directly to glycerol. The key to directing DHAP to G3P is a highly active glycerol 3-P dehydrogenase. G3P is converted directly to glycerol via G3P phosphatase, the rate-limiting step in the process. The transition to glycerol production is associated with increased activities of enzymes at key loci in the top part of glycogenolysis/glycolysis. Curtailment of the final section of glycolysis may reside at the level of pyruvate oxidation with an inactivation of pyruvate dehydrogenase (PDH) driven by increased levels of PDH kinase. Enzymes associated with amino acid trafficking are elevated as is the pivotal enzyme phosphoenolpyruvate carboxykinase.