Glycerol loss to water exceeds glycerol catabolism via glycerol kinase in freeze-resistant rainbow smelt (Osmerus mordax)

Abstract

Rainbow smelt accumulate high amounts of glycerol in winter. In smelt, there is a predictable profile of plasma glycerol levels that starts to increase in November (<5 μmol/ml), peaks in mid-February (>200 μmol/ml), and thereafter decreases to reach the initial levels in the beginning of May. The aim of this study was to investigate the respective role of the two main mechanisms that might be involved in glycerol clearance from mid-February: 1) breakdown of glycerol to glycerol-3-phosphate through the action of the glycerol kinase (GK) and 2) direct loss toward the environment. Over the entire glycerol cycle, loss to water represents a daily loss of ∼10% of the total glycerol content of fish. GK activities were very low in all tissues investigated and likely have a minor quantitative role in the glycerol cycle. These results suggest that glycerol levels are dictated by the rate of glycerol synthesis (accelerated and deactivated during the accumulation and decrease stages, respectively). Although not important in glycerol clearance, GK in liver might have an important metabolic function for other purposes, such as gluconeogenesis, as evidenced by the significant increase of activity at the end of the cycle.

Rainbow smelt: the unusual case of cryoprotection by sustained glycerol production in an aquatic animal

“…and are from Raymond (1992Raymond ( , 1993 is still below 0 °C. This pattern is highly reproducible and has been recorded in other independent experiments (Treberg et al 2002b;Robinson et al 2011;Ditlecadet et al 2011). The trigger to glycerol accumulation is clearly the decrease in temperature, per se, as it can be further elevated, late in the season, with imposed decreases in temperature (Lewis et al 2004) or induced in animals maintained at high non-glycerol-accumulating temperatures and subsequently challenged with a controlled decrease in temperature ( Fig.…”

“…The maximal activity of liver G3P Pase assayed at 15 °C and normalized to whole animal mass was 43 µmol g fish −1 day −1 (reported as 30 mU/g fish) occurring between a plasma glycerol level of 50 to 175 mmol l −1 (Ditlecadet and Driedzic 2013). The in vitro enzyme activity once adjusted to 0 °C compares very favorably with the rate of diffusive loss of about 6 µmol g fish −1 day −1 (Ditlecadet et al 2011).…”

“…The movement of glycerol out of the animal may be detected on absorbent paper placed against gill and skin tissue (Raymond 1993). The loss of glycerol to sea water is directly proportional to plasma glycerol (Ditlecadet et al 2011). The mechanism of glycerol movement, be it paracellular or transcellular (via simple or facilitated diffusion), is not known.…”

“…The later could theoretically be carried out by reversal of the cytosolic G3PDH discussed to this point or by the mitochondrial GPDH (mG3PDH) that reduces FAD to FADH 2 . Both GK and mG3PDH increase substantially at the end of the glycerol cycle but only well after glycerol levels have decreased (Ditlecadet et al 2011;Robinson et al 2011). Rates of appearance of glycerol into water are high enough to account for the decrease of glycerol in the animal (Ditlecadet et al 2011); however, this must represent a caloric loss to the species, the complexities of which have not been assessed.…”

“…Both GK and mG3PDH increase substantially at the end of the glycerol cycle but only well after glycerol levels have decreased (Ditlecadet et al 2011;Robinson et al 2011). Rates of appearance of glycerol into water are high enough to account for the decrease of glycerol in the animal (Ditlecadet et al 2011); however, this must represent a caloric loss to the species, the complexities of which have not been assessed. The concerted activity of GK and mG3PDH at the end of the cycle may serve to decrease glycerol to very low levels in the absence of a large diffusion gradient.…”