Accelerated hepatic glycerol synthesis in rainbow smelt (Osmerus mordax) is fuelled directly by glucose and alanine: a1H and13C nuclear magnetic resonance study

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Abstract
At seawater temperatures below 1°C, rainbow smelt (Osmerus mordax) accumulate plasma levels of glycerol up to 400 mM. Aspects of the synthesis of glycerol in liver and its regulation were previously investigated, but the pathways leading to glycerol synthesis remained unconfirmed. Here, we report nuclear magnetic resonance (NMR) studies which elucidate, in more detail, the fuel sources for rapid glycerol synthesis in rainbow smelt. Initial NMR analysis of liver homogenates from fish held at cold (−1°C) temperatures and from fish transferred from 8°C to −1°C showed elevated glycerol, whereas those from fish held at 8°C had far lower glycerol levels. These results confirm a temperature-responsive glycerol synthesis and show that NMR is a suitable approach to investigate the phenomenon. Further studies with fish held at low temperature and injected with labelled L-[2,3-13C2] alanine or D-[U-13C6]glucose revealed conversion of both alanine and glucose to glycerol. 13C spectra showed satellites (1JCC=41.1 Hz) about the glycerol resonances indicating intact incorporation of a 13C–13C unit in liver glycerol of fish injected with L-[2,3-13C2]alanine and a 13C–13C–13C unit in liver glycerol of fish injected with D[U-13C6]glucose. Thus, glycerol can be efficiently produced directly from amino acid precursors by glyceroneogenesis, which is an abbreviated gluconeogenesis process leading to glycerol through dihydroxyacetone phosphate (DHAP). Glucose can also be metabolised to glycerol via an abbreviated form of glycolysis that similarly leads to glycerol through DHAP. J. Exp. Zool. 305A, 2006.