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  • Results of studies on fish FBPase are

    2021-10-19

    Results of studies on fish FBPase are somewhat confusing: although fishes skeletal muscles possess relatively high FBPase activity (Knox et al., 1980, Lushchak et al., 2001) and the isozyme present therein has kinetic properties typical to all vertebrates muscle FBPases (Rosenmann et al., 1977), investigation designed to detect muscle-type FBPase mRNA in fish has failed (Tillmann et al., 2002). Furthermore, separation of liver and muscle FBPase in mammals, isrib and amphibia was estimated to have happened much later than the separation of fish and terrestrial vertebrates. Therefore, it is speculated that during evolution two independent FBPase gene duplication events might have occurred, leading to fish and to terrestrial vertebrates' liver and muscle FBPase genes (Tillmann et al., 2002). As in other vertebrates, physiological role of fish liver FBPase is well documented (Walton and Cowey, 1979, Biswas and Majumder, 1985, Mommsen et al., 1985, Mommsen, 1985, Lushchak et al., 2001). On the other hand, the role of muscle FBPase for many years was rather an enigma. In recent years, evidence has accumulated indicating that, in mammalian muscle, up to 50% of lactate is converted to glycogen in a process called glyconeogenesis (Fournier et al., 2002). In such a pathway FBPase is indispensable. The same might be true for the fish muscle. Fish white muscle is known to produce a large amount of lactate and to accumulate large amount of glycogen. On the other hand, absence of phosphoenolpyruvate carboxykinase (PEPCK) in the muscle type apparently eliminates resynthesis of glucose 6-phosphate in situ from lactate (Knox et al., 1980, Suarez and Mommsen, 1987). Therefore, it has been proposed that in carp lactate produced in white muscle is directly (without the involvement of the liver) transported to the red muscle capable of converting lactate to glucose, which is subsequently transported back to white muscle for glycogen synthesis (Knox et al., 1980). Recently it has been reported that reaction catalyzed by pyruvate kinase is reversible (Fournier et al., 2004), therefore neither carboxylase nor phosphoenolpyruvate carboxykinase is needed for glyconeogenesis. For a long time FBPase was regarded to be a cytosolic enzyme. Recently we have studied the subcellular localization of FBPase in muscle tissue of mammals. We have demonstrated that, in striated muscle, FBPase accumulates in close proximity to the Z-line, where it binds to alpha-actinin (Gizak et al., 2003, Rakus et al., 2003a), and we have postulated the existence of glyconeogenic metabolon around the Z-line (Rakus et al., 2003a, Rakus et al., 2005) in analogy to the multi-enzyme complex formed by glycolytic enzymes bound to thin filaments. Additionally, in cardiac and smooth muscle, FBPase is present in cells' nuclei (Gizak and Dzugaj, 2003, Gizak et al., 2004, Gizak et al., 2005). Taking into account possibility that fish liver and muscle FBPase have arisen by gene duplication independent of such duplication in other vertebrates (Tillmann et al., 2002) it appeared of importance for comparative purposes to extend our studies to the localization of the enzyme in fish muscle tissue.
    Materials and methods
    Results
    Discussion Mechanisms controlling glucose metabolism in tissues of vertebrates (especially liver) have been the subject of intensive research. Regulation of activity of the key glycolytic and gluconeogenic enzymes requires several concerted mechanisms: allosteric regulation, covalent modification (protein phosphorylation and dephosphorylation), and enzymes binding to subcellular structures as well as to one another. Among these, binding of glycolytic enzymes to actin has been especially well documented and filamentous actin is presumed to be a structural basis for the glycolytic complex in muscle tissue of phylogenetically distinct groups of animals (Dolken et al., 1975, Pette, 1975, Wojtas et al., 1997). An increasing body of evidence suggests that association of muscle FBPase with subcellular elements and other proteins plays an important role in regulation of the enzyme activity in vertebrates (Rakus et al., 2003a, Rakus et al., 2003b). Recently we have presented evidence on the existence, in mammalian striated muscles, of a triple aldolase–FBPase–α-actinin complex in which FBPase is entirely desensitized by aldolase to AMP inhibition (Rakus et al., 2003a, Rakus et al., 2003b). Since Ca2+ concentrations characteristic for contracting muscle disrupt the complex, the association of FBPase with the cellular constituents seem to play a role in controlling the speed of glyconeogenesis (Mamczur et al., 2005). Furthermore, the discovery that FBPase–aldolase interaction results in the substrate channeling (Rakus et al., 2004), encouraged us to postulate existence of glyconeogenic metabolon in the region of the Z-line of mammalian striated muscles.