**3. Magnesium and hepatic glucose metabolism**

In liver cells, adrenergic stimulation of α1-and β-adrenergic receptors, and glucagon receptors elicit a Mg2+extrusion that is associated with activation of glycolysis and glucose output on functional and temporal bases [40]. Although the nature of this association requires further clarification, it is fairly evident that conditions that limit the amplitude of Mg2+extrusion decrease the amount of glucose outputted from liver cells, and vice versa [40]. This association is further supported by several pieces of observation. Overnight starvation, which depletes the liver of its glycogen content, decreases total hepatic Mg2+content by 10-15% [58] rendering liver cells unresponsive to any subsequent adrenergic stimulation [58]. Both type-1 and type-2 diabetes present with a marked decrease in hepatic Mg2+content [59], and treatment with the anti-diabetic drug metformin, which operates predominantly on liver metabolism, increases intra-hepatic Mg2+content [60]. The loss of hepatic Mg2+observed under diabetic conditions depends on the enhanced phosphorylation of the Na+ .Mg2+exchanger [61,62], and can be attenuated to a significant extent by the presence of glycogen, amylopectin, or glucose within liver plasma membrane vesicle [62].

The functional association between Mg2+and glucose is also observed for Mg2+accumulation. Insulin, one of the hormones involved in Mg2+accumulation, is also responsible for glucose accumulation and conversion to glycogen [58]. Following insulin administration, Mg2+accumulation is directly proportional to the amount of glucose present in the system [63]. Conversely, decreasing Mg2+content in the extracellular system decreased the accumulation of glucose within the cells [40,63]. In part, the limited accumulation of glucose into insulinstimulated cells in the presence of low extracellular Mg2+concentration can be explained with the reduced activation of the insulin receptor occurring in these cells as Mg2+is essential for the proper autophosphorylation of the insulin receptor and the subsequent recruitment of the insulin receptor substrate to the activated receptor [54]. All together, these pieces of evidence and observation support an essential role of Mg2+in glucose regulation and pose for the cation as an important player in the onset and development of insulin resistance and diabetes in human patients.

#### **3.1. Magnesium and enzyme activation in glucose metabolism**

state FoxO1 localizes in the nucleus, binds to the insulin response element sequence of gluconeogenesis-related genes, chiefly glucose 6 phosphatase and PEPCK, and increases their transcription rate, indirectly increasing the rate of hepatic glucose production. In its phos‐ phorylated state, FoxO1 is unable to translocate to the nucleus and to activate the gluconeo‐ genesis-related genes. Inhibition of FoxO1 could then improve hepatic metabolism in cases of

The human insulin receptor homodimer is heavily glycosylated and contains a total of 19 predicted N-linked glycosylation sites in each monomer. The presence of sialic acid residues on molecules and cells is critical to their biological function and the presence of sialic acid residues on glycoproteins is partly responsible for the binding and transport of molecules, masking of the surface charge, aggregation and shape of cells [56]. Most recently, neuramini‐ dase-1(Neu-1) an enzyme responsible for hydrolyzing sialic acid (neuraminic acid), has been associated with the positive regulation of insulin signaling [57]. Neu-1 is transported to the cell surface and gets involved in the regulation of cell signaling. Insulin binding to its receptor rapidly induces interaction of the glycan chains of the receptor with Neu-1 which hydrolyzes sialic acid residues in the glycan chains of the receptor consequently inducing activation of the insulin receptor. Impaired insulin-induced phosphorylation of Akt, thus identifies Neu1 as a novel component of the signaling pathways of energy metabolism and glucose uptake. Insulin binding to the insulin receptor has been shown to induce the interaction of the receptor with a pool of Neu-1 near the cell surface [57]. Also, insulin signaling is partially impaired in tissues of Neu-1-deficient mice [3], and desialylation of the insulin receptor by Neu1 promote the receptor activation [57]. While CaCl2 has no significant effect on human liver neuraminidase activity, 10mM MnCl2 or MgCl2 shows a mild stimulatory effect (112% and 125% over control

Additional experimental evidence indicates that Mg2+is required for the activated insulin

In liver cells, adrenergic stimulation of α1-and β-adrenergic receptors, and glucagon receptors elicit a Mg2+extrusion that is associated with activation of glycolysis and glucose output on functional and temporal bases [40]. Although the nature of this association requires further clarification, it is fairly evident that conditions that limit the amplitude of Mg2+extrusion decrease the amount of glucose outputted from liver cells, and vice versa [40]. This association is further supported by several pieces of observation. Overnight starvation, which depletes the liver of its glycogen content, decreases total hepatic Mg2+content by 10-15% [58] rendering liver cells unresponsive to any subsequent adrenergic stimulation [58]. Both type-1 and type-2 diabetes present with a marked decrease in hepatic Mg2+content [59], and treatment with the anti-diabetic drug metformin, which operates predominantly on liver metabolism, increases

insulin resistance and metabolic syndrome [55].

102 Glucose Homeostasis

activity, respectively) [56].

receptor to phosphorylate IRS [54].

**3. Magnesium and hepatic glucose metabolism**

**2.1. Role of Mg2+on insulin receptor activation and signaling**

The physiological role of magnesium is principally related to enzyme activity. All enzymes utilizing ATP require Mg for substrate formation. Intracellular free magnesium also acts as an allosteric activator of enzyme action including critical enzyme systems such as adenylate cyclase, phosphofructokinase, phospholipase C, and Na+ /K+ -ATPase [64]. Magnesium is an enzyme substrate (ATPMg, GTPMg) to enzymes such as ATPase (Na+ , K+ ATPase, Ca2+ATPase), cyclases (adenylate cyclase, guanylate cyclase), and the kinases (hexokinase, protein kinase) [64]. Recently, our laboratory has provided evidence that Mg2+also modulates the amount of glucose 6-phosphate being routed into the endoplasmic reticulum (E.R) to be hydrolyzed to glucose plus Pi by the glucose 6-phosphatase, or to be converted to 6-phosphogluconolactone by the hexose 6-phosphate dehydrogenase, the reticular version of the G6PD. Moreover, our laboratory has provided significant evidence that both glucose and Mg2+homeostasis are altered under pathological conditions such as diabetes [61] and alcoholic liver disease [65].

Many of the enzymes of glycolytic pathway that utilizes glucose have a requirement for Mg2+ [26] and utilize MgATP2-as a cofactor [66]. The Km values for Mg2+in the glycolytic enzymes of the human erythrocyte are between 1 and 2.3 mM for hexokinase, 0.025 mM for phospho‐ fructokinase (PFK), 0.3 mM for phosphoglycerate kinase (PGK), and 1 mM for pyruvate kinase [26]. Magnesium ions (Mg2+) and MgATP2-regulate the most important glycolytic enzymes, namely hexokinase, phosphofructokinase, aldolase, phosphoglycerate kinase, and pyruvate kinase [66]. Glucokinase (Hexokinase IV or D), an enzyme expressed predominantly in liver and pancreatic β-cells of vertebrates, shows marked deviations from Michaelis-Menten kinetics when the glucose concentration is varied at a constant MgATP2-concentration, but shows no deviations from Michaelis-Menten kinetics with respect to MgATP2-[26,67]. Com‐ pared to the other hexokinase isoenzymes, this isoform has a low affinity for glucose (Table 1). Maximum binding of glucokinase and its regulatory protein to the hepatocyte matrix occurs at low [glucose] (<5mM) in a Mg2+-dependent manner (Table 2, [68]). The regulatory protein binds to the hepatocyte matrix with ionic characteristics similar to those of glucokinase but, unlike glucokinase, it does not translocate from the binding site. Since the binding of gluco‐ kinase to its regulatory protein is associated with a decrease in the affinity of the enzyme for glucose, the bound enzyme in the presence of Mg2+represents an inactive state and the translocated enzyme a more active state [69].


**Table 2.** Effect of [Substrate] on Kd of the high affinity binding sites of Glucokinase
