Stores of readily available glucose to supply the tissues with an oxidizable energy source are found principally in the liver, as glycogen. Glycogen is a polymer of glucose residues linked by α-(1,4)- and α-(1,6)-glycosidic bonds. A second major source of stored glucose is the glycogen of skeletal muscle. However, muscle glycogen is not generally available to other tissues, because muscle lacks the enzyme glucose-6-phosphatase.
The major site of daily glucose consumption (75%) is the brain via aerobic pathways. Most of the remainder of is utilized by erythrocytes, skeletal muscle, and heart muscle. The body obtains glucose either directly from the diet or from amino acids and lactate via gluconeogenesis. Glucose obtained from these two primary sources either remains soluble in the body fluids or is stored in a polymeric form, glycogen. Glycogen is considered the principal storage form of glucose and is found mainly in liver and muscle, with kidney and intestines adding minor storage sites. With up to 10% of its weight as glycogen, the liver has the highest specific content of any body tissue. Muscle has a much lower amount of glycogen per unit mass of tissue, but since the total mass of muscle is so much greater than that of liver, total glycogen stored in muscle is about twice that of liver. Stores of glycogen in the liver are considered the main buffer of blood glucose levels.back to the top
Degradation of stored glycogen, termed glycogenolysis, occurs through the action of glycogen phosphorylase. There are two distinct human genes encoding proteins with glycogen phosphorylase activity. One gene (PGYL) expresses the hepatic form of the enzyme and the other (PGYM) the muscle form. The PGYM gene is located on chromosome 11q13.1 and is composed of 20 exons encoding a protein of 842 amino acids. The PGYL gene is located on chromosome 14q22.1 with similar structure to the PGYM gene but encoding a protein of 845 amino acids. The functions and regulation of these two gene products are identical but defects in one or the other explain the tissue-specific nature of several of the glycogen storage diseases discussed below.
The action of phosphorylase is to phosphorolytically remove single glucose residues from α-(1,4)-linkages within the glycogen molecules. The product of this reaction is glucose-1-phosphate. The advantage of the reaction proceeding through a phosphorolytic step is that:
1. The glucose is removed from glycogen is an activated state, i.e. phosphorylated and this occurs without ATP hydrolysis.
2. The concentration of Pi in the cell is high enough to drive the equilibrium of the reaction in the favorable direction since the free energy change of the standard state reaction is positive.
The glucose-1-phosphate produced by the action of phosphorylase is converted to glucose-6-phosphate by phosphoglucomutase: this enzyme, like phosphoglycerate mutase (of glycolysis), contains a phosphorylated amino acid in the active site (in the case of phosphoglucomutase it is a Ser residue). The enzyme phosphate is transferred to C-6 of glucose-1-phosphate generating glucose-1,6-phosphate as an intermediate. The phosphate on C-1 is then transferred to the enzyme regenerating it and glucose-6-phosphate is the released product.
As mentioned above the phosphorylase mediated release of glucose from glycogen yields a charged glucose residue without the need for hydrolysis of ATP. An additional necessity of releasing phosphorylated glucose from glycogen ensures that the glucose residues do not freely diffuse from the cell. In the case of muscle cells this is acutely apparent since the purpose in glycogenolysis in muscle cells is to generate substrate for glycolysis.
The conversion of glucose-6-phosphate to glucose, which occurs in the liver, kidney and intestine, by the action of glucose-6-phosphatase does not occur in skeletal muscle as these cells lack this enzyme. Therefore, any glucose released from glycogen stores of muscle will be oxidized in the glycolytic pathway. In the liver the action of glucose-6-phosphatase allows glycogenolysis to generate free glucose for maintaining blood glucose levels.
Glycogen phosphorylase cannot remove glucose residues from the branch points (α-1,6 linkages) in glycogen. The activity of phosphorylase ceases 4 glucose residues from the branch point. The removal of the these branch point glucose residues requires the action of debranching enzyme (also called glucan transferase) which contains 2 activities: glucotransferase and glucosidase. The transferase activity removes the terminal 3 glucose residues of one branch and attaches them to a free C-4 end of a second branch. The glucose in α-(1,6)-linkage at the branch is then removed by the action of glucosidase. This glucose residue is uncharged since the glucosidase-catalyzed reaction is not phosphorylytic. This means that theoretically glycogenolysis occurring in skeletal muscle could generate free glucose which could enter the blood stream. However, the activity of hexokinase in muscle is so high that any free glucose is immediately phosphorylated and enters the glycolytic pathway. Indeed, the precise reason for the temporary appearance of the free glucose from glycogen is the need of the skeletal muscle cell to generate energy from glucose oxidation, thereby, precluding any chance of the glucose entering the blood.
Glycogen phosphorylase is a homodimeric enzyme that exist in two distinct conformational states: a T (for tense, less active) and R (for relaxed, more active) state. Phosphorylase is capable of binding to glycogen when the enzyme is in the R state. This conformation is enhanced by binding of AMP and inhibited by binding ATP or glucose-6-phosphate. The enzyme is also subject to covalent modification by phosphorylation as a means of regulating its activity. The relative activity of the un-modified phosphorylase enzyme (given the name phosphorylase-b) is sufficient to generate enough glucose-1-phosphate for entry into glycolysis for the production of sufficient ATP to maintain the normal resting activity of the cell. This is true in both liver and muscle cells.
Pathways involved in the regulation of glycogen phosphorylase. See the text for details of the regulatory mechanisms. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Green arrows denote positive effects on any enzyme is indicated. Red T-lines indicate inhibitory actions. Briefly, phosphorylase b is phosphorylated, and rendered highly active, by phosphorylase kinase (glycogen synthase-glycogen phosphorylase kinase, GS/GP kinase). Phosphorylase kinase is itself phosphorylated, leading to increased activity, by PKA (itself activated through receptor-mediated mechanisms). PKA also phosphorylates PPI-1 leading to an inhibition of phosphate removal allowing the activated enzymes to remain so longer. Calcium ions can activate phosphorylase kinase even in the absence of the enzyme being phosphorylated. This allows neuromuscular stimulation by acetylcholine to lead to increased glycogenolysis in the absence of receptor stimulation.
In response to lowered blood glucose the α cells of the pancreas secrete glucagon which binds to cell surface receptors on liver and several other cells. Liver cells are the primary target for the action of this peptide hormone. The response of cells to the binding of glucagon to its cell surface receptor is the activation of the enzyme adenylate cyclase which is associated with the receptor. Activation of adenylate cyclase leads to a large increase in the formation of cAMP. cAMP binds to an enzyme called cAMP-dependent protein kinase, PKA (see Figure below). Binding of cAMP to the regulatory subunits of PKA leads to the release and subsequent activation of the catalytic subunits. The catalytic subunits then phosphorylate a number of proteins on serine and threonine residues.
Representative pathway for the activation of cAMP-dependent protein kinase (PKA). In this example glucagon binds to its' cell-surface receptor, thereby activating the receptor. Activation of the receptor is coupled to the activation of a receptor-coupled G-protein (GTP-binding and hydrolyzing protein) composed of 3 subunits. Upon activation the alpha subunit dissociates and binds to and activates adenylate cyclase. Adenylate cyclase then converts ATP to cyclic-AMP (cAMP). The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. Once released the catalytic subunits of PKA phosphorylate numerous substrate using ATP as the phosphate donor.
Of significance to this discussion is the PKA-mediated phosphorylation of phosphorylase kinase (PHK) as shown in the diagram above. Hepatic phosphorylase kinase is a multi-subunit (hexadecameric) enzyme composed of four copies of each of the unique subunits: α, β, γ, and δ. The α and β subunits (encoded by the PHKA2 and PHKB genes, respectively) are the regulatory subunits that are phosphorylated. The γ subunit (encoded by the PHKG2 gene) is the catalytic subunit and the δ subunit is calmodulin (as described below: encoded by the CALM1 gene). Phosphorylation of phosphorylase kinase activates the enzyme which in turn phosphorylates the b form of phosphorylase. Phosphorylation of phosphorylase-b greatly enhances its activity towards glycogen breakdown. The modified enzyme is called phosphorylase-a. The net result is an extremely large induction of glycogen breakdown in response to glucagon binding to cell surface receptors.
This identical cascade of events occurs in skeletal muscle cells as well. However, in these cells the induction of the cascade is the result of epinephrine binding to receptors on the surface of muscle cells. Epinephrine is released from the adrenal glands in response to neural signals indicating an immediate need for enhanced glucose utilization in muscle, the so called fight or flight response. Muscle cells lack glucagon receptors. The presence of glucagon receptors on muscle cells would be futile anyway since the role of glucagon release is to increase blood glucose concentrations and muscle glycogen stores cannot contribute to blood glucose levels.
Regulation of phosphorylase kinase activity is also affected by two distinct mechanisms involving Ca2+ ions. The ability of Ca2+ ions to regulate phosphorylase kinase is through the function of one of the subunits of this enzyme. One of the subunits of this enzyme is the ubiquitous protein, calmodulin. Calmodulin is a calcium binding protein. Binding induces a conformational change in calmodulin which in turn enhances the catalytic activity of the phosphorylase kinase towards its substrate, phosphorylase-b. This activity is crucial to the enhancement of glycogenolysis in muscle cells where muscle contraction is induced via acetylcholine stimulation at the neuromuscular junction. The effect of acetylcholine release from nerve terminals at a neuromuscular junction is to depolarize the muscle cell leading to increased release of sarcoplasmic reticulum stored Ca2+, thereby activating phosphorylase kinase. Thus, not only does the increased intracellular calcium increase the rate of muscle contraction it increases glycogenolysis which provides the muscle cell with the increased ATP it also needs for contraction.
The second Ca2+ ion-mediated pathway to phosphorylase kinase activation is through activation of α1-adrenergic receptors by epinephrine or norepinephrine.
Pathways involved in the regulation of glycogen phosphorylase by epinephrine activation of α1-adrenergic receptors. See the text for details of the regulatory mechanisms. PLC-β is phospholipase C-β. The substrate for PLC-β is phosphatidylinositol-4,5-bisphosphate (PIP2) and the products are IP3 (inositol-1,4,5-trisphosphate) and DAG (diacylglycerol). GS-GP kinase is glycogen synthase-glycogen phosphorylase kinase, often just referred to as phosphorylase kinase.
Unlike β-adrenergic receptors which are coupled to activation of adenylate cyclase, α1-adrenergic receptors are coupled through G-proteins (Gq) that activate phospholipase-C-β (PLC-β). However, it is important to note that α2-adrenergic receptors couple to a Gi-type G-protein and, as a result of ligand binding, inhibit the activaiton of adenylate cyclase. Activation of PLC-β leads to increased hydrolysis of membrane phosphatidylinositol-4,5-bisphosphate (PIP2), the products of which are inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). DAG binds to and activates protein kinase C (PKC) an enzyme that phosphorylates numerous substrates, one of which is glycogen synthase (see below). IP3 binds to receptors on the surface of the endoplasmic reticulum leading to release of Ca2+ ions. The Ca2+ ions then interact the calmodulin subunits of phosphorylase kinase resulting in its' activation. Additionally, the Ca2+ ions activate PKC in conjunction with DAG.
In order to terminate the activity of the enzymes of the glycogen phosphorylase activation cascade, once the needs of the body are met, the modified enzymes need to be un-modified. In the case of Ca2+ induced activation, the level of Ca2+ ion release from muscle stores will terminate when the incoming nerve impulses cease. The removal of the phosphates on phosphorylase kinase and phosphorylase-a is carried out by phosphoprotein phosphatase-1 (PP-1). In order that the phosphate residues placed on these enzymes by PKA and phosphorylase kinase are not immediately removed, the activity of PP-1 must also be regulated. This is accomplished by the binding of PP-1 to phosphoprotein phosphatase inhibitor (PPI-1). This protein also is phosphorylated by PKA and dephosphorylated by PP-1 (see diagram above). The phosphorylation of PPI allows it to bind to PP-1, an activity it is incapable of carrying out when not phosphorylated. When PPI binds PP-1 its phosphorylations are removed by PP-1 but at a much reduced rate than by free PP-1 thus temporarily trapping PP-1 from other substrates. The effects of the activation of this regulatory phosphorylation cascade on the rate of glycogen synthesis is described below.back to the top
For de novo glycogen synthesis to proceed the first glucose residue is attached to a protein known as glycogenin. Glycogenin has the unusual property of catalyzing its own glycosylation, attaching C-1 of a UDP-glucose to a tyrosine residue on the enzyme. The attached glucose then serves as the primer required by glycogen synthase to attach additional glucose molecules via the mechanism described below.
Synthesis of glycogen from glucose is carried out by the enzyme glycogen synthase. This enzyme utilizes UDP-glucose as one substrate and the non-reducing end of glycogen as another. The activation of glucose to be used for glycogen synthesis is carried out by the enzyme UDP-glucose pyrophosphorylase. This enzyme exchanges the phosphate on C-1 of glucose-1-phosphate for UDP. The energy of the phospho-glycosyl bond of UDP-glucose is utilized by glycogen synthase to catalyze the incorporation of glucose into glycogen. UDP is subsequently released from the enzyme. The α-1,6 branches in glucose are produced by amylo-(1,4–1,6)-transglycosylase, also termed the branching enzyme. This enzyme transfers a terminal fragment of 6-7 glucose residues (from a polymer at least 11 glucose residues long) to an internal glucose residue at the C-6 hydroxyl position.
Glycogen synthase is a tetrameric enzyme consisting of 4 identical subunits. The liver and muscle glycogen synthase proteins are derived from different genes and share only 46% amino acid identity. The liver glycogen synthase gene is located on chromosome 12p12.2 and is identified by the gene symbol GYS2. The muscle (cardiac and skeletal) glycogen synthase gene (gene symbol = GYS1) is located on chromosome 19q13.3 and encodes a protein of 737 amino acids.
The activity of glycogen synthase is regulated by phosphorylation of serine residues in the subunit proteins. Phosphorylation of glycogen synthase reduces its activity towards UDP-glucose. When in the non-phosphorylated state, glycogen synthase does not require glucose-6-phosphate as an allosteric activator, when phosphorylated it does. The two forms of glycogen synthase are identified by the same nomenclature as used for glycogen phosphorylase. The unphosphorylated and most active form is synthase-a and the phosphorylated glucose-6-phosphate-dependent form is synthase-b.
Numerous kinases have been shown to phosphorylate and regulate both hepatic and muscle forms of glycogen synthase. Most detailed analyses have been carried out using glycogen synthase in isolated hepatocytes. At least five sites of phosphorylation have been identified in hepatic glycogen synthase that are the targets of at least seven kinases. Regulation of glycogen synthase by phosphorylation occurs via both primary and secondary phosphorylation events. The seven kinases that regulate glycogen synthase activity are PKA, PKC, glycogen synthase kinase-3 (GSK-3), glycogen synthase-glycogen phosphorylase (GS-GP) kinase (commonly just called phosphorylase kinase), calmodulin-dependent protein kinase-II (CaMPK-II), casein kinase I (CK-I), and casein kinase II (CK-II). Primary phosphorylation events are initiated by phosphorylase kinase, PKA, PKC, CaMPK-II, and CK-II. Secondary phosphorylation events are the result of GSK-3 and CK-I. When glucagon binds its' receptor on hepatocytes the resultant rise in activity of PKA leads to increased phosphorylation of glycogen synthase directly by PKA as well as through the PKA-mediated activation of phosphorylase kinase (PhK). In addition, glucagon effects an increase in the activity of casein kinase II (CK-II). Thus, the net effect of glucagon action on hepatocytes is activation of three distinct kinases that phosphorylate and inhibit glycogen synthase.
Since insulin and glucagon are counter-regulatory hormones it should be clear that they will exert opposing effects on the rate and level of glycogen synthase phosphorylation. As described above, when glucagon binds its receptor on hepatocytes there is a resultant rise in cAMP and a concommitant increase in the activity of PKA. PKA has been shown to phosphorylate glycogen synthase on at least four different sites. In addition, PKA activity results in an increase in the activity of phosphorylase kinase which in turn phosphorylates glycogen synthase at one of the same sites as PKA. The action of insulin, at the level of PKA, is to increase the activity of phosphodiesterase which hydrolyzes cAMP to AMP thereby reducing the level of active PKA. Insulin also exerts a negative effect on the activity of GSK-3 such that there is a reduced level of phosphorylation of glycogen synthase by this kinase. To see more on the action of insulin at the level of GSK-3 visit the Insulin Action page.
Pathways involved in the regulation of glycogen synthase. See the text for details of the regulatory mechanisms. PKA is cAMP-dependent protein kinase. PPI-1 is phosphoprotein phosphatase-1 inhibitor. Green arrows denote positive effects on any indicated enzyme. Red T-lines indicate inhibitory actions. Briefly, glycogen synthase a is phosphorylated, and rendered much less active and requires glucose-6-phosphate to have any activity at all. Phosphorylation of glycogen synthase is accomplished by several different enzymes. The most important is synthase-phosphorylase kinase (GS/GP kinase) the same enzyme responsible for phosphorylation (and activation) of glycogen phosphorylase. PKA (itself activated through receptor mediated mechanisms) also phosphorylates glycogen synthase directly. The effects of PKA on PPI-1 are the same as those described above for the regulation of glycogen phosphorylase. The other enzymes shown to directly phosphorylate glycogen synthase are protein kinase C (PKC), calmodulin-dependent protein kinase (CaMK1), glycogen synthase kinase-3 (GSK3). The enzyme PKC is activated by Ca2+ ions and phospholipids, primarily diacylglycerol, DAG. DAG is formed by receptor-mediated hydrolysis of membrane phosphatidylinositol bisphosphate (PIP2).
Hormones and neurotransmitters that result in release of stored intracellular Ca2+ also effect negative regulation of glycogen synthase activity. As described above Ca2+ ions bind to the calmodulin subunit of GS-GP kinase and result in its activation leading to increased phosphorylation of glycogen synthase. Activation of α1-adrenergic receptors in skeletal muscle results in activation of PLC-β leading to increased levels of IP3 and DAG. The action of IP3 results in increased release of stored Ca2+ with the same net effect at the level of glycogen synthase. The released Ca2+ ions, in conjunction with DAG, in turn activate PKC which phosphorylates glycogen synthase in the same domain of the enzyme that is one target for PKA and the site for CaMPK-II and CK-I phosphorylation.
Pathways involved in the regulation of glycogen synthase by epinephrine activation of α1-adrenergic receptors. See the text for details of the regulatory mechanisms. PKC is protein kinase C. PLC-β is phospholipase C-β. The substrate for PLC-β is phosphatidylinositol-4,5-bisphosphate (PIP2) and the products are IP3 and DAG.
When α1-adrenergic receptors are stimulated there is an increase in the activity of PLC-β with a resultant increase in PIP2 hydrolysis. The products of PIP2 hydrolysis are DAG and IP3. As described above for glycogen phoshorylase, DAG and the Ca2+ ions released by IP3 activate PKC which phosphorylates and inactivates glycogen synthase. Additional responses of calcium are the activation of calmodulin-dependent protein kinase (calmodulin is a component of many enzymes that are responsive to Ca2+) which also phosphorytes glycogen synthase.
The effects of these phosphorylations leads to:
1. Decreased affinity of synthase for UDP-glucose.
2. Decreased affinity of synthase for glucose-6-phosphate.
3. Increased affinity of synthase for ATP and Pi.
Reconversion of synthase-b to synthase-a requires dephosphorylation. This is carried out predominately by the serine/threonine phosphatase identified as protein phosphatase-1 (PP-1) the same phosphatase involved in the dephosphorylation of phosphorylase described above. Although another serine/threonine phosphatase, namely protein phosphatase-2A (PP-2A), has been shown to dephosphorylate glycogen synthase in vitro, its role in vivo is significantly less than that of PP-1.
The activity of PP-1 is also affected by insulin. The pancreatic hormone exerts an opposing effect to that of glucagon and epinephrine. This should appear obvious since the role of insulin is to increase the uptake of glucose from the blood.back to the top
Since glycogen molecules can become enormously large, an inability to degrade glycogen can cause cells to become pathologically engorged; it can also lead to the functional loss of glycogen as a source of cell energy and as a blood glucose buffer. Although glycogen storage diseases are quite rare, their effects can be most dramatic. The debilitating effect of many glycogen storage diseases depends on the severity of the mutation causing the deficiency. In addition, although the glycogen storage diseases are attributed to specific enzyme deficiencies, other events can cause the same characteristic symptoms. For example, Type I glycogen storage disease (von Gierke disease) is attributed to lack of glucose-6-phosphatase. However, this enzyme is localized on the cisternal surface of the endoplasmic reticulum (ER); in order to gain access to the phosphatase, glucose-6-phosphate must pass through a specific translocase in the ER membrane (see Figure below). Mutation of either the phosphatase or the translocase makes transfer of liver glycogen to the blood a very limited process. Thus, mutation of either gene leads to symptoms associated with von Gierke disease, which occurs at a rate of about 1 in 200,000 people.
The metabolic consequences of the hepatic glucose-6-phosphate deficiency of von Gierke disease extend well beyond just the obvious hypoglycemia that results from the deficiency in liver being able to deliver free glucose to the blood. The inability to release the phosphate from glucose-6-phopsphate results in diversion into glycolysis and production of pyruvate as well as increased diversion onto the pentose phosphate pathway. The production of excess pyruvate, at levels above of the capacity of the TCA cycle to completely oxidize it, results in its reduction to lactate resulting in lactic acidemia. In addition, some of the pyruvate is transaminated to alanine leading to hyperalaninemia. Some of the pyruvate will be oxidized to acetyl-CoA which can't be fully oxidized in the TCA cycle and so the acetyl-CoA will end up in the cytosol where it will serve as a substrate for triglyceride and cholesterol synthesis resulting in hyperlipidemia. The oxidation of glucose-6-phophate via the pentose phosphate pathway leads to increased production of ribose-5-phosphate which then activates the de novo synthesis of the purine nucleotides. In excess of the need, these purine nucleotides will ultimately be catabolized to uric acid resulting in hyperuricemia and consequent symptoms of gout. The interrelationships of these metabolic pathways is diagrammed in the Figure below.
The glycogen storage diseases are divided into two primary categories: those that result principally from defects in liver glycogen homeostasis and those that represent defects in muscle glycogen homeostasis. The liver glycogen storage diseases result in hepatomegaly and hypoglycemia or cirrhosis, whereas the muscle glycogen storage diseases result in skeletal and cardiac myopathies and/or energy impairment. The most notable muscle glycogen storage disease is Pompe disease (type II GSD) due to it being featured in the recent movie "Extraordinary Measures".
Several glycogenoses are the result of deficiencies in enzymes of glycolysis whose symptoms and signs are similar to those seen in McArdle disease (type V GSD). These include deficiencies in muscle phosphoglycerate kinase and muscle pyruvate kinase as well as deficiencies in fructose 1,6-bisphosphatase, lactate dehydrogenase and phosphoglycerate mutase.back to the top
|Type: Name||Enzyme Affected||Primary Organ||Manifestations|
|GSD0a||liver isozyme of glycogen synthase, called glycogen synthase-2 (GYS2)||liver||hypoglycemia, early death, hyperketonia, low blood lactate and alanine|
|glucose-6-phosphatase||liver||hepatomegaly, kidney failure, thrombocyte dysfunction|
|GSD1b||microsomal glucose-6-phosphate translocase (G6PT1): this protein is a member of the solute carrier protein family and is identified as SLC37A4||liver||like Ia, also neutropenia, bacterial infections|
|GSD1c||microsomal Pi transporter||liver||like Ia|
|lysosomal acid α-glucosidase
also called acid maltase
|skeletal and cardiac muscle||infantile form = death by 2
juvenile form = myopathy
adult form = muscular dystrophy-like
Cori or Forbes
|liver and muscle debranching enzyme||liver, skeletal and cardiac muscle||infant hepatomegaly, myopathy|
|branching enzyme||liver, muscle||hepatosplenomegaly, cirrhosis|
|muscle phosphorylase||skeletal muscle||excercise-induced cramps and pain, myoglobinuria|
|liver phosphorylase||liver||hepatomegaly, mild hypoglycemia, hyperlipidemia and ketosis, improvement with age|
|GSD7: Tarui||muscle PFK-1||muscle, RBC's||like V, also hemolytic anemia|
|α2-subunit of hepatic phosphorylase kinase (PHK)
β-subunit hepatic PHK
γ2-subunit hepatic PHK
α-subunit muscle PHK
|liver, leukocytes, muscle||like VI|
|glucose transporter-2 (GLUT-2)||liver||failure to thrive, hepatomegaly, rickets, proximal renal tubular dysfunction; also associated with a form of permanent neonatal diabetes mellitus|