pyrophosphate and Glycogen-Storage-Disease-Type-I

There now is compelling evidence that hydrolysis of glucose-6-phosphate (Glc-6-P) in intact hepatic endoplasmic reticulum (ER) membrane preparations involves four integral components of the membrane: a Glc-6-P specific transporter (T1), a nonspecific enzyme (E) with its active site facing the lumen, and two other transport systems to mediate rapid and reversible fluxes of the hydrolytic products, inorganic phosphate (Pi) and glucose, i.e. (T2) and (T3), respectively. T2 also mediates transport of inorganic pyrophosphate (PPi) and carbamylphosphate. This concept readily and completely reconciles all known characteristics of the glucose-6-phosphatase (Glc-6-P'ase) system provided appropriate considerations are given to: (1) the quantitative contribution of E residing in membranes lacking a permeability barrier; (2) the kinetic restrictions imposed by T1 and T2; and (3) the influences of the endocrine, developmental and nutritional state on the kinetic relationship between the capacities to transport and hydrolyze. A broader-based understanding and application of these principles in the study of Glc-6-P'ase is needed to ensure accurate diagnosis of type 1 glycogen storage disease (GSD) and minimize unnecessary controversy. The view that the enzyme in native ER membranes is conformationally constrained is not supported by direct measurements of the catalytic turnover number. Finally, we describe the marked deficiencies of rapid filtration assays of Glc-6-P and PPi "uptake" as a direct method of diagnosis of types 1b and 1c GSD.

Topics: Animals; Biological Transport; Carbamyl Phosphate; Diphosphates; Endoplasmic Reticulum; Glucose-6-Phosphatase; Glycogen Storage Disease Type I; Hydrolysis; Microsomes, Liver; Phosphates; Phosphoric Monoester Hydrolases; Rats

Topics: Diphosphates; Glucose-6-Phosphatase; Glucose-6-Phosphate; Glucosephosphates; Glycogen Storage Disease Type I; Humans; Microsomes, Liver; Protein Conformation; Substrate Specificity

Hepatic microsomal glucose-6-phosphatase (Glc-6-P'ase) is a complex multicomponent system containing at least three transport proteins, in addition to the catalytic subunit and a Ca2+ binding regulatory protein. The transport proteins have been designated T1 the glucose-6-phosphate transport protein, T2 a phosphate/pyrophosphate transport protein and T3 a glucose transport protein. Diagnosis of the genetic deficiencies of these transport proteins at present requires a complex kinetic analysis of the Glc-6-P'ase system as a whole. Here we describe the progress to date in our attempts to identify, purify and clone each transport protein with the ultimate aim of isolating specific cDNA probes for each transport protein which can be used for the diagnosis of types 1b, 1c and the putative 1d glycogen storage diseases.

Topics: Animals; Antiporters; Carrier Proteins; Diphosphates; DNA Probes; Glucose-6-Phosphatase; Glycogen Storage Disease Type I; Humans; Membrane Proteins; Microsomes, Liver; Monosaccharide Transport Proteins; Phosphate-Binding Proteins; Phosphotransferases

We demonstrate that glucose-6-phosphatase, pyrophosphate-glucose phosphotransferase, carbamyl phosphate-glucose phosphotransferase and inorganic pyrophosphatase activities are deficient in livers of patients with type I glycogen storage disease. This provides strong genetic evidence that these enzymatic activities reside in a single protein or share a common polypeptide chain.

Topics: Carbamyl Phosphate; Diphosphates; Glucose; Glucose-6-Phosphatase; Glycogen Storage Disease Type I; Humans; Liver; Phosphotransferases; Pyrophosphatases