Glucose-6-phosphatase and type 1 glycogen storage disease: some critical considerations.

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.