Estimation of glucose carbon recycling in children with glycogen storage disease: A 13C NMR study using [U-13C]glucose.

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Abstract
A stable isotope procedure to estimate hepatic glucose carbon recycling and thereby elucidate the mechanism by which glucose is produced in patients lacking glucose 6-phosphatase is described. A total of 10 studies was performed in children with glycogen storage disease type I (GSD-I) and type III (GSD-III) and control subjects. A primed dose-constant nasogastric infusion of D-[U-13C]glucose (greater than 99% 13C-enriched) or an infusion diluted with nonlabeled glucose solution was administered following different periods of fasting. Hepatic glucose carbon recycling was estimated from 13C NMR spectra. The recycling parameters were derived from plasma beta-glucose C-1 splitting pattern, doublet/singlet values of plasma glucose C-1 in comparison to doublet/singlet values of known mixtures of [U-13C]glucose and unlabeled glucose as a function of 13C enrichment of glucose C-1. The fractional glucose C-1 enrichment of plasma glucose samples was analyzed by 1H NMR spectroscopy and confirmed by gas chromatography/mass spectroscopy. The values obtained for GSD-I patients coincided with the standard [U-13C]glucose dilution curve. These results indicate that the plasma glucose of GSD-I subjects comprises only a mixture of 99% 13C-enriched D-[U-13C]glucose and unlabeled glucose but lacks any recycled glucose. Significantly different glucose carbon recycling values were obtained for two GSD-III patients in comparison to GSD-I patients. Our results eliminate a mechanism for glucose production in GSD-I children involving gluconeogenesis. However, glucose release by amylo-1,6-glucosidase activity would result in endogenous glucose production of non-13C-labeled and nonrecycled glucose carbon, as was found in this study. In GSD-III patients gluconeogenesis is suggested as the major route for endogenous glucose synthesis. The contribution of the triose-phosphate pathway in these patients has been determined. The significant difference of the glucose C-1 splitting pattern in plasma GSD-III and control subjects, in comparison to GSD-I plasma, can be used as a parameter for estimating glucose recycling. This approach can be developed as a noninvasive diagnostic test for inborn enzymatic defects involving gluconeogenesis.

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