Factors influencing glycogen branching enzyme activity in mouse liver

Abstract

Glycogen is the storage polysaccharide in a spectrum of organisms ranging from yeast to humans. Glycogen has a highly branched structure somewhat resembling a tree or shrub, with the branch points formed by glucose molecules being joined by α 1,6-glycosidic linkages. These linkages are formed by the action of branching enzyme. Branching of glycogen is critical to health, with a lack of branching causing glycogen storage disease type IV, which is fatal in humans by age 2. While glycogen branching enzyme has been studied in clinical medicine, modulation of the enzyme has received comparatively little attention. This dissertation undertook three experiments to address whether or not branching enzyme changes in response to dietary intervention and supplementation in mouse liver. Experiment one focused on the application and refinement of a quantitative assay technique for the measurement of branching enzyme activity. The experiment tested the reliability of the assay for the first time on a large number (n=68) of subjects. Intraclass correlation coefficients revealed that the technique demonstrated high reliability (ICC’s of 0.848, 0.925, 0.99, and 0.998) for the various assay components. A simplification of the assay method was also introduced that increases the economy of the testing protocol. Experiment two examined whether or not a dietary intervention resulted in different degrees of branching in the liver glycogen of two groups of mice. The results did not show a significant difference between mice fed on low-carbohydrate versus high-carbohydrate diets. The third experiment investigated whether diet combined with quercetin supplementation had an effect on the degree of branching of mouse liver glycogen. Of the nine cohorts under study, only two groups were significantly different from one another statistically (p = .049). These two groups differed only in the time of day the animals were euthanized. Glycogen concentration was not different between the two groups. Taken together, the experiments provide data indicating that the assay technique is reliable. New data pertaining to norms in the degree of branching in mouse liver glycogen are reported as well. Future experiments could apply the assay to other tissues such as skeletal muscle in conjunction with exercise.