One of the major metabolic defects associated with diabetes is an impaired ability to synthesize glycogen. There are two key enzymes involved in the synthesis pathway: glycogenin is the primer or initiation protein to which the first five glucose residues are attached, and glycogen synthase is the enzyme responsible for elongating the chain. In the normal animal, synthase is activated by insulin to a glucose-6-phosphate independent form. In the diabetic, insulin activation is lost. However, in the diabetic animal, total synthase activity (G-6-P dependent and G-6-P independent) and the amount of enzyme present are both increased compared to those in the normal. Synthase, however, cannot function in the absence of a primer.
The aim of this proposal is to determine what effect the diabetic state has on the primer protein, glycogenin. This protein has been identified as a primer since 1985, but changes in the protein as a result of diabetes have not been investigated. Because of the dependence of synthase on glycogenin for chain initiation, alterations in the amount or activity (glycogenin can gain up to five glucosyl residues by autoglucosylation) of glycogenin could have a profound effect on synthase activity in vivo, and therefore on glycogen synthesis. Preliminary studies suggest that such changes do occur. Using specific antibodies and bioassays for glycogenin and glycogen synthase, changes in each enzyme, as well as changes in their stoichiometric ratios, which may affect their function, will be assessed. Once these data have been correlated, the effects of hormonal regulators on enzyme activity and levels will be investigated to determine which specific effectors may produce the changes seen in diabetes, and which potentially mediate recovery of enzyme function. Future investigations of the defect in glycogen metabolism will include defining where the metabolic regulators are working (protein synthesis or degradation, transcription, translation, post translation) and the mechanism for coordinate regulation of the enzymes involved in glycogen metabolism.
