"Double Barrel" Glucagon Test

Abstract

STRUCTURALLY, glycogen is a complex branched molecule composed of glucosyl units. 1 The branching points are formed by 1,6-bonds, while all other glucosyl units are attached by 1,4-bonds. Liver phosphorylase, an enzyme activated by glucagon, splits the 1,4-bonds, releasing glucose-1-phosphate. This, in turn, is metabolized to glucose-6-phosphate and then to glucose. The debrancher enzyme (amylo-1,6-glucosidase) splits the 1,6-bonds; if this enzyme is deficient, the degradation of glycogen will cease at the branching points. Theoretically, a 14-hour fast in a patient with debrancher enzyme deficiency, type 3 glycogen storage disease by Cori's classification, 1 would be associated with the degradation of the outer branches of glycogen by liver phosphorylase to the points of 1,6-bonds; glucagon administration to such a patient would not result in a hyperglycemic response. If, however, such a patient were fasted for 14 hours and then fed a regular meal, outer branches might be resynthesized sufficiently so