Effects of novel analogues of D-glucose on glycogen phosphorylase activities in crude extracts of liver and skeletal muscle.

The inhibitory properties of a series of both N-linked and C-linked C1-substituted glucose derivatives towards glycogen phosphorylase (GP) activity from crude extracts of rat liver and muscle have been measured. The most effective inhibitor was N-acetyl-beta-D-glucopyranosylamine, which has Kis of 51 microM (muscle GPa), 30 microM (muscle GPb), 2.7 mM (liver GPa) and 4 mM (liver GPb). All analogues tested inhibit muscle GP more potently than liver GP, highlighting some differences between the two isoenzymes, which are nearly 80% similar. The human liver GP enzyme has been modelled on the basis of the rabbit muscle structure and, together with comparison of structures of muscle GPa and GPb, has provided some insights into possible explanations for the different properties of the two isoenzymes. Maximal activities of GP have also been measured in tissues from diabetic (db/db) and wild-type (db/+) mice. Liver GP from db/db mice exhibits higher activity [132% (a)-67% (b)] than from db/+ controls, although similar activities were observed for muscle GP from both db/db and db/+ animals.