The mechanism of caffeine-enhanced glucose stimulation of liver glycogen synthase phosphatase activity

Abstract

Our report that glucose within its physiological range stimulates glycogen synthase phosphatase activity, provided an appropriate second effector is present, has been expanded. The nature of the stimulatory process, particularly the roles of glucose, and of caffeine which represents the potential second effectors, has been studied. Glucose and caffeine stimulated synthase phosphatase activity in a synergistic manner. With 0.5 mM caffeine the A0.5 for glucose was 11 mM (from 27 mM), whereas in the presence of 30 mM glucose the A0.5 for caffeine was 0.06 mM (from 0.7 mM). At 10 mM glucose the A0.5 for caffeine was 0.1 mM. Glucose stimulation remained non-cooperative, unaffected by the presence of caffeine, whereas the cooperative stimulation of caffeine was unaffected by glucose. Some slight stimulation of synthase activity was observed with caffeine and with glucose over a wide concentration range. However, they did not act synergistically to influence the measurement of synthase activity. Glucose-6-phosphate, which also stimulates synthase phosphatase activity, acted independently, not synergistically with caffeine. All the methylxanthines were tested as potential second effectors in an effort to discover the essential structural elements of the agent. All dimethylxanthines, 3- and 7-methylxanthine and 1-methyl-3-isobutylxanthine enhanced glucose stimulation but none of them alone was stimulatory. Judged from the half-maximal concentrations, in the presence of 10 mM glucose, caffeine was the most potent second effector by a significant margin. The maximum velocity was also greatest with caffeine, whereas that with other methylxanthines was generally lower, and varied. 1-Methylxanthine with increased concentration was slightly inhibitory even in the presence of 10 mM glucose. Xanthine (0.5 mM), itself, strongly inhibited synthase phosphatase activity, an effect not influenced by glucose. Xanthine did not influence the measurement of synthase or phosphorylase phosphatase activity with or without glucose. In general, conditions of methylxanthine-enhanced, glucose stimulation of synthase phosphatase and phosphorylase phosphatase activities differed markedly, confirming that separate, distinct mechanisms are involved.