PT - JOURNAL ARTICLE AU - Ercan-Fang, Nacide AU - Nuttall, Frank Q. TI - The Effect of Caffeine and Caffeine Analogs on Rat Liver Phosphorylase <em>a</em> Activity DP - 1997 Mar 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 1312--1318 VI - 280 IP - 3 4099 - http://jpet.aspetjournals.org/content/280/3/1312.short 4100 - http://jpet.aspetjournals.org/content/280/3/1312.full SO - J Pharmacol Exp Ther1997 Mar 01; 280 AB - Liver phosphorylase a is stimulated by adenosine monophosphate. It is inhibited by adenosine diphosphate, adenosine triphosphate and glucose. Using these effectors as well as other potential in vivo effectors at concentrations approximating those present in hepatocytes, we previously reported that the net effect was nil, i.e., at estimated in vivo concentration, the inhibitors neutralized the stimulatory effect of adenosine monophosphate in a phosphorylase apreparation. In addition, a concentration dependent inhibition by glucose was not present. Therefore, we were interested in determining if addition of caffeine, an inhibitor that synergizes with glucose, would result in a reduction in activity in the presence of the other effectors and restore regulation by physiological concentrations of glucose. The effect of xanthine and xanthine analogs also were studied. Purified liver phosphorylase a was used. Activity was measured in the direction of glycogenolysis at 37°, pH 7.0 and under initial rate conditions. Caffeine (1 mM) was added to individual and various combinations of other effectors. The interactions among the potential in vivo effectors when caffeine was present were complex. However, when caffeine was present glucose again regulated activity. This most likely was due to a synergistically facilitated reduction in binding affinity for AMP by caffeine and glucose. Theophylline and adenosine did not inhibit activity but reduced AMP stimulation and facilitated glucose inhibition. Xanthine and the other xanthine derivatives all strongly inhibited activity and the inhibition was independent of other effectors. The American Society for Pharmacology and Experimental Therapeutics