A comparison of the activator sites of liver and muscle glycogen phosphorylase b.

Characteristics of the activator sites of liver and muscle phosphorylase b were probed by using AMP and AMP analogs in kinetic studies, by quantitative affinity chromatography, and by reaction with an affinity-labeling reagent. Activation of liver phosphorylase b by N6-(6-aminohexyl)AMP in comparison with AMP and other analogs is explained by preferential binding to the activator site. The KM value for glucose-1-P of liver phosphorylase b activated with N6-(6-aminohexyl)AMP is considerably higher than that of muscle phosphorylase b. Affinity chromatography utilizing AMP-Sepharose suggests that the activator site is less well formed in liver phosphorylase than in muscle phosphorylase. Reaction with 8-[m-(m-fluorosulfonylbenzamido)benzylthio]adenine activates liver phosphorylase b and is consistent with the reaction at the activator site. The results suggest that part of the reason that liver phosphorylase b is not activated by AMP and AMP analogs is due to a poor coupling between the activator and active sites. Lack of good activation by AMP also can be explained by binding at the inhibitor site.