The interaction of phosphorylase a with D-glucose displays alpha-stereospecificity

Half-maximal inhibition of phosphorylase a required a much lower concentration of alpha-D-glucose (4 mM) than of the beta-anomer (14 mM) and of 1-deoxyglucose (about 25 mM). beta-D-Glucose was almost ineffective at concentrations of 1-2.5 mM, but at 50 mM the two anomers were equipotent. A similar picture emerged when the stimulatory effects of the glucose anomers and of 1-deoxyglucose were investigated on the inactivation of phosphorylase by phosphorylase phosphatase. However, upon addition of either glucose anomer (5-20 mM) to a suspension of isolated hepatocytes, the inactivation of phosphorylase occurred at the same rate. It is shown that, in the latter conditions, the rate of intracellular mutarotation considerably exceeds the rate of glucose transport. This results presumably in a rapid anomeric equilibrium in the liver cells.