A kinetic re-interpretation of the regulation of rabbit skeletal-muscle phosphorylase kinase activity by Ca2+ and phosphorylation

The regulation of phosphorylase kinase has been proposed to occur physiologically under conditions of zero-order ultrasensitivity [Meinke & Edstrom (1991) J. Biol. Chem. 266, 2259-2266]. This is also one of the conditions that recent theoretical approaches have indicated to be essential in order for an interconvertible enzyme cascade to generate a sensitive response to an effector [Cardenas & Cornish-Bowden (1989) Biochem. J. 257, 339-345]. In contrast, all published kinetic data to date have strongly suggested that activation of phosphorylase kinase by Ca2+ or phosphorylation is attributable solely to a change in affinity for phosphorylase, with no effect on the Vmax. of the reaction. In this study an attempt is made to resolve this conflict. Findings suggest that changes in Vmax. can fully account for the activation of phosphorylase kinase by the physiological mechanisms of cyclic AMP-dependent phosphorylation and increase in Ca2+ concentration.