Cooperative Behavior in the Thiol Oxidation

Glycogen phosphorylase a and b are irreversibly inactivated by oxidation with the disulfide cystamine. The mechanism is complex and involves oxidation of at least two classes of sulfhydryl groups. The oxidation of one or more of the first class of 4 f 1 sulfhydryl groups is reversible, but the equilibrium constant for the oxidation is so unfavorable (1 X that the micromolar concentrations of cysteamine released stoichiometrically with enzyme oxidation are sufficient to prevent complete oxidation even in the presence of 100 mM cystamine. The rapid phase of inactivation of phosphorylase b, which is first order in cystamine (k = 2.9 f 0.3 M” min-’), is followed by the oxidation of 5 f 1 groups in an irreversible process that is second order in cystamine concentration (k = 3.9 f M-’ min-l). Similar behavior is observed for phosphorylase a, although the behavior is complicated by association/dissociation equilibrium. The second-order dependence of the rate of irreversible inactivation on cystamine concentration is interpreted in terms of a “cooperative” model in which a rapidly reversible thermodynamically unfavorable equilibrium oxidation of one or more sulfhydryl groups must precede the irreversible oxidation of one or more additional sulfhydryl groups. The thiol/ disulfide oxidation equilibrium constant for the initial reversible reaction is estimated to be at least lo4 less favorable than that for the reversible oxidation of phosphofructokinase.