Dynamics of platelet tubulin in response to changes in free sulfhydryl groups.

Pools of polymerized and total tubulin were measured in human platelets as a function of free sulfhydryl groups both in acid-soluble and acid-precipitable cell fractions. Changes in free thiols were produced either by storage of platelets at room temperature or by addition of the potent oxidizing agent diazene dicarboxylic acid (diamide) and were correlated with shifts in the dynamic equilibrium between assembled and disassembled microtubules and platelet aggregation. Diamide at concentrations of 0.5 to 5 mM depleted acid soluble SH groups and reduced protein thiols while causing a progressive decrease in polymerized tubulin. Similar changes, although not as severe, were initiated by storage of platelets at room temperature. Platelet aggregation especially that induced by collagen showed a positive correlation with the pool of polymerized tubulin. Our results indicate that the state of oxidation of sulfhydryl groups especially in the acid-precipitable fraction plays an important role in determining the position of equilibrium between polymerized and depolymerized tubulin.