The role of protein sulfhydryl groups and protein disulfides of the platelet surface in aggregation processes involving thiol exchange reactions

Blood platelets are central to haemostasis and platelet aggregation is considered to be a direct index of platelet function. Although protein disulfides (PSSP) are structural components of most proteins, current evidence suggests that PSSP work together with protein SH groups (PSH) to activate various platelet functions in dynamic processes involving thiol/disulfide exchange reactions.Based on these assumptions, we performed experiments to demonstrate how PSH and PSSP are involved in platelet aggregation and how modifications of PSH and PSSP concentrations on the platelet surface by N-ethylmaleimide (NEM) (a PSH-blocking reagent) and dithiothreitol (DTT) (a PSSP-reducing reagent), respectively, may condition platelet susceptibility in protein rich plasma and washed platelets and integrin αIIbβ3 conformation. Our data strongly suggest that the PSH blockage and the PSSP reduction of the platelet surface are deeply involved in aggregation processes evoked in protein rich plasma and washed platelets by ADP and collagen; that endogenous thiols (e.g. GSH) may interfere with NEM actions; that NEM and DTT, acting on preexisting PSH and PSSP of active platelets have opposite conformational changes on integrin αIIbβ3 conformation. Although the precise mechanism and the populations of specific PSH and PSSP involved remain unresolved, our data support the notion that PSH and PSSP of the platelet surface are involved in platelet activation by thiol exchange reactions. A plausible molecular mechanism of PSH and PSSP recruitment during thiol exchange reactions is here proposed.

Platelet stimulation by agonists exerts conformational changes of platelet PSH and PSSP by PSH/PSSP thiol exchange reactions and ROS eventually produced during platelet activation may be involved in these processes (A). Platelet pre-exposure to electrophilic (NEM) may inhibit the thiol exchange reactions reducing the agonist effect (B). The exposure of platelets to reducing agents (DTT) induces a partial platelet activation in the absence of agonists (C).Figure optionsDownload as PowerPoint slide