Protein disulfide isomerase and sulfhydryl-dependent pathways in platelet activation

The inhibition of blood platelet aggregation and secretion was studied usin g covalent thiol reagents, maleimides, or mercuribenzoates, or using inhibi tors of protein disulfide isomerase (PDI), bacitracin or antibodies to PDI. As expected, both types of inhibitors were effective against stimulation b y normal physiologic stimuli. On the other hand, when stimulation was initi ated with the peptide LSARLAF, that specifically activates the integrin alp ha IIb beta3 (the fibrinogen receptor), the PDI inhibitors were without eff ect. LSARLAF-induced aggregation was, however, inhibited by the sulfhydryl reagents. To further investigate the role of sulfhydryl-containing proteins and alpha IIb beta3, platelets were labeled with membrane-impermeant sulfh ydryl reagents. Nine bands were found labeled on gel electrophoresis. Two o f the labeled bands were identified as alpha IIb and beta3. The conclusions are that while PDI is required for platelet aggregation and secretion, an additional sulfhydryl-dependent step or protein is also required. This latt er reaction occurs at the level of alpha IIb beta3. In distinction to most literature reports, at least a subpopulation of alpha IIb beta3 contains fr ee sulfhydryl groups, consistent with the possibility that it is a substrat e for PDI or part of the sulfhydryl-dependent response.