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.