Hermansky-Pudlak syndrome in humans: impaired α granules and PDI secretion in a patient with Hermansky-Pudlak syndrome. Comparing normal human platelets and human Hermansky-Pudlak syndrome platelets, thrombin-induced granule exocytosis was studied in vitro to characterize α granule release, thiol isomerase secretion, and PDI antigen secretion. Platelet activation was performed with indicated amounts of thrombin. (A) P-selectin (α granules) expression by flow cytometry in platelets from a Hermansky-Pudlak syndrome patient and from a normal subject. (B) PDI antigen in lysate of resting normal and Hermansky-Pudlak syndrome platelets. (C) PDI antigen secretion monitored by western blot analysis of the releasate of thrombin-activated normal and Hermansky-Pudlak syndrome platelets. (D) Thiol isomerase activity monitored by the reduction of a di-E-GSSG as a substrate. Normal platelets, closed bars; Hermansky-Pudlak syndrome platelets, open bars. N = 2 experiments; mean ± SD; *P = .02, **P < .01.