Subcellular Localization and Purification of

A protease inhibitor has been purified from platelets by ammonium sulfate fractionation, QAE Sephadex chromatography, Sephadex G-200 gel filtration, and affinity chromatography on concanavalin A-Sepharose to apparent homogeneity on disc gel electrophoresis. The following properties of the inhibitor suggest its relationship to plasma a1 -antitrypsin: antigenic identity with a antitrypsin, molecular weight of 60,000 estimated from gel filtration, a-globulin mobility on electrophoresis, and adsorption to concanavalin A-Sepharose with elution by a-methyl-D-glucoside. The protein inhibits the proteolytic and amidolytic activities of trypsin, the esterolytic activity of chymotrypsin, and the coagulation activity of factor Xla. Exposure of platelets to aggregating agents such as thrombin, epinephrine, and ADP causes the release of protease-inhibitory activity in parallel with the release of serotonin, suggesting that the inhibitors may be released from the dense granules. Thrombin also causes the release of 37% of a,-antitrypsin antigen from platelets. Subcellular fractionation confirms the localization in platelet granules but also reveals considerable membrane-bound -antitrypsin. Platelets have been studied from two a1-antitrypsin deficient patients: A with 10% and B with 0% plasma a1-antitrypsin. In patient A, normal concentrations of a,-antitrypsin antigen and trypsin-inhibitory activity were found in gel-filtered platelets as well as in the granular and soluble subcellular fractions, further suggesting that a,-antitrypsin is an intrinsic platelet protein. However, unlike normal individuals, platelets from patient A contamed no membrane-bound a1-antitrypsin, indicating that its origin may be from plasma. In contrast, the platelets from patient B, like the plasma, contained no demonstrable a,-antitrypsin antigen in any subcellular fraction. However, the trypsin-inhibitory activity was 50% of normal, corresponding to the occurrence in normal platelets of inhibitory activity distinct from a,-antitrypsin.