Thrombospondin (TSP) forms sodium dodecyl sulfate (SDS)-stable complexes with thrombin and other proteases, apparently by thiol-disulfide ex change. It also forms complexes with other mole cules of TSP by a similar mechanism. TSP has a conformation-sensitive thiol and intrachain disul fide bond. Thus, TSP has the potential for covalently cross-linking itself and other proteins.In studies with labeled thrombin derivatized with a photoactivatable cross-linking reagent, Danishefsky and Detwiler 1 observed that after photoactivation a labeled complex with a mass greater than 500 kd was recovered in the supernatant solution. When the derivatized thrombin was added to the supernatant solution of platelets activated with the ionophore A23187 and photoactivated, there was incorporation of label into a similar complex, indi cating that the complex included a protein released from activated platelets. The complex formed within 20 sec, the period of photoactivation. It did not form with fibrinogen or α 2 -macroglobulin, but it did form with purified TSP. The significance of this study was the demonstration that thrombin and TSP formed a complex with sufficient affinity (ap parently greater than that with fibrinogen or α 2macroglobulin) to permit cross-linking by the photoactivatable reagent.

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