SODIUM DODECYL SULFATE-STABLE COMPLEXES BETWEEN SERPINS AND ACTIVE ORINACTIVE PROTEINASES CONTAIN THE REGION COOH-TERMINAL TO THE REACTIVE-SITE LOOP

Recently inhibitors of the serpin family were shown to form complexes with dichloroisocoumarine (DCI)-inactivated proteinases under native c onditions (Enghild, J. J., Valnickova, Z., Thogersen I., and Pizzo, S. V. (1994) J. Biol. Chem. 269, 20159-20166). This study demonstrates t hat serpin-DCI/proteinase complexes resist dissociation when analyzed in reduced SDS-polyacrylamide gel electrophoresis. Previously, SDS-sta ble serpin-proteinase complexes have been observed only between serpin s and catalytically active proteinases. The stability of these complex es is believed to result from an acyl-ester bond between the active si te Ser(195) of the proteinase and the alpha-carbonyl group of the scis sile bond in the reactive site loop. We have further analyzed the stru cture of the SDS-stable serpin-proteinase and serpin-DCI/proteinase co mplexes. The results of these studies demonstrate the presence of the COOH-terminal region of the serpin in both complexes. Since (i) modifi cation of Ser(195) does not prevent formation of SDS-stable complexes and (ii) COOH-terminal peptides are present in both complexes, the pre viously described mechanism does not sufficiently explain the formatio n of SDS-stable complexes.