A NOVEL METHOD FOR CHEMICAL MODIFICATION OF FUNCTIONAL-GROUPS OTHER THAN A CARBOXYL GROUP IN PROTEINS BY N-ETHYL-5-PHENYLISOOXAZOLIUM-3'-SULFONATE (WOODWARDS REAGENT-K) - INHIBITION OF ADP-INDUCED PLATELET RESPONSES INVOLVES COVALENT MODIFICATION OF AGGREGIN, AN ADP RECEPTOR

The chemical reaction of N-ethyl-5-phenylisooxazolium-3'-sulfonate (Wo odward's Reagent-K, WR-K) with a carboxyl group yields an enol ester t hat cannot be reduced by sodium borohydride in an aqueous solution, wh ile other nucleophiles such as sulfhydryl, hydroxyl, amino, and imidaz ole groups, react with WR-K to yield unsaturated ketones that are capa ble of being reduced by sodium borohydride in an aqueous medium. Aggre gin, a 100-kDa protein on the surface of human blood platelets has bee n identified as an ADP receptor. Autoradiography of the gels obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the sa mples of solubilized human blood platelets modified by WR-K and then r educed by tritiated sodium borohydride (NaB[H-3](4)) showed the presen ce of a prominent band corresponding to a 100-kDa radio-labeled protei n. Labeling of platelets by WR-K and NaB[H-3](4) was inhibited by ADP, ATP, and thiol group modifying reagents. WR-K blocked completely labe ling of platelets by [beta-P-32] romo-2,3-dioxo-butylthio)adenosine-5' -diphosphate, an ADP-affinity analog that selectively and covalently l abels aggregin (Puri, R. N., Kumar, A., Chen, H., Colman, R. F., and C olman, R. W. (1995) J. Biol. Chem. 256, 24482-24488). WR-K also inhibi ted ADP-induced platelet shape change, aggregation, and mobilization o f intracellular Ca2+ and blocked ADP-induced inhibition of stimulated adenylate cyclase activity. The results show conclusively that WR-K in hibited ADP-induced platelet responses by preventing binding of ADP to aggregin and suggest that ADP binding domain of aggregin contains an essential thiol group. The method of labeling proteins by WR-K and NaB [H-3](4), hitherto not used to distinguish among functional groups mod ified by WR-K, offers a useful and convenient alternative to previousl y used ultraviolet spectral methods which cannot be used to investigat e the modified proteins in intact cellular systems. (C) 1996 Academic Press, Inc.