The reaction of bovine alpha-thrombin with tetranitromethane. Characterization of the modified protein

Previous studies from several laboratories have shown that thrombin is inactivated by tetranitromethane with the formation of nitrotyrosine. The inactivation is characterized by an apparently greater loss of fibrinogen-clotting activity than activity toward synthetic ester substrates, suggesting that the residues modified by tetranitromethane are involved in the interaction of thrombin with fibrinogen. This study was designed 1) to determine the effect of solvent conditions on the rate of modification and the stoichiometry of the reaction of tetranitromethane with bovine alpha-thrombin; 2) to identify the residue(s) modified; and 3) to characterize the modified enzyme with respect to its interaction with peptide nitroanilide substrates and fibrinogen. The inactivation of thrombin by tetranitromethane proceeded more rapidly in 50 mM Tris, pH 8.0, than in 50 mM sodium phosphate, 100 mM NaCl, pH 8.0. Approximately 10% fibrinogen-clotting activity remained at maximal inactivation. A study of the effect of tetranitromethane concentration on the rate of inactivation suggested that the loss of activity was the result of the modification of 1 mol of tyrosine/mol of thrombin. A similar result was obtained from the analysis of the extent of inactivation as a function of the extent of protein modification. Structural analysis of the modified protein showed substantial modification at both Tyr71 and Tyr85. Enzyme kinetic studies were performed with the modified protein and a control thrombin with N2-tosylglycylprolylarginine p-nitroanilide. H-D-phenylalanylpipecolylarginine p-nitronailide, and purified bovine fibrinogen. With all three substrates, a substantial decrease in kcat was observed, whereas there was essentially no change in Km. These results suggest that, contrary to previous suggestions, the modification of Tyr71 and Tyr85 in thrombin does not influence the binding of substrates, but rather influences active site reactivity.