ph 6 the graphically determined inhibition constants are given in table i the positive charge in the p isomer is essential for the inactivation of thrombin since its substitution by methyl resulted in a loss of biological activity table i______________________________________inhibition constants of nitrophenylesters i and ii determined fromdixon plotsenzyme i p isomer ii m isomer ______________________________________ k sub i m x 10 sup 5 u03b2 trypsin 7 0 0 9 u03b1 chymotrypsin 3 6 1 3thrombin bovine 12 sup a 41 5thrombin human 5 3plasmin human 50 4 2plasma kallikrein human 4 7 1 9______________________________________ sup a determined from presteady state kinetics the results obtained in accordance with the present invention offer a remarkable example of selective inactivation of enzymes the two reagents examined the p and m isomer are substrate analogs for trypsin like enzymes and in fact form a complex with trypsin thrombin plasmin and plasma kallikrein with k 1 values in the 10 4 to 10 5 m range however in only one case i e the complex resulting from the p isomer and thrombin is the geometry favorable for a transfer of the sulfonyl group to the enzyme resulting in inactivation it is clear that in spite of the structural homology prevailing among trypsin homologs particularly in the region of the active center that small variations exist which permit a qualitative difference in reactivity such a result is important from the point of view of chemotherapy since although quantitative differences may be found.