Calculation of kinetic constants of an enzymatic reaction in organic solvents requires knowledge of the functional active-site concentration in organic solvents, and this can be significantly different than that in water. An experimental method for active-site titration of serine proteases in organic media has been developed based on the kinetics of inhibition by phenylmethanesulfonyl fluoride (PMSF), a serine-specific inhibitor (or suicide substrate). This kinetic approach is fundamentally different from othertechniques that require complete titration of all accessible enzyme active sites. This active site titration method was applied to subtilisins BPN' and Carlsberg and achymotrypsin and resulted in fractions of active sites that ranged from 8 to 62% (of the fraction active in water) depending on the enzyme, the method of enzyme preparation, and the organic solvent used. The active-site concentration of subtilisin BPN' and Carlsberg increased with increasing hydrophobicity of the solvent and with increasing solvent hydration in tetrahydrofuran. The dependence of the fraction of active sites on the nature of the organic solvent appears to be governed largely by solvent-induced inactivation caused by direct interaction of a hydrophilic solvent with the enzyme. 0 1996