DIRECT FLUOROMETRIC-DETERMINATION OF A DISSOCIATION-CONSTANT AS LOW AS 10-10M FOR SUBTILISIN BPN'-PROTEIN PROTEINASE-INHIBITOR (STREPTOMYCES SUBTILISIN INHIBITOR) COMPLEX BY A SINGLE PHOTON-COUNTING TECHNIQUE

It was found that an increase in fluorescence intensity at 340 nm is observed on the binding of Streptomyces subtilisin inhibitor (SSI) with subtilisin BPN' in the pH range 6-10.The dissociation constant, K1, of the enzyme-inhibitor complex was determined as a function of pH and temperature by direct fluorometric titration utilizing the single photon counting technique in the protein concentration range of 10-9 M. K1 values as low as 10-10 M could be obtained with reasonable accuracy by this high-sensitivity detection method.From the temperature dependence of K1 it was found that the binding is endothermic, and is entirely entropy-driven" in nature.The effect of pH on K1 suggested the participation of an ionizable group with pKapp+ 8.5 in the binding. © 1978 By The Journal of Biochemistry."