Specificity of esterases. III. Critical concentrations of the substrate.

On the basis of this plot it was found (1) that, with a preparation of hog pancreas as the source of esterase, an apparent deviation from theoretical behavior was the result of the simultaneous action of two enzymes on the substrate. In the present investigation it is found that the curves obtained with horse liver esterase display two rather sharp discontinuities that cannot be ascribed to simultaneous action of two or more enzymes. Evidence is presented that one of these discontinuities is caused by a change in the substrate and the other by a change in the kinetic constants of a single enzyme system at a definite (critical) substrate concentration.