Kinetic characterization of N-1-Naphthylphthalamic acid binding sites from maize coleoptile homogenates

The association of NPA with particulate material from corn coleoptile homogenates and the dissociation of the complexes formed are characterized. Using a filtration method, a single rate constant for the association at 0° C was determined as 2.7×10(-6) M(-1) min(-1), and for the dissociation of the complex a rate constant of 0.086 min(-1) was obtained with the same method. From these a dissociation constant of the complex of 3.4×10(-8) M can be calculated, which is in good agreement with the dirct determination of this constant in equilibrium measurements. This is taken as an indication for a simple reaction schema describing the reaction of NPA with its particle-bound receptor, and it can be inferred that membrane permeation does not impose a limitation on this reaction under the conditions used in this investigation. This view is strengthened by the absence of any indication for a carrier mechanism in "counterflow" experiments. A few minor deviations from quantitative predictions of the simple binding reaction schema are discussed.