ABSTRACT
The activity of alkaline phosphatase (AP) shows a change in optimum pH in the opposite direction to the applied change in storage pH. Typically, a change in storage pH from 9.8 to 8.5 results in a (reversible) change of the pH-optimum from 10.0 to 10.8. Protein fluorescence analysis shows that this response is probably due to conformational changes induced by the different storage conditions. As storage pH increases, a more 'open' or less 'compact' conformation is attained. Analysis of the diprotic model (a model which describes possible pH-responses of enzymes) indicates, that, as the AP conformation is getting more 'open' an increase in the dissociation of activity-regulating protons of AP occurs. This leads to a decrease in pH-optimum, precisely as found in the experiment. The prerequisite for such a response, however, is that the conformational adaptation to environmental assay pH is slow (hysteretic) when compared with assay time (400 s). The relaxation time of this adaptation was found to be in the order of 2 h.
