Alkaline phosphatase activity and pH optima.

High pH values have been found necessary for optimum activity of alkaline phosphatase in vitro. The pH values in the living cell structures, on the other hand, are near neutrality: cytoplasm 6.9, nucleus 7.6 (l-3). This definite difference between the pH required for optimum activity and the pH of the living cell indicates that alkaline phosphatase is not optimally active in viva. However, the pH required for optimum activity in vivo may not be the same as that in vitro. The pH optimum for alkaline phosphatase activity has been shown to vary with the substrate concentration (4, 5), the type of substrate (6), and the buffer employed (7, 8). A pH optimum less than 8.0, however, had never been reported for alkaline phosphatase. The possibility of lowering the pH optimum to physiological values by lowering the substrate concentration was investigated. As the concentrations of the substrates were lowered, the pH optima for alkaline phosphatase activity were found to be nearer neutral pH values.