Serum alkaline phosphatases—Effect of pH and buffer on optimum substrate concentration

Human liver, bone and intestinal alkaline phosphatases show differences in behaviour towards increasing substrate concentration which are dependent on pH and buffer type. At pH 10.5, serum containing phosphatases of each type showed increasing activity towards p-nitrophenyl phosphate and α-naphthyl phosphate when the substrate concentration was increased from 0.55 to 55 mM. This increase was more marked with bicarbonate buffer than with methyl aminopropanol. At pH 9.5, the phosphatases showed little variation in activity with increasing substrate concentration irrespective of which substrate or buffer was used. Possible implications of these results have been discussed.