Studies on alkaline phosphatases. 2. Factors influencing pH optima and Michaelis constant

Abstract

Marked differences, and in some cases unexpected similarities, were found in the pH optima (for the respective concentrations of substrate) required for the activities of the alkaline phosphatases from various tissues of different species of birds and mammals. Linear relationships were shown to exist between the initial concentrations of substrate, expressed logarithmically, and the pH optima for the activities of the phosphatases from various sources. The optimum pH shifted to higher values with increaseing concentration of substrate. The addition of magnesium concentrations (5 m[image] for plasma phosphatase and 0. 01[image] for intestinal phosphatase) for maximum activation caused the respective pH optima to shift to higher values for the plasma phosphatase and to lower values for the intestinal enzyme of fowls when compared with pH optima without the addition of magnesium. By the use of velocities at the optimum pH for each concentration of substrate, values for Km1 from data with dilute substrate, and Km2 from data with high concentrations of substrate, were calculated for the phosphatases of various tissues and plasma of fowls, geese, pigeons, rats, rabbits, pigs and calves. The average values for Km1 and Km2 with [beta]glycerophosphate were 2. 1 m[image] and 22. 5 m[image] for 12 preparations of plasma, and 2. 2 m[image] and 9 m[image] for six preparations of intestinal mucosa of chicks. With phenyl phosphate, the average values for Km1 and Km2 for 4 preparations of intestinal mucosa of chicks were 1 and 3 m[image]. Variations in the values for Km1 and Km2 were quite small. More data are required on the phosphatases of other sources with different substrates to evaluate the similarities and differences in the constants recorded. There was considerable variation in Km derived from velocities measured at a constant pH for all concentrations of substrate.