Alkaline p-nitrophenylphosphate phosphatase activity from Halobacterium halobium. Selective activation by manganese and effect of other divalent cations.

1. Alkaline p-nitrophenylphosphate phosphatase (pNPPase) activity of Halobacterium halobium is selectively stabilized and stimulated by Mn2+ ions. 2. Mn2+ binding to native pNPPase is characterized by a dissociation constant of 0.35 mM at pH 8.5, 37 degrees C, with a Hill coefficient of 0.988. 3. Mn2+ behaves as a mixed type nonessential activator, increasing the Vmax value (beta = 6.09, pH 8.5) and decreasing the Km value for pNPP (alpha = 0.56, pH 8.5). The Ki value for inorganic phosphate (a competitive inhibitor) was also decreased in the presence of Mn2+. 4. Activation of native pNPPase by preincubation with Mn2+ is a slow temperature-dependent process, which can be described by an exponential relationship vs time. However, a weak but immediate activation was also detected. 5. Zn2+, Cu2+ and Ni2+ were found to inhibit both native and Mn(2+)-stimulated pNPPase, whereas Co2+ and Cd2+ inhibited the Mn(2+)-stimulated pNPPase but had no effect on the native enzyme form.