Abstract
1. 1. Alkaline p-nitrophenylphosphate phosphatase (pNPPase) activity of Halobacterium halobium is selectively stabilized and stimulated by Mn 2+ ions. 2. 2. Mn 2+ binding to native pNPPase is characterized by a dissociation constant of 0.35 mM at pH 8.5, 37°C, with a Hill coefficient of 0.988. 3. 3. Mn2'1' behaves as a mixed type nonessential activator, increasing the V max value (β = 6.09, pH 8.5) and decreasing the K m value for pNPP (α = 0.56, pH 8.5). The K i value for inorganic phosphate (a competitive inhibitor) was also decreased in the presence of Mn 2+. 4. 4. Activation of native pNPPase by preincubation with Mn 2+ 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. 5. Zn 2+, Cu 2+ and Ni 2+ were found to inhibit both native and Mn 2+-stimulated pNPPase, whereas Co 2+ and Cd 2+ inhibited the Mn 2+-stimulated pNPPase but had no effect on the native enzyme form.
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