The effect of substrate and potassium on the inhibitory kinetics of MnCl2 on the enzyme K+-p-nitrophenyl phosphatase in rat brain.

The effect of Mn2+, a divalent metal, on the enzyme K+-p-nitrophenyl phosphatase (K+-PNPPase) was studied in rat brain. The metal was found to be a moderate inhibitor of the enzyme, with an I50 of approximately 480 microM. The inhibition was pH dependent, but not temperature dependent. On measurement of the inhibition with varying concentrations of PNPP (1-5 mM), the I50 value remained constant. However, when the inhibition was measured with K+ (5-20 mM), the I50 value increased from 130 microM to 490 microM, suggesting that K+ antagonized the effect of Mn2+. In kinetic studies, Mn2+ inhibited the enzyme in a non-competitive manner with respect to PNPP. The Km remained constant (2.9), but the Vmax was decreased from 5.0 to 1.6. However, with respect to K+, the inhibition was competitive, as the concentration for half maximal activation (K0.5) increased from 1.3 to 8.9 mmol l-1 with 1 mM of MnCl2, suggesting that the apparent affinity of K+ for the enzyme was decreased. The apparent Vmax was not affected. The degree of cooperactivity (n) measured as the slope of the Hill plot remained unaltered (1.9 +/- 0.2) over the entire concentration range of MnCl2 tested.