The effect of substrate and potassium on the inhibitory kinetics of mnci 2 on the enzyme k + ‐ p ‐nitrophenyl phosphatase in rat brain

The effect of Mn 2+ , 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 I 50 of approximately 480 μ M . The inhibition was pH dependent, but not temperature dependent. On measurement of the inhibition with varying concentrations of PNPP (1–5 m M ), the I 50 value remained constant. However, when the inhibition was measured with K + (5–20 m M ), the lso value increased from 130 μ M to 490 μ M , suggesting that K + antagonized the effect of Mn 2+ . In kinetic studies, Mn 2+ inhibited the enzyme in a non‐competitive manner with respect to PNPP. The K m remained constant (2.9), but the F max was decreased from 5.0 to 1.6. However, with respect to K + , the inhibition was competitive, as the concentration for half maximal activation (K 0.5 ) increased from 1.3 to 8.9 mmol I −1 with 1 m M of MnCl 2 , suggesting that the apparent affinity of K + for the enzyme was decreased. The apparent V max was not affected. The degree of cooperativity ( n ) measured as the slope of the Hill plot remained unaltered (1.9 ± 0.2) over the entire concentration range of MnCl 2 tested.