The effect of magnesium, ATP, Pi, and sodium on the inhibition of the (Na+ + K+)-activated enzyme system by g-strophanthin

Add time:08/18/2019    Source:sciencedirect.com

The influence of magnesium, sodium, ATP, and Pi on the reaction of the (Na+ + K+)-activated enzyme system with g-strophanthin has been investigated.Magnesium is required for the reaction, but with magnesium alone the rate of reaction with g-strophanthin is very low.With magnesium, ATP has an effect both on the rate and on the steady-state level of inhibition. With ATP in concentrations which are low relative to the concentration of magnesium, it increases the rate of inhibition. With a non-limiting concentration of magnesium, the concentration of ATP for half-maximum inhibition by 1 μM g-strophanthin is about 1 μM. When the ATP concentration is increased above the magnesium concentration, i.e. with decreasing concentrations of uncomplexed magnesium (Mg2+), both the rate and the steady-state level of inhibition is decreased.With magnesium, Pi increases the rate of inhibition just as ATP, but the concentration of Pi to obtain a certain inhibition by 1 μM g-strophanthin is higher than the concentration of ATP. The maximum rate of inhibition obtained with magnesium and Pi is higher than with magnesium and ATP. Pi in concentrations higher than the concentration of magnesium (3 mM with 2.5 mM magnesium and 2 mM EDTA) did not decrease the rate and the steady-state level of inhibition as did ATP.With ATP, sodium decreases the requirement for magnesium for the inhibition. With 1 μM g-strophanthin the concentration of Mg2+ for 50 % inhibition and the MgATP/ATPi × K ratio is decreased by a factor of 103–104 by an increase in the sodium concentration from 0 to 10 mM (Mg2+ decreased from 4 μM to 0.7 nM). Sodium increases the rate of inhibition and with sub-optimal concentrations of magnesium it also increases the steady-state level of inhibition. The maximal rate of inhibition with magnesium, ATP, and sodium is faster than with magnesium and Pi, and also than with magnesium and ATP; but even under optimal conditions for the reaction the rate of inhibition with 1 μM g-strophanthin is a relatively slow process.Without ATP and with or without Pi, sodium has the opposite effect. It decreases the requirement for Mg2+ for a given inhibition. With 1.5 mM Pi, the concentration of Mg2+ necessary for 50 % inhibition by 1 μM g-strophanthin is increased from 5 μM to 3 mM when the sodium concentration is increased from 0 to 50 mM.The concentration of g-strophanthin to give 50 % inhibition under steady-state conditions with 3 mM ATP and 100 mM sodium decreases when the Mg2+ concentration is increased and goes towards a minimum value of 10 nM. With non-limiting concentrations of magnesium and Pi for inhibition, the value was 11 nM and with magnesium, sodium and Pi 440 nM. The results suggest that with ATP, sodium increases the affinity for Mg2+ while with Pi, sodium decreases the affinity for g-strophanthin.

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