Kinetics of the Mechanism of Action of Monoamine Oxidase in the Regulation of Na⁺, K⁺‐ATPase Activity in Rat Brain

Kinetic studies on the action of monoamine oxidase (MAO) in the regulation of Na⁺,K⁺‐ATPase were performed using 3‐methoxy‐4‐hydroxybenzaldehyde (MHB), which is an analogue of 3‐methoxy‐4‐hydroxyphenylacetylaldehyde (product of MAO‐catalysed reaction with dopamine as substrate). It was observed that at 2.6 μM MHB, the activation of Na⁺,K⁺‐ATPase may be the result of the removal of the inhibitory Ca²⁺, thereby increasing the V_max. Double‐reciprocal plots of P_i versus MHB showed that Ca²⁺ counteracted the effect of the aldehyde not by changing the K_m, but be decreasing the V_max of the Na⁺,K⁺‐ATPase stimulation. The removal of 3′,5′‐cyclic AMP‐dependent protein kinase from the microsomes by sodium dodecyl sulphate treatment abolished the activation and/or inhibition of the Na⁺,K⁺‐ATPase by aldehyde; it can therefore be inferred that 3′,5′‐cyclic AMP‐dependent protein kinase is involved in the regulation of Na⁺,K⁺‐ATPase.