Calcium ions alter monoamine oxidase A activity

Activation of the flavoprotein monoamine oxidase A (MAO A) by calcium ions has been reported in isolated mitochondria, in cell lines, and in vivo. Enhanced MAO A activity has been associated with a rise in oxidative damage (1,2), and with apoptosis in a neuronal cell line (5). Here, purified human MAO A and membrane-bound MAO A and B were used to investigate the direct regulation of MAO activity by Ca2+ ions.
At fixed substrate concentration the activity of purified MAO A was stimulated (<20%) by Ca2+ over a physiological range, a much smaller effect than observed in cells; Mg2+ and Mn2+ had no effect. However, when chelating agents (EDTA or EGTA) were added to remove divalent ions present in the water, MAO A activity was increased, but addition of 1 mM Ca2+ to the chelating buffer resulted in inhibition of activity requiring 10 min to reach the minimum. When assayed in HEPES buffer (50 mM, pH 7.4) without chelator, Ca2+ caused mixed-type inhibition of kynuramine oxidation with a Ki of 0.3 mM for binding to free enzyme. We conclude that Ca2+ (but not Mg2+ or Mn2+) interacts directly with purified MAO A to change the kinetic parameters resulting in decreased activity. In contrast, with membrane-bound MAO A (but not membrane-bound MAO B), Ca2+ enhances activity by doubling the Vmax without a change in KM. Recently, molecular dynamics simulations suggested that membrane attachment altered the conformational dynamics of MAO A and facilitated the opening of the substrate tunnel. The data here are consistent with a conformational change induced by Ca2+, the effect of which is different when the enzyme is stabilised in the membrane.