Aluminum inhibits the plasma membrane and sarcoplasmic reticulum Ca2+-ATPases by different mechanisms

Abstract

Aluminum (Al 3+ ) is involved in the pathophysiology of neurodegenerative disorders. The mechanisms that have been proposed to explain the action of Al 3+ toxicity are linked to changes in the cellular calcium homeostasis, placing the transporting calcium pumps as potential targets. The aim of this work was to study the molecular inhibitory mechanism of Al 3+ on Ca 2+ -ATPases such as the plasma membrane and the sarcoplasmic reticulum calcium pumps (PMCA and SERCA, respectively). These P-ATPases transport Ca 2+ actively from the cytoplasm towards the extracellular medium and to the sarcoplasmic reticulum, respectively. For this purpose, we performed enzymatic measurements of the effect of Al 3+ on purified preparations of PMCA and SERCA. Our results show that Al 3+ is an irreversible inhibitor of PMCA and a slowly-reversible inhibitor of SERCA. The binding of Al 3+ is affected by Ca 2+ in SERCA, though not in PMCA. Al 3+ prevents the phosphorylation of SERCA and, conversely, the dephosphorylation of PMCA. The dephosphorylation time courses of the complex formed by PMCA and Al 3+ (EPAl) in the presence of ADP or ATP show that EPAl is composed mainly by the conformer E 2 P. This work shows for the first time a distinct mechanism of Al 3+ inhibition that involves different intermediates of the reaction cycle of these two Ca 2+ -ATPases.