Catalyzes the decarboxylation of oxaloacetate into pyruvate. Seems to play a role in maintaining cellular concentrations of bicarbonate and pyruvate. H(+) + oxaloacetate = CO2 + pyruvate Binds 1 Mg(2+) ion per subunit. Homotetramer; dimer of dimers. Belongs to the isocitrate lyase family. Oxaloacetate decarboxylase subfamily.