Aconitase is an iron-sulfur protein containing a [Fe4S4] 2+ cluster that catalyzes the stereo-specific isomerization of citrate to isocitrate via cis-aconitate. Aconitase functions in both the tricarboxylic acid (Krebs) and glyoxylate cycles. Unlike the majority of iron-sulfur proteins that function as electron carriers, the [Fe4S4] 2+ cluster of aconitase reacts directly with its substrate. In eukaryotes, there are two forms of the enzyme, a cytosolic (cAcn) and a mitochondrial aconitase (mAcn), which are encoded by two different genes. In bacteria, there are also two forms, aconitase A (AcnA) and B (AcnB). Eukaryotic aconitases contain the [Fe4S4] 2+ cluster which is required for their enzymatic activities. It has been shown that exposure of aconitase to oxidants, particularly superoxide and hydrogen peroxide, renders the enzyme inactive. Loss of aconitase activity in cells or in biological samples treated with pro-oxidants has been interpreted as a measure of oxidative damage. Oxidative damage during aging targets mitochondrial aconitase. It has been demonstrated in vitro that reactivation of aconitase can occur upon removal of oxidants and reinsertion of a ferrous ion into the [Fe4S4] 2+ cluster. Aconitase activity is reported to decline during cardiac ischemia/reperfusion events.