Acetoacetate decarboxylase from "Clostridium acetobutylicum" catalyzes the decarboxylation of acetoacetate to yield acetone and carbon dioxide (Figure 1). The reaction mechanism proceeds via the formation of a Schiff base intermediate, which is covalently attached to lysine 115 in the active site. The first line of support for this mechanism came from a radiolabeling experiment in which researchers labeled the carbonyl group of acetoacetate with 18O and observed that oxygen exchange to water, used as the solvent, is a necessary part of decarboxylation step. These results provided support that the mechanism proceeds through a Schiff base intermediate between the ketoacid and an amino acid residue on the enzyme.