sentences:  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.Further research led to the isolation of an active site peptide sequence and identification of the active site lysine, Lys115, that is involved in the formation of the Schiff base intermediate.