Hydrogen cyanide seems to be an uncompetitive inhibitor, combining with Schiff's base compounds formed at the active site. Addition of carbonyl compounds to the enzyme, in the presence of hydrogen cyanide, increases hydrogen cyanide's ability to inhibit acetoacetate decarboxylase, suggesting that carbonyl compounds readily form Schiff's bases at the active site. Hydrogen cyanide is most potent as an inhibitor at pH 6, the optimum pH for the enzyme, suggesting that the rate-limiting step of catalysis is the formation of the Schiff base intermediate.