NADH cannot replace NADPH as reductant. The ¦¤8,9-double bond of (+)-cis-isopulegone and the ¦¤1,2-double bond of (¡À)-piperitone are not substrates. The enzyme from peppermint (Mentha x piperita) converts (+)-pulegone into both (?)-menthone and (+)-isomenthone at a ratio of 70:30 for native enzyme but it does not catalyse the reverse reaction. This enzyme is a member of the medium-chain dehydrogenase/reductase superfamily.