AbstractThe subcellular distribution of aldehyde dehydrogenase activity was determined in human liver biopsies by analytical sucrose density‐gradient centrifugation. There was bimodal distribution of activity corresponding to mitochondral and cytosolic localizations. At pH 9.6 cytosolic aldehyde dehydrogenase had a lower apparentK mappfor NAD (0.03 mmol l−1), than the mitochrondrial enzyme (K mappNAD = 1.1 mmol l−1). Also, the pH optimum for cytosolic aldehyde dehydrogenase activity (pH 7.5) was lower than that for the mitochondrial enzyme activity (pH 9.0), and the cytosolic enzyme activity was more sensitive to inhibition by disulfiram in vitro. Disulfiram (40 μmol l−1) caused a 70% reduction in cytosolic aldehyde dehydrogenase activity, but only a 30% reduction in mitochondrial enzyme activity after 10 min incubation. The liver cytosol may therefore be the major site of acetaldehyde oxidationi