1. Kinetic experiments suggested the possible existence of at least two different NAD+-dependent aldehyde dehydrogenases in rat liver. Distribution studies showed that one enzyme, designated enzyme I, was exclusively localized in the mitochondria and that another enzyme, designated enzyme II, was localized in both the mitochondria and the microsomal fraction. 2. A NADP+-dependent enzyme was also found in the mitochondria and the microsomal fraction and it is suggested that this enzyme is identical with enzyme II. 3. The Km for acetaldehyde was apparently less than 10μm for enzyme I and 0.9–1.7mm for enzyme II. The Km for NAD+was similar for both enzymes (20–30μm). The Km for NADP+was 2–3mm and for acetaldehyde 0.5–0.7mm for the NADP+-dependent activity. 4. The NAD+-dependent enzymes show pH optima between 9 and 10. The highest activity was found in pyrophosphate buffer for both enzymes. In phosphate buffer there was a striking difference in activity between the two enzymes. Compared with the activity in pyrophosphate buffer, the activity of enzyme II was uninfluenced, whereas the activity of enzyme I was very low. 5. The results are compared with those of earlier investigations on the distribution of aldehyde dehydrogenase and with the results from purified enzymes from different sources.

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