Steady-state and pre-steady kinetic studies on mitochondrial sheep liver aldehyde dehydrogenase. A comparison with the cytoplasmic enzyme.

Kinetic studies were carried out on mitochondrial aldehyde dehydrogenase (EC 1.2.1.3) isolated from sheep liver. Steady-state studies over a wide range of acetaldehyde concentrations gave a non-linear double-reciprocal plot. The dissociation of NADH from the enzyme was a biphasic process with decay constants 0.6s-1 and 0.09s-1. Pre-steady-state kinetic data with propionaldehyde as substrate could be fitted by using the same burst rate constant (12 +/- 3s-1) over a wide range of propionaldehyde concentrations. The quenching of protein fluorescence on the binding of NAD+ to the enzyme was used to estimate apparent rate constants for binding (2 X 10(4) litre.mol-1.s-1) and dissociation (4s-1). The kinetic properties of the mitochondrial enzyme, compared with those reported for the cytoplasmic aldehyde dehydrogenase from sheep liver, show significant differences, which may be important in the oxidation of aldehydes in vivo.