Aldehyde Dehydrogenases in Rat Brain. Subcellular Distribution and Properties

Abstract

Kinetic studies suggested the presence of several forms of NAD-dependent aldehyde dehydrogenase (ALDH) in rat brain. A subcellular distribution study showed that low- and high-K_m activities with acetaldehyde as well as the substrate-specific enzyme succinate semialdehyde dehydrogenase were located mainly in the mitochondrial compartment. The low-K_m activity was also present in the cytosol (K_m activity in the homogenate was only 10–15% of the total activity with acetaldehyde as the substrate. Two K_m values were obtained with both acetaldehyde (0.2 and 2000 μm) and 3,4-dihydroxyphenylacetaldehyde (DOPAL) (0.3 and 31 μm), and one K_m value with succinate semialdehyde (5 μm). The main part of the aldehyde dehydrogenase activities with acetaldehyde, DOPAL, and succinate semialdehyde, but only little activity of the marker enzyme for the outer membrane (monoamine oxidase, MAO), was released from a purified mitochondrial fraction subjected to sonication. Only small amounts of the ALDH activities were released from mitochondria subjected to swelling in a hypotonic buffer, whereas the main part of the marker enzyme for the intermembrane space (adenylate kinase) was released. These results indicate that the ALDH activities with acetaldehyde, DOPAL and succinate semialdehyde are located in the matrix compartment. The low-K_m activity with acetaldehyde and DOPAL, but not the high-K_m activities and succinate semialdehyde dehydrogenase, was markedly stimulated by Mg²⁺ and Ca²⁺ in phosphate buffer. The low- and high-K_m activities with acetaldehyde showed different pH optima in pyrophosphate buffer.