Human aldehyde dehydrogenase. Purification and characterization of a third isozyme with low Km for gamma-aminobutyraldehyde.

An enzyme which catalyzes dehydrogenation of gamma-aminobutyraldehyde has been purified to homogeneity from human liver and identified as an isozyme of aldehyde dehydrogenase (EC 1.2.1.3); two other isozymes, previously obtained in a homogeneous form, are known as E1 and E2. Affinity chromatography on NAD-agarose (N6 with 8 carbon spacer) yields homogeneous enzyme which migrates as two components on isoelectric focusing with pI = 5.3 and 5.45. These two components, separated by fast protein liquid chromatography on a Mono-P HR 5/20 column, have similar Km values for gamma-aminobutyraldehyde, acetaldehyde, propionaldehyde, and NAD. The Km value for gamma-aminobutyraldehyde is 8.0-14.0 microM versus 760 microM for E1 and 512 microM for E2. The enzyme's molecular weight, subunit molecular weight, and amino acid composition are similar to those of the E1 and E2 isozymes. The enzyme also interacts with anti-E1 and anti-E2 antibodies; it is relatively insensitive to disulfiram inhibition and is neither activated nor inhibited by magnesium. Its absorption spectrum, where the ratio of 280/260 nm is 1.1 and a weak absorption is seen in the 340 nm range (Racker band), suggests the presence of bound coenzyme. gamma-Aminobutyraldehyde dehydrogenase (with Km value of 15 microM for gamma-aminobutyraldehyde) was previously partially purified from Pseudomonas fluorescens (Jakoby, W.B., and Fredericks, J. (1959) J. Biol. Chem. 234, 2145-2150) but never from a mammalian organism.