Purification and Properties of Sheep‐Liver Aldehyde Dehydrogenases

Abstract

Sheep liver cytoplasmic aldehyde dehydrogenase [EC 1.2.1.3] was purified to homogeneity to give a sample with a specific activity of 380 nmol NADH min-1 mg-1. An amino acid analysis of the enzyme gave results similar to those reported for aldehyde dehydrogenases from other sources. The isoelectric point was at pH 5.25 and the enzyme contained no significant amounts of metal ions. On the binding of NADH to the enzyme there was a shift in absorption maximum of NADH to 344 nm and a 5.6-fold enhancement of nucleotide fluorescence. The protein fluorescence (.lambda.excit = 290 nm, .lambda.emission = 340 nm) was quenched on the binding of NAD+ and NADH. The enhancement of nucleotide fluorescence on the binding of NADH was utilized to determine the dissociation constant for the enzyme.cntdot.NADH complex (Kd = 1.2 .+-. 0.2 .mu.M). A Hill plot of the data gave a straight line with a slope of 1.0 .+-. 0.3, indicating the absence of cooperative effects. Ellman''s reagent reacted only slowly with the enzyme, but in the presence of sodium dodecylsulfate complete reaction occurred within a few minutes to an extent corresponding to 36 thiol groups/enzyme. MW were determined for cytoplasmic and mitochondrial aldehyde dehydrogenases and were 212,000 .+-. 8,000 and 205,000, respectively. Each enzyme consisted of 4 subunits with a MW of 53,000 .+-. 2000. Properties of the cytoplasmic and mitochondrial aldehyde dehydrogenases from sheep liver were compared with other mammalian liver aldehyde dehydrogenases.