title: Kinetics of inhibition and hysteresis of sheep liver cytoplasmic aldehyde dehydrogenase with glyoxylic acid: further evidence relating to the two-site model for aldehyde oxidation.
abstract: Despite the fact that it is an aldehyde, glyoxylic acid is not a substrate for sheep liver cytoplasmic aldehyde dehydrogenase; instead it functions as an inhibitor of both the esterase and dehydrogenase activities. From a consideration of the inhibition patterns it is concluded that glyoxylic acid does not bind in the catalytic propionaldehyde-binding domain, thus confirming the two-site model as proposed previously. Since the corresponding neutral methyl ester is a substrate it is suggested that the catalytic binding domain must contain a negatively charged group which prevents the binding of glyoxylic acid. Steady-state and pre-steady-state kinetic studies indicate that glyoxylic acid inhibits the dehydrogenase activity by converting the enzyme into a dead-end form which cannot undergo the catalytically essential conformational change. Incubation of the enzyme with NAD+ and glyoxylic acid for 10 min before the addition of propionaldehyde gave rise to hysteresis effects which can be explained on the basis of a slow isomerization of the enzyme X NAD+ X glyoxylic acid complex.
mesh_terms: D000444:Aldehyde Dehydrogenase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D000447:Aldehydes / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D003593:Cytoplasm / Q000201:enzymology; D006038:Glyoxylates / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008956:Models, Chemical; D009243:NAD / Q000378:metabolism; D010084:Oxidation-Reduction; D012756:Sheep; D013050:Spectrometry, Fluorescence
chemical_list: D000447:Aldehydes; D006038:Glyoxylates; D009243:NAD; C005556:propionaldehyde; D000444:Aldehyde Dehydrogenase; C031150:glyoxylic acid