The inhibition measurements carried out with pea alcohol dehydrogenase (PADH) in the presence of acids, acid amides, and dimethyl sulfoxide show that the binding sites for ethanol and acetaldehyde are not identical. The binding site for ethanol lies in the hydrophobic domain of the PADH molecule as follows from the inhibitory action of acids: with the increasing hydrophobicity of the organic acid the inhibitory power also increases. The inhibitory effect of amides indicates that the binding site for acetaldehyde is most likely localized in the part of the enzyme molecule which is not hydrophobic. The difference in the binding sites for ethanol and acetaldehyde has been postulated before on the basis of the measurements with chloride ions, which inhibit the two substrates to different degrees, and on the basis of differences in pH-optima for ethanol oxidation and acetaldehyde reduction.
