Specificity of alcohol dehydrogenases for sulfoxides

Sulfoxides inhibit horse liver alcohol dehydrogenase (EqADH) by binding to the enzyme-NADH complex. X-ray crystallography suggests that sulfoxides make a cation-pi interaction with the benzene ring of Phe-93 [Cho et al. (1997) Biochemistry 36, 382-389]. Structure-function relationships were examined with seven different sulfoxides binding to five human enzymes (alpha, beta(1), gamma(2), pi, and sigma) and three mutated forms of the horse enzyme. The human gamma(2) enzyme, EqADH, and EqADH with Phe-93 replaced with Trp were selectively and strongly inhibited (K-i less than or equal to 1 mu M) by the 3-butyl or hexyl derivatives of thiolane 1-oxide. The other human enzymes (all with Thr-48) and EqADH with Ser-48 substituted with Thr had relatively lower affinities for the thiolane 1-oxides due to close contact of the methyl group of Thr-48 with a carbon adjacent to the sulfoxide sulfur. EqADH binds the S isomers of 3-butylthiolane 1-oxides, hexyl methyl sulfoxide, and phenyl methyl sulfoxide more tightly than the R isomers, but EqADH with Phe-93 substituted with Ala and the human a enzyme (with Ala-93) prefer (R)-phenyl methyl sulfoxide, apparently because the phenyl ring fits into the space near residue 93. EqADH and the enzymes with Phe-93 replaced with Ala or Trp had similar affinities for sulfoxides, indicating that the contribution of the cation-pi interaction to binding is small or compensated for by altered interactions. Ab initio calculations also suggest that the interaction of a sulfoxide with benzene is relatively weak.