Investigation on the biochemical isomerization of ibuprofen led us to the successful purification of “2-arylpropionyl-CoA epimerase” from rat liver cytosol and mitochondria. The purified enzymes from both subcellular fractions exhibit similar physical and catalytic properties and are distinctly different from rat liver methylmalonyl-CoA epimerase. Both are monomeric proteins with an apparent molecular mass of 42 kDa and show similar contents of most amino acids. Their UV spectra gave no indication of any bound cofactors, and their enzyme activities were not affected by exposure to EDTA or metal ions (except Cu2+). These results suggest that the cytosolic and mitochondrial epimerases may be structurally related. The purified enzymes catalyze the epimerization of various 2-arylpropionyl-CoAs with some degree of stereochemical differentiation. For 2- (4-isobutylphenyl) propionyl-CoA, the equilibrium constant was estimated to be 1.5 in favor of the R-isomer. Evidence indicated that the proton exchange may be mediated by a 2-base mechanism and that a carboxylic residue in the active site may serve as a general base for proton abstractio
