Studies on the mechanism and enzymology of metabolic ibuprofen isomerization constituted the focus of this investigation. Comparative in vivo studies revealed that this biotransformation proceeded via a proton abstraction mechanism in all tested species of mammals, which is in agreement with the previous reports. Direct evidence supporting this conclusion stemmed from the in vitro epimerization of ibuprofen-CoA thioester in rat liver homogenates. Chemically synthesized (R)-ibuprofen-CoA thioester was rapidly transformed to its (S)-counterpart by subcellular hepatic preparations. Examination of this epimerase activity in various rat tissue homogenates indicated that this enzyme was highly tissue specific. This biochemical reaction mainly took place in the liver and kidney, whereas low levels of enzyme activity were associated with other tissues. Nevertheless, the liver and kidney homogenates failed to invert (R)-ibuprofen directly even in the presence of all the necessary cofactors. Presumably, the failure to characterize this bioconversion was due to the lack of enzymatic acyl-CoA synthesis in these homogenates. It is noteworthy that the '2-arylpropionyl-CoA epimerase' catalyzed the transformation from either direction and with high turnover rates. The catalytic efficiency of (S)-ibuprofen CoA epimerization appeared to be greater than that of the (R)-counterpart. These in vitro findings suggest that the step of acyl-CoA formation assume a pivotal role in controlling the stereoselectivity and efficiency of the in vivo metabolism. As the responsible acyl-CoA synthetase(s) in different species of animals may exert the reaction with different degrees of enantiomeric preference and efficiency, the resulting stereochemical outcome and metabolic rates of this bioinversion vary accordingly. Consequently, in guinea pigs, this biotransformation proceeds in both directions with nearly equal efficiency, whereas it is virtually unidirectional and slow in humans. Currently, the purification and characterization of this novel '2-arylpropionyl-CoA epimerase' from rat livers constitute the focus of this investigation.