Microsomal long chain fatty acid CoA ligase (EC 22.214.171.124) has been implicated in the formation of CoA thioesters of xenobiotics containing a carboxylic acid moiety. In this study we have demonstrated that the microsomal enzyme from rat liver exhibits biphasic kinetics for the formation of palmitoyl-CoA, i.e. there are high affinity low capacity Kmhigh, 1.6 μM, Vmaxhigh, 12.9 nmol/mg/min) and low affinity high capacity (Kmlow, 506μM, Vmaxlow, 58.3 nmol/mg/min) components. Inhibition of the high affinity isoform was studied using the R and S enantiomers of ibuprofen, fenoprofen, ketoprofen and naproxen. The high affinity component of palmitoyl-CoA formation was competitively inhibited by R-fenoprofen(Ki 15.4μM) while R-ibuprofen exhibited mixed inhibition kinetics. In contrast the R and S enantiomers of ketoprofen and naproxen were non-competitive inhibitors. This diversity of inhibition kinetics observed argues in favour of a binding site in addition to the catalytic site. A competitive interaction with the high affinity form correlated with literature evidence of enantiospecific chiral inversion and "hybrid" triglyceride formation for the R enantiomers of fenoprofen and ibuprofen. Paradoxically, R-ketoprofen which is extensively inverted in rats was a non-competitive inhibitor of palmitoyl-CoA formation by the high affinity isoform suggesting that it may not act as an alternate substrate. The results of this study clearly indicate that formation of R-2-arylpropionate-CoAs is not fully explained by interaction with the high affinity isoform of a microsomal long chain (palmitoyl) CoA ligase and therefore the involvement of other isofonns cannot be discounted.