Multiplicity of hepatic microsomal coenzyme A ligases catalyzing acyl-CoA thioester formation is an important factor for consideration in relation to the metabolism of xenobiotic carboxylic acids. In this study the kinetic characteristics of rat hepatic microsomal nafenopin-CoA ligase were studied and compared with those of long-chain fatty acid (palmitoyl) CoA ligase. The high affinity component of palmitoyl-CoA formation was inhibited by nafenopin (Ki 53 microM) and ciprofibrate (Ki 1000 microM). Analagous to palmitoyl-CoA, nafenopin-CoA formation was catalyzed by an apparent high affinity low capacity isoform (Km 6 +/- 2.5 microM, Vmax 0.33 +/- 0.12 nmol/mg per min) which was inhibited competitively by palmitic acid (mean Ki 1.7 microM, n = 5) and R-ibuprofen (mean Ki 10.8 microM, n = 5) whilst ciprofibrate and clofibric acid were ineffective as inhibitors. The intrinsic metabolic clearance of nafenopin to nafenopin-CoA (Vmax/Km 0.057 +/- 0.011 nmol/mg/min/ +/- M) was similar to that reported recently for the formation of ibuprofenyl-CoA by rat liver microsomes. Evidence of both a substantial difference between the Km and Ki for nafenopin and lack of commonality with regard to xenobiotic inhibitors suggests that the high affinity microsomal nafenopin-CoA and long-chain fatty acid-CoA ligases are kinetically distinct. Thus until the current 'long-chain like' xenobiotic-CoA ligases are fully characterised in terms of substrate specificity, inhibitor profile, etc, it will be impossible to rationalize (and possibly predict) the metabolism and hence toxicity of xenobiotic carboxylic acids forming acyl-CoA thioester intermediates.