Neither salicylate nor ibuprofen was a substrate or inhibitor of the long-chain fatty acid:CoA ligase. In contrast, all three xenobiotic-metabolizing medium-chain fatty acid:CoA ligases (XL-I, XL-II, and XL-III) had activity toward salicylate. The K(m) value for salicylate was similar for all three forms (2 to 3 microM), but XL-II and XL-III had higher activity at Vmax. For ibuprofen, only XL-III catalyzed its activation, and it had a K(m) for ibuprofen of 36 microM. Studies of salicylate inhibition of XL-I, XL-II, and XL-III revealed that it inhibited the benzoate activity of all three forms with K1 values of ca. 2 microM, which is in agreement with the K(m) values obtained with salicylate as substrate. Kinetic analysis revealed that salicylate conjugation by all three forms is characterized by substrate inhibition when salicylate exceeds ca. 20 microM. Substrate inhibition was more extensive with XL-I and XL-III. Previous work on the ligases employed assay concentrations of salicylate in the range of 0.1 to 1.0 mM, which are clearly inhibitory, particularly toward XL-I and XL-III. Thus, activity was not properly measured in previous studies, which accounts for the fact that salicylate conjugation was only found with one form, which is most likely XL-II since it has the highest Vmax activity and shows the least amount of substrate inhibition. Studies with ibuprofen indicated that it inhibited XL-I, XL-II, and XL-III, with KI values being in the range of 75-125 microM. The short-chain ligase was inhibited by both salicylate and ibuprofen with KI values of 93 and 84 microM, respectively. It was concluded that pharmacological doses of salicylate, but not ibuprofen, will affect the metabolism of medium-chain fatty acids and carboxylic acid xenobiotics and that the previously described mitochondrial ibuprofen:CoA ligase activity is attributable to XL-III.