Inhibition of rat peroxisomal palmitoyl-CoA ligase by xenobiotic carboxylic acids

Enantioselective Pharmacokinetics of Ibuprofen and Involved Mechanisms

Although dexibuprofen (S-ibuprofen) was marketed in Austria and Switzerland, the racemate at various formulations is still extensively used worldwide, and there are no indications that the racemate will be replaced by the single enantiomer. Thus, elucidation of the characteristics and involved mechanisms of the chiral pharmacokinetics of racemic ibuprofen is of special importance for the understanding of the pharmacological and toxicological consequences, and for prediction of the clinically potential drug interactions and influence of the pathological states. Stereoselective pharmacokinetics and metabolism are common features for chiral nonsteroidal antiinflammatory drugs (NSAIDs) and especially for 2-arylpropionic acid derivatives characterized with a chiral center adjacent to the carboxyl group. Although the enantioselective pharmacokinetic characteristics of different NSAIDs should be treated case by case, they share similar mechanisms underlying the protein binding, metabolism and chiral inversion. Ibuprofen was the most extensively researched drug in terms of chiral characteristics and mechanisms. Therefore, elucidation of the mechanisms derived from research on ibuprofen may provide better understanding and prediction of other chiral drugs. This article attempts to elucidate the chiral pharmacokinetics and involved mechanisms of ibuprofen in comparison with other NSAIDs based on recent developments. Topics on history of ibuprofen, enantioselective analysis method, absorption, protein binding, conventional metabolism, metabolic chiral inversion, gene polymorphism, and biochemical developments were included. It is worth mentioning that some underlying biochemical mechanisms, especially for the metabolic chiral inversion and ethnic differences still remain to be seen. Further research is required to develop human-resourced researching model and to provide more evidence concerning the site of inversion, species variation, CYP450 gene polymorphisms, and biochemical mechanisms.

Nafenopin-, ciprofibroyl-, and palmitoyl-CoA conjugation in vitro: kinetic and molecular characterization of marmoset liver microsomes and expressed MLCL1

Acyl-CoA conjugation of xenobiotic carboxylic acids is catalyzed by hepatic microsomal long-chain fatty acid CoA ligases (LCL, EC 6.2.1.3). Marmosets (Callithrix jacchus) are considered genetically closer to humans than rodents and are used in pharmacological and toxicological studies. We have demonstrated that marmoset liver microsomes catalyze nafenopin-, ciprofibroyl-, and palmitoyl-CoA conjugation and that only palmitoyl-CoA conjugation is significantly upregulated (1.7-fold, P < 0.02) by a high fat diet. Additionally, the apparent C(50) values for nafenopin-, ciprofibroyl-, and palmitoyl-CoA conjugation of 149.7, 413.4, and 3.4 microM were comparable to those reported for human liver microsomes viz, 213.7, 379.8, and 3.4 microM, respectively. Comparison with human data was enabled by the cloning of a full-length marmoset cDNA (MLCL1) that encoded a 698-amino-acid protein sharing 83% similarity with rat liver acyl-CoA synthetase (ACS1) and 93 and 90% similarity with human liver LCL1 and LCL2, respectively. MLCL1 transiently expressed in COS-7 cells activated nafenopin (C(50) 192.9 microM), ciprofibrate (C(50) 168.7 microM), and palmitic acid (C(50) 4.5 microM) to their respective CoA conjugates. This study also demonstrated that the sigmoidal kinetics observed for nafenopin- and ciprofibroyl-CoA conjugation were not unique to human liver microsomes but were also characteristic of marmoset liver microsomes and recombinant MLCL1. More extensive characterization of the substrate specificity of marmoset LCL isoforms will aid in determining further the suitability of marmosets as a model for human xenobiotic metabolism via acyl-CoA conjugation.

Clinical Pharmacokinetics of Dexketoprofen

Dexketoprofen trometamol is a water-soluble salt of the dextrorotatory enantiomer of the nonsteroidal anti-inflammatory drug (NSAID) ketoprofen. Racemic ketoprofen is used as an analgesic and an anti-inflammatory agent, and is one of the most potent in vitro inhibitors of prostaglandin synthesis. This effect is due to the (S)-(+)-enantiomer (dexketoprofen), while the (R)-(-)-enantiomer is devoid of such activity. The racemic ketoprofen exhibits little stereoselectivity in its pharmacokinetics. Relative bioavailability of oral dexketoprofen (12.5 and 25mg, respectively) is similar to that of oral racemic ketoprofen (25 and 50mg, respectively), as measured in all cases by the area under the concentration-time curve values for (S)-(+)-ketoprofen. Dexketoprofen trometamol, given as a tablet, is rapidly absorbed, with a time to maximum plasma concentration (tmax) of between 0.25 and 0.75 hours, whereas the tmax for the (S)-(+)-enantiomer after the racemic drug, administered as tablets or capsules prepared with the free acid, is between 0.5 and 3 hours. The drug does not accumulate significantly when administered as 25mg of free acid 3 times daily. The profile of absorption is changed when dexketoprofen is ingested with food, reducing both the rate of absorption (tmax) and the maximal plasma concentration. Dexketoprofen is strongly bound to plasma proteins, particularly albumin. The disposition of ketoprofen in synovial fluid does not appear to be stereoselective. Dexketoprofen trometamol is not involved in the accumulation of xenobiotics in fat tissues. It is eliminated following extensive biotransformation to inactive glucuroconjugated metabolites. No (R)-(-)-ketoprofen is found in the urine after administration of dexketoprofen, confirming the absence of bioinversion of the (S)-(+)-enantiomer in humans. Conjugates are excreted in urine, and virtually no drug is eliminated unchanged. The analgesic efficacy of the oral pure (S)-(+)-enantiomer is roughly similar to that observed after double dosages of the racemic compound. At doses above 7mg, dexketoprofen was significantly superior to placebo in patients with moderate to severe pain. A dose-response relationship between 12.5 and 25mg could be seen in the time-effects curves, the superiority of the 25mg dose being more a result of an extended duration of action than of an increase in peak analgesic effect. A plateau in the analgesic activity of dexketoprofen trometamol at the 25mg dose is suggested. The time to onset of pain relief appeared to be shorter in patients treated with dexketoprofen trometamol. The drug was well tolerated.

In vitro covalent binding of nafenopin-CoA to human liver proteins

Endogenous fatty acyl-CoAs play an important role in the acylation of proteins. A number of xenobiotic carboxylic acids are able to mimic fatty acids, forming CoA conjugates and acting as substrates in pathways of lipid metabolism. In this study nafenopin, a substrate for human hepatic fatty acid-CoA ligases, was chosen as a model compound to study xenobiotic acylation of human liver proteins. (3)H-nafenopin (+/- unlabeled palmitate) or (14)C-palmitate (+/- unlabeled nafenopin) were incubated for up to 120 min at 37 degrees C with ATP, CoA, and homogenate protein (1 mg/ml) from four individual human livers. Nafenopin covalently bound to proteins was detectable in all human livers and increased with time. Nafenopin adduct formation was directly proportional to nafenopin-CoA formation (r = 0.985, p < 0.05). Attachment of nafenopin to proteins involved both thioester and amide linkages with 76 and 24% of adducts formed with proteins > 100 and 50-100 kDa, respectively. Protein acylation by palmitate was also demonstrated. Palmitate significantly inhibited nafenopin-CoA formation by 29% but had no effect on nafenopin-CoA-mediated protein acylation. In contrast, nafenopin significantly inhibited protein palmitoylation by palmitoyl-CoA. This is the first study to demonstrate a direct relationship between xenobiotic-CoA formation, acylation of human liver proteins, and inhibition of endogenous palmitoylation. The ability of xenobiotics to acylate tissue proteins may have important biological consequences including perturbation of endogenous regulation of protein localization and function.

Role of hepatic fatty acid:coenzyme A ligases in the metabolism of xenobiotic carboxylic acids

1. Formation of acyl-coenzymes (Co)A occurs as an obligatory step in the metabolism of a variety of endogenous substrates, including fatty acids. The reaction is catalysed by ATP-dependent acid:CoA ligases (EC 6.2.1.1-2.1.3; AMP forming), classified on the basis of their ability to conjugate saturated fatty acids of differing chain lengths, short (C2-C4), medium (C4-C12) and long (C10-C22). The enzymes are located in various cell compartments (cytosol, smooth endoplasmic reticulum, mitochondria and peroxisomes) and exhibit wide tissue distribution, with highest activity associated with liver and adipose tissue. 2. Formation of acyl-CoA is not unique to endogenous substrates, but also occurs as an obligatory step in the metabolism of some xenobiotic carboxylic acids. The mitochondrial medium-chain CoA ligase is principally associated with metabolism via amino acid conjugation and activates substrates such as benzoic and salicylic acids. Although amino acid conjugation was previously considered an a priori route of metabolism for xenobiotic-CoA, it is now recognized that these highly reactive and potentially toxic intermediates function as alternative substrates in pathways of intermediary metabolism, particularly those associated with lipid biosyntheses. 3. In addition to a role in fatty acid metabolism, the hepatic microsomal and peroxisomal long-chain-CoA-ligases have been implicated in the formation of the acyl-CoA thioesters of a variety of hypolipidaemic and peroxisome proliferating agents (e.g. clofibric acid) and of the R(-)-enantiomers of the commonly used 2-arylpropionic acid non-steroidal anti-inflammatory drugs (e.g. ibuprofen). In vitro kinetic studies using rat hepatic microsomes and peroxisomes have alluded to the possibility of xenobiotic-CoA ligase multiplicity. Although cDNA encoding a long-chain ligase have been isolated from rat and human liver, there is currently no molecular evidence of multiple isoforms. The gene has been localized to chromosome 4 and homology searches have revealed a significant similarity with enzymes of the luciferase family. 4. Increasing recognition that formation of a CoA conjugate increases chemical reactivity of xenobiotic carboxylic acids has led to an awareness that the relative activity, substrate specificity and intracellular location of the xenobiotic-CoA ligases may explain differences in toxicity. 5. Continued characterization of the human xenobiotic-CoA ligases in terms of substrate/inhibitor profiles and regulation, will allow a greater understanding of the role of these enzymes in the metabolism of carboxylic acids.

Sequestration of coenzyme A by the industrial surfactant, Toximul MP8. A possible role in the inhibition of fatty-acid beta-oxidation in a surfactant/influenza B virus mouse model for acute hepatic encephalopathy

We have investigated the mechanistic basis of our recent observation that exposing young mice to an industrial surfactant potentiates the inhibition of fatty-acid beta-oxidation that occurs with subsequent virus infection (Murphy et al., Biochim. Biophys. Acta 1315, 208-216, 1996). In our mouse model for acute hepatic encephalopathy (AHE), neonatal mice were painted on their abdomens from birth to postnatal day 12 with nontoxic amounts of the industrial surfactant, Toximul MP8 (Tox), and then infected with a sublethal dose (LD30) of mouse-adapted human Influenza B (Lee) virus (FluB). Mortality in mice treated with Tox + FluB was significantly higher than that in mice treated with FluB alone. In vitro assays of hepatic beta-oxidation of [1-(14)C]palmitic and [1-(14)C]octanoic acids in the presence or absence of exogenous coenzyme A (CoA) indicated that Tox-mediated inhibition of oxidation was masked when CoA was added to the assays. FluB also inhibited beta-oxidation by 20-30%, however this effect was independent of exogenous CoA which suggested that it involved a different mechanism. Tox-mediated potentiation of the inhibitory effect was most obvious (> 80% inhibition) when assays were done without added CoA. Analysis of hepatic CoA and its esters indicated that levels of both free CoA and acetyl-CoA were significantly lower in mice that were painted with Tox for 12 days. Tox-dependent reductions of acetyl-CoA were transient and returned to normal values after cessation of painting, whereas those of CoA persisted. FluB infection alone significantly reduced hepatic acetyl-CoA and the magnitude of this reduction (> 30%) was not affected by pre-exposing the mice to Tox. Relative to control mice, levels of acid insoluble acyl-CoA esters were elevated significantly in FluB and Tox + FluB treated mice. Activation of both [1-(14)C]palmitic and [1-(14)C]octanoic acids was reduced in Tox-exposed mice at experimental day 12, but only when exogenous CoA was not included in the assay media; this effect appeared to persist after cessation of painting. Collectively, these data support the concept that Tox and FluB have independent effects on hepatic CoA metabolism that are associated with abnormalities in fatty-acid beta-oxidation. However, these do not fully explain the synergistic effect of the virus and chemical on beta-oxidation inhibition, which is a candidate co-mechanism for potentiation of mortality in this mouse model of AHE.

Isolation and characterization of rat liver microsomal R-ibuprofenoyl-CoA synthetase

Microsomal long-chain acyl-CoA synthetase (EC 6.1.2.3.) has been suggested to be involved in the stereoselective formation of the CoA thioester of ibuprofen. In this study, we demonstrated that the microsomal enzyme from rat liver responsible for palmitoyl-CoA synthesis also catalyzes the formation of R-ibuprofenoyl-CoA in a Mg(2+)- and ATP-dependent process. Long-chain acyl-CoA synthetase from rat liver microsomes was purified to homogeneity as evidenced by SDS-gel electrophoresis. Simultaneous measurements of palmitoyl-CoA and R-ibuprofenoyl-CoA formation with HPLC in various fractions and purification steps during protein isolation revealed a high correlation between both activities. The purification procedure included solubilization of the microsomes obtained from rat livers with Triton X-100 and subsequent chromatography of the 100,000 x g supernatant on blue-sepharose, hydroxyapatite, and phosphocellulose. The purified enzyme exhibited an apparent molecular weight of 72 kDa as estimated by SDS gel electrophoresis, with specific activities of 71 nmol.min-1.mg-1 protein and 901 nmol.min-1.mg-1 protein for formation of R-ibuprofenoyl-CoA and palmitoyl-CoA, respectively. Palmitoyl-CoA formation catalyzed by the purified enzyme exhibited biphasic kinetics indicative of two isoforms, a high-affinity (KM 0.13 +/- 0.11 microM), low-capacity form and a low-affinity (KM 81 +/- 11.5 microM), high-capacity form. In contrast, measurement of R-ibuprofenoyl-CoA synthesis over a concentration range from 5 to 3000 microM showed the participation of a single CoA ligase with a KM of 184 +/- 19 microM, corresponding to the low-affinity isoform of palmitoyl-CoA synthesis with a marked enantioselectivity towards the R-form of ibuprofen. R-ibuprofenoyl-CoA formation of the enzyme preparation was inhibited by palmitic acid (KI 13.5 +/- 0.5 microM) and S-ibuprofen (KI 405 +/- 10 microM). In summary, these data give strong evidence for the identity of R-ibuprofenoyl-CoA and long-chain acyl-CoA synthetase.

Sex differences in the metabolism of (+)-S-145, a novel thromboxane A2 receptor antagonist in rat

1. After the oral administration of 5 mg/kg S-1452 to rat, the plasma levels of (+)-S-145 were similar between the male and female, but there were sex differences in the profiles of its beta-oxidized and hydroxylated metabolites in plasma. 2. beta-Oxidation of (+)-S-145 determined in vitro was slightly higher in the female than in the male, and agreed with the plasma levels of the beta-oxidized metabolites. 3. 5-Hydroxylation activities of (+)-S-145 and beta-oxidized metabolites by rat liver microsomes were significantly higher in the male than in the female, but marked sex differences were not observed in 6-hydroxylation activities. These results revealed that differences in monooxygenase activities directly account for the sex differences in the plasma level of 5-hydroxylated metabolites, and that the peroxisomal beta-oxidation enzyme system also affected the plasma level of 6-hydroxylated metabolites. 4. Biliary excretion was higher in the male than in the female, and quantitative identification of metabolites in bile indicated that this was based on the prominent excretion of taurine conjugates in the male rat. This conclusion was supported by the fact that taurine conjugation activity was higher in male liver homogenates than in the female.

Abnormalities in hepatic fatty-acid metabolism in a surfactant/influenza B virus mouse model for acute encephalopathy

Abnormalities in fatty-acid metabolism are believed to play a role in nonspecific acute encephalopathy (AE) with hepatomegaly, although the specific nature of these abnormalities and their temporal relationship to the pathology are not well defined. We have examined hepatic fatty-acid beta-oxidation and metabolism in a mouse model for AE in which neonatal mice were exposed dermally to nontoxic doses of the industrial surfactant, Toximul MP8 (Tox), daily from days 1 to 12 after birth, and then infected with a sublethal dose (LD30) of mouse-adapted human influenza B (Lee) virus (FluB). The number of deaths in the group treated with Tox + FluB were significantly higher than those in the group infected with virus alone. Under optimal in vitro assay conditions, beta-oxidation of [1-14C]palmitic acid was approximately 15% higher in liver homogenates from mice painted with Tox for 12 days (P < 0.02); catabolism of [1-14C]octanoic acid to 14C-labelled water-soluble products (14C-WSP) and 14CO2 was unaltered by Tox. Infecting Tox-free mice with FluB inhibited beta-oxidation of both [1-14C]palmitate and [1-14C]octanoate by 20-30% (P < 0.001). On days 18-19, when most Tox + FluB-dependent deaths occurred, the inhibition of oxidation was increased to approximately 50% in mice given the combined treatment. Treatment of the mice with Tox/FluB also altered the pattern of incorporation of fatty acids into complex lipids. Hepatic levels of thiobarbituric acid reactive substance (TBARS), a marker for lipid peroxides, were approximately 15% higher in Tox-painted than in control mice (P < 0.01); FluB alone had no effect. In Tox + FluB-treated animals, TBARS levels were > 2-fold higher than in any other experimental group (P < 0.001). These studies demonstrated that nasally-administered FluB has profound effects on hepatic fatty-acid metabolism, particularly beta-oxidation. Exacerbation of this and related effects by exposing young animals to xenobiotic surfactants could be the basis of surfactant-mediated potentiation of virus-induced mortality.

Preclinical and clinical development of dexketoprofen

Dexketoprofen trometamol is a water-soluble salt of the dextrorotatory enantiomer of the nonsteroidal anti-inflammatory drug (NSAID) ketoprofen. Racemic ketoprofen is used as an analgesic and an anti-inflammatory agent, and is one of the most potent in vitro inhibitors of prostaglandin synthesis. This effect is due to the S(+)-enantiomer (dexketoprofen), while the R(-)-enantiomer is devoid of such activity. The pharmacokinetic profile of ketoprofen and its enantiomers was assessed in several animals species and in human volunteers. In humans, the relative bioavailability of oral dexketoprofen trometamol (12.5 and 25 mg, respectively) is similar to that of oral racemic ketoprofen (25 and 50 mg, respectively), as measured in all cases by the area under the concentration-time curve values for S(+)-ketoprofen. Dexketoprofen trometamol, given as a tablet, is rapidly absorbed, with a time to maximum plasma concentration (tmax) of between 0.25 and 0.75 hours, whereas the tmax for the S-enantiomer after the racemic drug, administered as tablets or capsules prepared with the free acid, is between 0.5 and 3 hours. Peak plasma concentrations of 1.4 and 3.1 mg/L are reached after administration of dexketoprofen trometamol 12.5 and 25 mg, respectively. From 70 to 80% of the administered dose is recovered in the urine during the first 12 hours, mainly as the acyl-glucuronoconjugated parent drug. No R(-)-ketoprofen is found in the urine after administration of dexketoprofen [S(+)-ketoprofen], confirming the absence of bioinversion of the S(+)-enantiomer in humans. in animal studies, the anti-inflammatory potency of dexketoprofen was always equivalent to that demonstrated by twice the dose of ketoprofen. Similarly, animal studies showed a high analgesic potency for dexketoprofen trometamol. The R(-)-enantiomer demonstrated a much lower potency, its analgesic action being apparent only in conditions where the metabolic bioinversion to the S(+)-enantiomer was significant. The gastric ulcerogenic effect of dexketoprofen at various oral doses (1.5 to 6 mg/kg) in the rat do not differ from those of the corresponding double doses (3 to 12 mg/kg) of racemic ketoprofen. Repeated (5-day) oral administration of dexketoprofen as the trometamol salt causes less gastric ulceration than was observed after the acid form of both dexketoprofen and the racemate. In addition, single dose dexketoprofen as the free acid at 10 to 20 mg/kg does not show a significant intestinal ulcerogenic effect in rats, while racemic ketoprofen 20 or 40 mg/kg is clearly ulcerogenic to the small intestine. The analgesic efficacy of oral dexketoprofen trometamol 10 to 20 mg is superior to that of placebo and similar to that of ibuprofen 400 mg in patients with moderate to serve pain after third molar extraction. The time to onset of pain relief appeared to be shorter in patients treated with dexketoprofen trometamol than in those treated with ibuprofen 400 mg. Dexketoprofen trometamol was well tolerated, with a reported incidence of adverse events similar to that of placebo.

Mechanistic studies on the ethyl-esterification of acitretin by human liver preparations in vitro

Studies have been performed with human liver microsome preparations in vitro, to investigate the reaction mechanisms involved in the conversion of acitretin to the corresponding ethyl ester, etretinate. The results indicate that: Three fresh samples of human liver, which had been stored in liquid nitrogen for up to 8 months, all produced traces of etretinate (5.8 +/- 0.8 ng/ml) in the presence of ethanol but not when the acitretin was added in acetone, or when the sample was denatured by preheating. Studies with pooled human liver microsomes, to identify the cellular location of the enzymes and the co-factors involved in this esterification, indicate a primary requirement for both ethanol and CoA + ATP with a secondary potentiation in the presence of an NADPH regenerating system. A possible explanation for these finding is that the microsomal ligase enzymes form an intermediate ester between CoA and acitretin, which is then trans-esterified by the ethanol. The low formation with CoA + ATP may indicate that second stage of this process occurs spontaneously, with the NADPH potentiation suggesting that it could also be mediated enzymically.

Kinetic characteristics of rat liver peroxisomal nafenopin-CoA ligase

In this study we have demonstrated that rat hepatic peroxisomes catalyse the formation of nafenopin-CoA. The process is mediated by apparent high affinity (Km 6.7 microM), low capacity (Vmax 0.31 nmol/mg/min) and low affinity, high capacity isoforms. Palmitic acid (Ki 1.1 microM), R(-) ibuprofen (Ki 7.9 microM), ciprofibrate (Ki 60.2 microM) and clofibric acid (Ki 86.8 microM) competitively inhibited nafenopin-CoA formation catalysed by the apparent high affinity isoform. An antibody raised against the microsomal palmitoyl-CoA ligase inhibited the equivalent peroxisomal enzyme significantly (P < 0.001) but did not inhibit peroxisomal nafenopin-CoA ligase activity. These data suggest that nafenopin-CoA formation is catalysed by a peroxisomal CoA ligase which differs from the peroxisomal long chain fatty acid-CoA ligase in relation to its xenobiotic/antibody inhibitor profile and kinetic characteristics.

Differential induction of rat hepatic microsomal and peroxisomal long-chain and nafenopin-CoA ligases by clofibric acid and di-(2-ethylhexyl)phthalate

1. Activity of rat hepatic microsomal and peroxisomal long-chain (palmitoyl) and nafenopin-CoA ligases were studied following administration of either clofibric acid, di-(2-ethylhexyl)phthalate (DEHP) or phenobarbitone. 2. Clofibric acid significantly induced the peroxisomal palmitoyl and nafenopin-CoA ligases, whilst no induction of the equivalent enzymes was observed in the microsomal fraction. 3. DEHP induced only palmitoyl-CoA formation in peroxisomes, whilst all enzymes were refractory to phenobarbitone treatment. 4. The enzyme-specific patterns of inductions both intra- and inter-organelle suggest that the palmitoyl and nafenopin-CoA ligases are under different regulatory control. 5. Modulation of both the rate and extent of nafenopin- and palmitoyl-CoA formation was both agent and organelle specific. 6. This study highlights the difficulty in delineating the individual roles of both fatty acyl-CoAs and xenobiotic-CoAs in peroxisome proliferation.

The role of coenzyme A in the biotransformation of 2-arylpropionic acids

A number of 2-arylpropionic acid non-steroidal anti-inflammatory drugs ('profens') undergo highly enantioselective inversion from the (R)- to (S)-enantiomer. The mechanism of this inversion reaction involves the initial enantioselective formation of a coenzyme A thioester followed by epimerization and finally hydrolysis to regenerate free acids. Long-chain fatty acyl-CoA synthetase appears to mediate the initial thioester formation and an epimerase of an unknown physiologic function effects the second step. The hydrolases mediating the final step are poorly defined. Available evidence suggests that the liver is quantitatively the most important tissue site of inversion but local tissue inversion may influence the pharmacological and toxicological response of a given organ. Data from isolated rat hepatocytes indicate that other xenobiotics can modulate the formation and hydrolysis of ibuprofenyl-CoA by influencing inversion pathways, non-inversion pathways or both. Interactions between xenobiotics may therefore accentuate inter-individual variability in response to 2-aryl-propionic acids. The formation of 2-arylpropionyl-CoA thioesters in vivo has the potential to disrupt numerous biochemical pathways in addition to enhancing individual exposure to the potent anti-inflammatory (S)-enantiomers.

Xenobiotic acyl-CoA formation: evidence of kinetically distinct hepatic microsomal long-chain fatty acid and nafenopin-CoA ligases

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.

Studies on the effect of fenoprofen on the activation and oxidation of long chain and very long chain fatty acids in hepatocytes and subcellular fractions from rat liver

We studied the effect of fenoprofen on the activation of palmitic acid (C16:0), lignoceric acid (C24:0) and cerotic acid (C26:0) in microsomal and peroxisomal fractions from rat liver. Fenoprofen was found to inhibit the formation of palmitoyl-CoA in both microsomal and peroxisomal fractions whereas the formation of lignoceroyl-CoA and cerotoyl-CoA was not inhibited at all. In freshly isolated rat hepatocytes palmitic acid beta-oxidation was progressively inhibited at increasing concentrations of fenoprofen, most probably due to its inhibitory effect on palmitoyl-CoA synthetase activity. On the other hand, fenoprofen was also found to inhibit the beta-oxidation of lignoceric acid and cerotic acid in rat hepatocytes. It is shown that the acyl-CoA oxidase activity with lignoceroyl-CoA as substrate was inhibited by fenoprofen whereas the palmitoyl-CoA and pristanoyl-CoA oxidase activities were not inhibited by fenoprofen. This finding provides an explanation for the inhibitory effect of fenoprofen on lignocerate and cerotate beta-oxidation in hepatocytes.