Effects of phthalate esters on the respiration of rat liver mitochondria.

The effects of phthalate esters on the oxidation of succinate, glutamate, beta-hydroxybutyrate and NADH by rat liver mitochondria were examined and it was found that di-n-butyl phthalate (DBP) strongly inhibited the succinate oxidation by intact and sonicated rat mitochondria, but did not inhibit the State 4 respiration with NAD-linked substrates such as glutamate and beta-hydroxybutyrate. However, oxygen uptake accelerated by the presence of ADP and substrate (State 3) was inhibited and the rate of oxygen uptake decreased to that without ADP (State 4). It was concluded that phthalate esters were electron and energy transport inhibitors but not uncouplers. Phthalate esters also inhibited NADH oxidation by sonicated mitochondria. The degree of inhibition depended on the carbon number of alkyl groups of phthalate esters, and DBP was the most potent inhibitor of respiration. The activity of purified beef liver glutamate dehydrogenase [EC 1.4.1.3] was slightly inhibited by phthalate esters.     

The pyruvate branch point in fish liver mitochondria: Effects of acylcarnitine oxidation on pyruvate dehydrogenase and pyruvate carboxylase activities

Summary Palmitoyldl-carnitine inhibits¹⁴CO₂ production from 1-[¹⁴C]-pyruvate and from 1-[¹⁴C]-alanine by mitochondria from rainbow trout liver. The inhibitory effect occurs in both respiratory states III and IV. Fixation of H¹⁴CO ₃ ^– into acid-stable products by intact mitochondria requires pyruvate and ATP and is inhibited by sodium arsenite. This inhibitory effect is completely abolished by acetyldl-carnitine. It is proposed that under these conditions, oxidation of palmitoyldl-carnitine results in inhibition of pyruvate dehydrogenase while oxidation of acetyldl-carnitine results in activation of pyruvate carboxylase in intact rainbow trout liver mitochondria.     

The effect of Ca2+ on the oxidation of exogenous NADH by rat liver mitochondria.

The respiratory rate of rat liver mitochondria in the presence of NADH as exogenous substrate is enhanced by the addition of CaCl₂ (> 50 μM) when inorganic phosphate is present in the medium. The Ca-induced oxidation of NADH is inhibited by rotenone but is not affected by uncoupling agents. EDTA, which does not reverse the swelling of mitochondria which occurs in the presence of Ca²⁺ and phosphate, is able to inhibit reversibly the Ca-stimulated NADH oxidation. A stimulation of the rate of oxidation of NADH by Ca²⁺ is also observed in mitochondria partially swollen in a hypotonic medium.     

Inhibition of oxidative phosphorylation by organotin thiocarbamates

A series of triphenyl-, tricyclohexyl- and tribenzyltin compounds have been synthesized and examined as inhibitors of mitochondrial oxidative phosphorylation. All compounds tested inhibit oxidative phosphorylation linked to succinate oxidation by potato tuber mitochondria. All of the organotin compounds inhibit ADP-stimulated O2 uptake linked to succinate oxidation with concentrations for 50% inhibition in the range 2-50 microM. This inhibition is not due to inhibition of electron transport from succinate to O2 per se: none of the organotin compounds at 50 microM substantially inhibit the rate of succinate oxidation in the presence of 2,4-dinitrophenol. Representative organotin compounds at 0.5-50 microM do not act as uncouplers of succinate oxidation. It is concluded that the organotin compounds act as energy transfer inhibitors to inhibit oxidative phosphorylation in potato tuber mitochondria. A similar mode of action of representative organotin compounds was found with rat liver mitochondria. These organotin compounds inhibit a hydrophobic Ca2+-dependent plant protein kinase in the absence but not in the presence of thiols.     

Ketone body and fatty acid metabolism in sheep tissues. 3-Hydroxybutyrate dehydrogenase, a cytoplasmic enzyme in sheep liver and kidney

1. 3-Hydroxybutyrate dehydrogenase (EC 1.1.1.30) activities in sheep kidney cortex, rumen epithelium, skeletal muscle, brain, heart and liver were 177, 41, 38, 33, 27 and 17μmol/h per g of tissue respectively, and in rat liver and kidney cortex the values were 1150 and 170 respectively. 2. In sheep liver and kidney cortex the 3-hydroxybutyrate dehydrogenase was located predominantly in the cytosol fractions. In contrast, the enzyme was found in the mitochondria in rat liver and kidney cortex. 3. Laurate, myristate, palmitate and stearate were not oxidized by sheep liver mitochondria, whereas the l-carnitine esters were oxidized at appreciable rates. The free acids were readily oxidized by rat liver mitochondria. 4. During oxidation of palmitoyl-l-carnitine by sheep liver mitochondria, acetoacetate production accounted for 63% of the oxygen uptake. No 3-hydroxybutyrate was formed, even after 10min anaerobic incubation, except when sheep liver cytosol was added. With rat liver mitochondria, half of the preformed acetoacetate was converted into 3-hydroxybutyrate after anaerobic incubation. 5. Measurement of ketone bodies by using specific enzymic methods (Williamson, Mellanby & Krebs, 1962) showed that blood of normal sheep and cattle has a high [3-hydroxybutyrate]/[acetoacetate] ratio, in contrast with that of non-ruminants (rats and pigeons). This ratio in the blood of lambs was similar to that of non-ruminants. The ratio in sheep blood decreased on starvation and rose again on re-feeding. 6. The physiological implications of the low activity of 3-hydroxybutyrate dehydrogenase in sheep liver and the fact that it is found in the cytoplasm in sheep liver and kidney cortex are discussed.     

Oxidative metabolism of long-chain fatty acids in mitochondria from sheep and rat liver. Evidence that sheep conserve linoleate by limiting its oxidation.

Mitochondria isolated from the livers of sheep and rats were shown to oxidize palmitate, oleate and linoleate in a tightly coupled manner, by monitoring the oxygen consumption associated with the degradation of these acids in the presence of 2mM-L-malate. Rat liver mitochondria oxidized linoleate and oleate at a rate 1.2-1.8 times that of palmitate. Sheep liver mitochondria had a specific activity for the oxidation of palmitate that was 50-80% of that of rats and a specific activity for the oxidation of oleate and linoleate that was 30-40% that of rats. This would indicate that sheep conserved linoleate by limiting its oxidation. Carnitine acyltransferase I (CAT I) actively esterified palmitoyl-CoA and linoleate to carnitine in both rat and sheep liver mitochondria, and in both cases the rate for linoleate was faster than for palmitate. The CAT I reaction in both rat and sheep liver was inhibited by micromolar amounts of malonyl-CoA. With 90 microM-palmitoyl-CoA as substrate, CAT I was inhibited by 50% with 2.5 microM-malonyl-CoA in rats, and in sheep, 50% inhibition was found with all malonyl-CoA concentrations tested (1-5 microM). With 90 microM-linoleate as substrate for CAT I, a much larger difference in response to malonyl-CoA was seen, the rat enzyme being 50% inhibited at 22 microM-malonyl-CoA, whereas sheep liver CAT I was 91% and 98% inhibited at 1 microM- and 5 microM-malonyl-CoA respectively. We propose that malonyl-CoA may act as an important regulator of beta-oxidation in sheep, discriminating against the use of linoleate as an energy-yielding substrate.     

The relationship between palmitoyl-coenzyme A synthetase activity and esterification of sn-glycerol 3-phosphate in rat liver mitochondria

1. The specific activities for palmitoyl-CoA synthetase and for sn-glycerol 3-phosphate esterification, with palmitoyl-CoA generated either by the endogenous synthetase or from palmitoyl-(−)-carnitine, CoA and excess of carnitine palmitoyltransferase, were measured with rat liver mitochondria. 2. The mean specific activity of palmitoyl-CoA synthetase was approximately five- and seven-fold the rates of sn-glycerol 3-phosphate esterification from palmitate and palmitoyl-(−)-carnitine respectively. No significant correlation was found in different rats between the activities of palmitoyl-CoA synthetase and sn-glycerol 3-phosphate esterification from either acyl precursor. However, there was a significant correlation (r=0.83, P<0.001) between the rates of glycerolipid synthesis from palmitate and palmitoyl-(−)-carnitine. 3. The mean molar composition of the glycerolipid synthesized from palmitate was 58% lysophosphatidate, 31% phosphatidate and 11% neutral lipid. With palmitoyl-(−)-carnitine the equivalent values were 70, 23 and 7%, which were significantly different. 4. When palmitoyl-CoA synthetase had been inactivated by 60–70% after preincubation of mitochondria at 37°C, it became rate-limiting in glycerolipid biosynthesis. Additions of 1–5mm-ATP prevented inactivation of palmitoyl-CoA synthetase. 5. Preincubation also inhibited the oxidation of palmitate, palmitoyl-CoA, palmitoyl-(−)-carnitine and malate plus glutamate. These inhibitions could not be prevented by addition of ATP. 6. Diversion of palmitoyl-CoA to form palmitoyl-(−)-carnitine did not inhibit sn-glycerol 3-phosphate esterification. 7. The palmitoyl-CoA pool synthesized by the palmitoyl-CoA synthetase was augmented by adding partially purified synthetase or carnitine palmitoyltransferase and palmitoyl-(−)-carnitine. No stimulation of palmitate incorporation into glycerolipids occurred. 8. At low concentrations of Mg²⁺, palmitate, ATP and CoA the velocity with palmitoyl-CoA synthetase decreased more than that of glycerolipid synthesis from palmitate. 9. It is concluded that in the presence of optimum substrate concentrations the activity of sn-glycerol 3-phosphate acyltransferase and not of palmitoyl-CoA synthetase is rate-limiting in the synthesis of phosphatidate and lysophosphatidate in isolated rat liver mitochondria.     

Substrate stereochemistry of 2-methyl-branched-chain acyl-CoA dehydrogenase: elimination of one hydrogen each from (pro-2S)-methyl and alpha-methine of isobutyryl-CoA

Dehydrogenation of (2S)-[3-13C]isobutyryl-CoA was carried out in vitro using 2-methyl-branched-chain acyl-CoA dehydrogenase purified from rat liver mitochondria. The product was subsequently hydrated by the addition of bovine crotonase. The resulting 3-hydroxyisobutyric acid was predominantly enriched with 13C at the beta-hydroxy position as determined as a methyl ester using electron ionization-gas chromatography-mass spectroscopy. This finding indicates that isobutyryl-CoA is dehydrogenated stereospecifically at the (pro-2S)-methyl and alpha-methine groups to form methacrylyl-CoA, which is later hydrated with the addition of hydrogen on the same side of the molecule from which it was subtracted to produce 3-hydroxyisobutyryl-CoA.     

Metabolism and excretion of carnitine and acylcarnitines in the perfused rat kidney

Rat kidneys were perfused for 30 min with a Krebs-Henseleit bicarbonate buffer with 5 mM glucose. Albumin proved superior to pluronic polyols as oncotic agent with regard to carnitine reabsorption in the perfused kidney. The reabsorption of 30 μM (−)-[methyl-³H]carnitine was approx. 96% during the first 10 min. At 750 μM the reabsorption decreased to 40%. The tubular reabsorptive maximum (T_max) was approx. 170 nmol/min per kidney. The fractional reabsorption and clearance of (+)-carnitine, γ-butyrobetaine, and carnitine esters did not deviate significantly from that of (−)-carnitine. (+)-Carnitine was not metabolized by the perfused kidney. In perfusions with (−)-carnitine or (−)-carnitine plus 10 mM α-ketoisocaproate or α-ketoisovalerate increased amounts of acetylcarnitine, isovalerylcarnitine and isobutyrylcarnitine were found. Propionate (5 mM) inhibited acetylcarnitine formation. Isovalerylcarnitine, isobutyrylcarnitine and propionylcarnitine were actively degraded to free (−)-carnitine. In urine, we found a disproportionally high excretion of carnitine or carnitine esters formed in the kidney, compared to the same derivatives when ultrafiltrated. Leakage of metabolites formed in the kidney into preurine may explain this phenomenon.     

Evidence that the development of hepatic fatty acid oxidation at birth in the rat is concomitant with an increased intramitochondrial CoA concentration

The development of hepatic fatty acid oxidation during the perinatal period in the rat was studied using isolated mitochondria. Ketone body synthesis from substrates entering at different levels of beta-oxidation was 2-3 times lower in mitochondria isolated from term-fetal liver than in 16-h-old newborn or adult liver mitochondria. The low rate of palmitoyl-L-carnitine oxidation in term-fetal mitochondria was linked neither to the low capacity of the respiratory chain nor to the removal of acetyl-CoA in the hydroxymethylglutaryl-CoA synthase pathway. The 2.5-times lower concentration of CoA found in term-fetal liver mitochondria when compared to 16-h-old or adult liver mitochondria might be the factor responsible for the low rate of fatty acid oxidation in term-fetal liver mitochondria.     

Effect of acetaldehyde on fatty acid oxidation and ketogenesis by hepatic mitochondria.

Acetaldehyde inhibited the oxidation of fatty acids by rat liver mitochondria as assayed by oxygen consumption and CO₂ production. ADP-stimulated oxygen uptake was more sensitive to inhibition by acetaldehyde than was uncoupler-stimulated oxygen uptake, suggesting an effect of acetaldehyde on the electron transport-phosphorylation system. This conclusion is supported by the decrease in the respiratory control ratio, associated with fatty acid oxidation. Acetaldehyde depressed ketone body production as well as the content of acetyl CoA during palmitoyl-1-carnitine oxidation. Acetaldehyde was considerably more inhibitory toward fatty acid oxidation than was acetate. Therefore, the inhibition by acetaldehyde is not mediated by acetate, the direct product of acetaldehyde oxidation by the mitochondria. Oxygen uptake was depressed by acetaldehyde to a slightly, but consistently, greater extent in the absence of fluorocitrate, than in its presence. This suggests inhibition of oxygen consumption from β-oxidation to acetyl CoA and that which arises from citric acid cycle activity. The inhibition of fatty acid oxidation is not due to any effect on the activation or translocation of fatty acids into the mitochondria.The depression of the end products of fatty acid oxidation (CO₂, ketones, acetyl CoA) as well as the greater sensitivity of palmitate oxidation compared to acetate oxidation, suggests inhibition by acetaldehyde of β-oxidation, citric acid cycle activity, and the respiratory-phosphorylation chain. Neither the activities of palmitoyl CoA synthetase nor carnitine palmitoyltransferase appear to be rate limiting for fatty acid oxidation.     

Calcium ion-induced uptakes and transformations of substrates in liver mitochondria

Substrate-depleted rat liver mitochondria will reaccumulate malate, succinate, oxoglutarate, beta-hydroxybutyrate and glutamate if provided with an energy source and Ca(2+) (or Ca(2+) and Mn(2+)). The energy requirement for ion uptake by fresh mitochondria causes a transient oxidation of their NADH and presumably this leads to an increased oxaloacetate concentration. A consequence is the promotion of formation of citrate, which tends to remain in the particles, provided the pH is above 7. Analyses made of systems blocked with fluorocitrate show that citrate accumulates when Ca(2+) is added with the following substrates; (a) pyruvate in the presence of ATP or malate, (b) palmitoyl-l(-)-carnitine in presence of malate and (c) oxoglutarate. Lowering the pH, even to 6.8, causes the citrate to emerge. This could be the basis of a cellular control mechanism. The generation of citrate in response to Ca(2+) can explain the stoichiometry of one proton ejected per Ca(2+) ion taken up. The new carboxyl group formed from acetyl-CoA when it condenses with oxaloacetate provides an internal anionic charge and a proton to emerge when Ca(2+) enters.     

High phosphate requirement for oxidative phosphorylation and low affinity for phosphate transport in newborn rat liver mitochondria

Rat liver mitochondria are not fully functional at birth. The relationship between this deficiency and the affinity for phosphate, in oxidative phosphorylation or in phosphate transport, have been studied.The phosphate concentration necessary to observe maximal rate of succinate oxidation in the presence of ADP was higher for newborn than for adult rat liver mitochondria. After preincubation of newborn rat liver mitochondria with ATP, the rate of succinate oxidation in the presence of ADP increased with phosphate concentration similarly for newborn and adult rat liver mitochondria. The maximal rate of phosphate-acetate exchange, which is an indirect measure of the rate of phosphate transport across the mitochondrial membrane, was not significantly different for adult and newborn rat liver mitochondria. On the contrary the apparent affinity for phosphate was about ten-fold lower for newborn than for adult mitochondria.     

High phosphate requirement for oxidative phosphorylation and low affinity for phosphate transport in newborn rat liver mitochondria

Rat liver mitochondria are not fully functional at birth. The relationship between this deficiency and the affinity for phosphate, in oxidative phosphorylation or in phosphate transport, have been studied. The phosphate concentration necessary to observe maximal rate of succinate oxidation in the presence of ADP was higher for newborn than for adult rat liver mitochondria. After preincubation of newborn rat liver mitochondria with ATP, the rate of succinate oxidation in the presence of ADP increased with phosphate concentration similarly for newborn and adult rat liver mitochondria. The maximal rate of phosphate-acetate exchange, which is an indirect measure of the rate of phosphate transport across the mitochondrial membrane, was not significantly different for adult and newborn rat liver mitochondria. On the contrary the apparent affinity for phosphate was about ten-fold lower for newborn than for adult mitochondria.     

Metabolism of N6,O2'-[3H]dibutyryl cyclic adenosine 3',5'-monophosphate and macromolecular interactions of the products perfused rat liver.

Mitochondria from liver, kidney, brain, and skeletal muscle metabolized acetaldehyde. Acetaldehyde oxidation by liver and kidney mitochondria was maximal at low levels of acetaldehyde and was sensitive to rotenone, suggesting the involvement of a NAD⁺-dependent aldehyde dehydrogenase with a high affinity for acetaldehyde. Acetaldehyde oxidation was stimulated 50% by ADP, suggesting that, in state 4, reoxidation of NADH is rate limiting for acetaldehyde oxidation. In state 4, acetaldehyde oxidation was decreased by NAD⁺-dependent substrates, as well as by succinate and ascorbate. The inhibition by the latter two substrates was prevented by ADP, dinitrophenol, valinomycin, and gramicidin, but not by oligomycin. Since these compounds are linked to energy transduction and utilization, the data suggest that the inhibition is mediated via energy-dependent reversed electron transport. In state 3, all of these substrates caused considerably less inhibition of acetaldehyde oxidation, suggesting that the activity of aldehyde dehydrogenase, and not of NADH reoxidation, is probably rate limiting for acetaldehyde oxidation. The ionophores valinomycin and gramicidin stimulated acetaldehyde oxidation to a greater extent than ADP. These ionophores also stimulated acetaldehyde oxidation in the presence of ADP. Stimulation by valinomycin occurred in the presence of monovalent cations transported by this ionophore, e.g., K⁺, Rb⁺, Cs⁺. Stimulation by gramicidin also occurred in the presence of these cations, but did not occur with Na⁺ or Li⁺. Na⁺ prevents the stimulation of acetaldehyde oxidation, which occurs in the presence of gramicidin and K⁺. The stimulation by valinomycin and gramicidin was energy dependent and required the presence of a permeant anion. In the absence of an ionophore, potassium phosphate had no effect on acetaldehyde oxidation. These data suggest that the oxidation of acetaldehyde by rat liver and kidney mitochondria is influenced by the oxidation-reduction state of the mitochondria and by the cationic environment. With brain and muscle mitochondria, the rate of acetaldehyde oxidation increased two- to threefold as the concentration of acetaldehyde was raised from 0.167 to 0.50 mm. Acetaldehyde oxidation in these mitochondria was also sensitive; to rotenone, indicating dependence on NAD⁺. ADP, valinomycin, gramicidin, and succinate, compounds which either increased or decreased the rate of acetaldehyde oxidation by liver and kidney mitochondria, had no effect on acetaldehyde oxidation by muscle or brain mitochondria. In state 4, mitochondria from Becker-transplantable hepatocellular carcinoma HC-252 oxidized acetaldehyde at the same rate as liver mitochondria. However, in the presence of ADP, dinitrophenol, valinomycin and gramicidin, the rate of acetaldehyde oxidation by the tumor mitochondria was two to three times greater than that of liver mitochondria, suggesting the presence of a more active; acetaldehyde-oxidizing system in tumor than in liver mitochondria.     

Organophosphorous plant growth regulator Melaphen: Resistance of plant and animal cells to stress factors

The addition of the organophosphorous plant growth regulator Melaphen (4 × 10^?12 M) to the incubation medium increases the maximum rate of oxidation of NAD-dependent substrates in rat liver and sugar beet root mitochondria. In addition, Melaphen stimulates electron transport during oxidation of succinate by rat liver mitochondria, but has no effect on the rate of this substrate oxidation in sugar beet root mitochondria. In storage organs of plants, the rate of oxidation of NAD-dependent substrates by mitochondria is relatively low. By stimulating the activity of NAD-dependent dehydrogenases, Melaphen stimulates energy metabolism in the cells and manifests adaptogenic activity by accelerating the germination of seeds. Melaphen does not influence the fluorescence of lipid peroxidation (LPO) products in mitochondria non-exposed to stress, but decreases 1.5–2 fold the LPO fluorescence in rat liver mitochondria exposed to cold stress and artificially “aged” sugar beet root mitochondria. Besides, Melaphen increases the rate of electron transport in a terminal site of respiratory chains of plant and animal mitochondria and decreases LPO. The data obtained testify to antistress activity of Melaphen.     

Inhibitory effect of glucose on the maturation of rat liver mitochondria at birth. Phospholipid and oxidative metabolism

(1) The rate of ATP synthesis coupled with succinate oxidation in rat liver mitochondria is low at birth and increases rapidly during the first postnatal hours (Nakazawa, T., Asami, K., Suzuki, H. and Yakawa, O. (1973) J. Biochem. 73, 397-406). A glucose injection given to newborn rats immediately after birth seemed to delay this maturation process. (2) Glucose administration specifically diminished the rate of 32Pi incorporation into phosphatidylcholine both in microsomes and in mitochondria while other phospholipids remained unaffected. (3) In newborn rat liver, 32Pi incorporation into phospholipids can be explained by de novo synthesis of phospholipids in microsomes followed by transfer to mitochondria with two exceptions phosphatidylserine and sphingomyelin. Indeed, after a 20-min incorporation of 32Pi into phospholipids, the specific radioactivity of phosphatidylserine and sphingomyelin was higher in mitochondria than in microsomes. (4) As far as phospholipid synthesis is concerned, no precursor-product relationship could be observed between light and heavy mitochondria.     

Preparation and properties of rainbow trout liver mitochondria

Summary Mitochondria prepared from rainbow trout liver consistently display high respiratory control and ADP/O ratios. These appear to possess a complete Krebs cycle, since pyruvate, palmitoyll-carnitine, citrate, and various Krebs cycle intermediates can all be oxidized. Rapid oxidation of pyruvate and palmitoyll-carnitine requires the pressence of malate. Oxidation of palmitoyll-carnitine appears to inhibit pyruvate oxidation. Malate stimulates -ketoglutarate oxidation while aspartate inhibits glutamate oxidation, indicating the presence of malate--ketoglutarate and glutamate-aspartate carriers. These properties are compared with those of liver mitochondria from other species of fish.     

Peroxisomal fatty acid oxidation in rat and human tissues. Effect of nutritional state, clofibrate treatment and postnatal development in the rat

Oxidation of palmitate 14C-labeled in different positions was assayed in the absence and presence of antimycin and rotenone in homogenates of various rat and human tissues to determine total and peroxisomal oxidation and acetyl group production. Total and antimycin-insensitive palmitate oxidation rates were higher in rat heart, liver and quadriceps muscle than in the corresponding human tissues. The proportion of antimycin-insensitive oxidation of [1-14C]palmitate was 17-35% in tissues of starved rats and in human muscles and fibroblasts, but peroxisomal production of acetyl groups amounted only to 5-11% of that by mitochondria. The mean number of peroxisomal beta-oxidation cycles was 1.5-2.5 per palmitate molecule. The nutritional state markedly influenced the total oxidation rate and the antimycin-insensitive proportion in rat liver. Clofibrate feeding increased total and antimycin-insensitive oxidation rates in liver, heart and kidney, but not in quadriceps muscle. Total oxidation capacity was maximal in rat liver at weaning, and in rat heart at an age of 70 days. Antimycin-insensitive oxidation rates increased in rat liver and heart at postnatal development up to weaning. A marked proportion of lignocerate oxidation was antimycin-sensitive in rat tissues.     

Peroxisomes contribute to the acylcarnitine production when the carnitine shuttle is deficient

Fatty acid β-oxidation may occur in both mitochondria and peroxisomes. While peroxisomes oxidize specific carboxylic acids such as very long-chain fatty acids, branched-chain fatty acids, bile acids, and fatty dicarboxylic acids, mitochondria oxidize long-, medium-, and short-chain fatty acids. Oxidation of long-chain substrates requires the carnitine shuttle for mitochondrial access but medium-chain fatty acid oxidation is generally considered carnitine-independent. Using control and carnitine palmitoyltransferase 2 (CPT2)- and carnitine/acylcarnitine translocase (CACT)-deficient human fibroblasts, we investigated the oxidation of lauric acid (C12:0). Measurement of the acylcarnitine profile in the extracellular medium revealed significantly elevated levels of extracellular C10- and C12-carnitine in CPT2- and CACT-deficient fibroblasts. The accumulation of C12-carnitine indicates that lauric acid also uses the carnitine shuttle to access mitochondria. Moreover, the accumulation of extracellular C10-carnitine in CPT2- and CACT-deficient cells suggests an extramitochondrial pathway for the oxidation of lauric acid. Indeed, in the absence of peroxisomes C10-carnitine is not produced, proving that this intermediate is a product of peroxisomal β-oxidation. In conclusion, when the carnitine shuttle is impaired lauric acid is partly oxidized in peroxisomes. This peroxisomal oxidation could be a compensatory mechanism to metabolize straight medium- and long-chain fatty acids, especially in cases of mitochondrial fatty acid β-oxidation deficiency or overload.