The oxidation of dicarboxylic acid CoA esters via peroxisomal fatty acyl-CoA oxidase

Evidence supporting a common peroxisomal beta-oxidation pathway for the coenzyme A thioesters of medium-chain-length dicarboxylic acids (DCn-CoA) and monocarboxylic acids (MCn-CoA) has been obtained. Using the mono-CoA esters of dodecanedioic acid (DC12-CoA) and lauroyl-CoA (MC12-CoA) as substrates, parallel inductions of activities and parallel increases in specific activities during purification of peroxisomal fatty acyl-CoA oxidase (EC 1.3.99.3) from rat liver after di(2-ethylhexyl)phthalate treatment were seen. The purified enzyme was used for antiserum production in rabbits; antiserum specificity was verified by immunoblot analysis. Coincident losses of oxidase activities with MC12-CoA and DC12-CoA were found in immunotitration experiments with rat liver homogenates, supporting the hypothesis that peroxisomal fatty acyl-CoA oxidase is solely responsible for the oxidation of medium-chain length dicarboxylic acid substrates. Kinetic studies with purified enzyme using the mono-CoA esters of sebacic (DC10-CoA), suberic (DC8-CoA), and adipic (DC6-CoA) acids along with DC12-CoA revealed substrate inhibition. Although these substrates exhibited similar calculated Vmax values, with decreasing chain length, the combination of increasing Km values and decreasing substrate inhibition constant (Ki) caused the maximum obtainable velocity to decrease. These studies offer an explanation for the previously observed limit of the ability of peroxisomes to chain-shorten dicarboxylates and increased urinary excretion of adipic acid when peroxisomal oxidation of dicarboxylic acids is enhanced.     

Hepatic peroxisomal and mitochondrial fatty acid oxidation in the riboflavin-deficient rat.

The effects of riboflavin deficiency on hepatic peroxisomal and mitochondrial palmitoyl-CoA oxidation were examined in weanling Wistar-strain male rats. The specific activities of peroxisomal catalase and palmitoyl-CoA-dependent NAD+ reduction were not affected by up to 10 weeks of riboflavin deficiency. In contrast, the specific activity of mitochondrial carnitine-dependent palmitoyl-CoA oxidation was depressed by 75% at 10 weeks of deficiency. The amount of peroxisomal protein per g of liver was not affected by riboflavin deficiency, whereas, expressed per liver, both riboflavin-deficient and pair-fed controls showed decreased peroxisomal protein compared with controls fed ad libitum. Hepatic mitochondria, but not peroxisomes, were sensitive to riboflavin deficiency.     

Simultaneous stimulation of fatty acid synthesis and oxidation in rat hepatocytes by vanadate

When added to the hepatocyte incubation medium, vanadate increased the rate of fatty acid synthesis de novo as well as the activity of acetyl-CoA carboxylase, whereas it had no effect on the activity of fatty acid synthase. On the other hand, and despite elevating the intracellular levels of malonyl-CoA, vanadate diverted exogenous fatty acids into the oxidation pathway at the expense of the esterification route. This was concomitant to an increase in carnitine palmitoyltransferase I activity. All these effects were not significantly different between periportal and perivenous hepatocytes and were also evident in cells incubated in Ca2(+)-free medium. Nevertheless, Ca2+ ions enhanced carnitine palmitoyltransferase I activity in isolated liver mitochondria. In addition, the effects of vanadate on acetyl-CoA carboxylase and carnitine palmitoyltransferase I were only evident in a permeabilized-cell assay, disappearing upon cell disruption and isolation of the corresponding cell subfraction for enzyme assay. Results show that vanadate exerts specific insulin-like and non-insulin-like effects on hepatic fatty acid metabolism, and suggest that the intracellular concentration of malonyl-CoA is not the only factor responsible for the regulation of the fatty-acid-oxidative process in the liver.     

The involvement of fatty acid binding protein in peroxisomal fatty acid oxidation

Rat liver fatty acid-binding protein (FABP) can function as a fatty acid donor protein for both peroxisomal and mitochondrial fatty acid oxidation, since 14C-labeled palmitic acid bound to FABP is oxidized by both organelles. FABP is, however, not detected in peroxisomes and mitochondria of rat liver by ELISA. Acyl-CoA oxidase activity of isolated peroxisomes was not changed by addition of FABP or flavaspidic acid, an inhibitor of fatty acid binding to FABP, nor by disruption of the peroxisomal membranes. These data indicate that FABP may transfer fatty acids to peroxisomes, but is not involved in the transport of acyl-CoA through the peroxisomal membrane.     

The effects of lactate and acetate on fatty acid and cholesterol biosynthesis by isolated rat hepatocytes

1. The present study demonstrates that lactate and acetate stimulate fatty acid synthesis and inhibit cholesterogenesis by isolated rat hepatocytes. 2. Exposure of the intact cells to lactate increases the activity of acetyl-CoA carboxylase, as can be measured in homogenates of these cells. A similar effect by acetate was not observed. 3. Both acetate and lactate drastically increase the cellular level of citrate. 4. Possible mechanisms underlying the difference in response of fatty acid and cholesterol synthesis to an increase in substrate availability are discussed. Futhermore, a mechanism is proposed for the lactate effect on acetyl-CoA carboxylase.     

Mitochondrial and peroxisomal fatty acid oxidation in liver homogenates and isolated hepatocytes from control and clofibrate-treated rats.

Mitochondrial and peroxisomal fatty acid oxidation were compared in whole liver homogenates. Oxidation of 0.2 mM palmitoyl-CoA or oleate by mitochondria increased rapidly with increasing molar substrate:albumin ratios and became saturated at ratios below 3, while peroxisomal oxidation increased more slowly and continued to rise to reach maximal activity in the absence of albumin. Under the latter condition mitochondrial oxidation was severely depressed. In homogenates from normal liver peroxisomal oxidation was lower than mitochondrial oxidation at all ratios tested except when albumin was absent. In contrast with mitochondrial oxidation, peroxisomal oxidation did not produce ketones, was cyanide-insensitive, was not dependent on carnitine, and was not inhibited by (+)-octanoylcarnitine, malonyl-CoA and 4-pentenoate. Mitochondrial oxidation was inhibited by CoASH concentrations that were optimal for peroxisomal oxidation. In the presence of albumin, peroxisomal oxidation was stimulated by Triton X-100 but unaffected by freeze-thawing; both treatments suppressed mitochondrial oxidation. Clofibrate treatment increased mitochondrial and peroxisomal oxidation 2- and 6- to 8-fold, respectively. Peroxisomal oxidation remained unchanged in starvation and diabetes. Fatty acid oxidation was severely depressed by cyanide and (+)-octanoylcarnitine in hepatocytes from normal rats. Hepatocytes from clofibrate-treated rats, which displayed a 3- to 4-fold increase in fatty acid oxidation, were less inhibited by (+)-octanoylcarnitine. Hydrogen peroxide production was severalfold higher in hepatocytes from treated animals oxidizing fatty acids than in control hepatocytes. Assuming that all H2O2 produced during fatty acid oxidation was due to peroxisomal oxidation, it was calculated that the contribution of the peroxisomes to fatty acid oxidation was less than 10% both in cells from control and clofibrate-treated animals.     

Developmental changes in the characteristics of peroxisomal fatty acid oxidation system in rat liver

Developmental changes in fatty acid oxidation system of rat liver peroxisomes were studied to compare with that of mitochondria. More apparent enhancement of peroxisomal palmitoyl-CoA oxidase was observed than mitochondrial palmitoyl-CoA dehydrogenase during prenatal (20-day fetal) to neonatal (1-day after birth) period. The characteristics of peroxisomal enzymes, fatty acyl-CoA oxidase and carnitime acyltransferase, on the bases of substrate specificities, were rapidly established within the 1 day after birth accompanied by the marked enhancement of these activities. These findings indicate that peroxisomal fatty acid oxidation system plays an important role for early growth of neonatal rats; this system may contribute to supplying short- to medium-chain fatty acyl-CoA and NADH₂ for mitochondrial energy formation system.     

Degradation of very long chain dicarboxylic polyunsaturated fatty acids in mouse hepatocytes, a peroxisomal process

Polyunsaturated fatty acids can be omega-oxidized to dicarboxylic polyunsaturated fatty acids (DC-PUFA), bioactive compounds which cause vasodilatation and activation of PPARalpha and gamma. DC-PUFA can be shortened by beta-oxidation, and to determine whether mitochondria and/or peroxisomes are responsible for this degradation 20-carboxy-[1-(14)C]-eicosatetraenoic acid (20-COOH-AA) was synthesized and given to hepatocytes from mouse models with peroxisomal dysfunctions. In contrast to wild type cells, hepatocytes from mice with liver-selective elimination of peroxisomes, due to Pex5p deficiency, failed to produce (14)CO(2) and labeled acid-soluble oxidation products, indicating that peroxisomes are involved in the degradation of 20-COOH-AA. Subsequently, the oxidation of 20-COOH-AA was analyzed in hepatocytes lacking multifunctional protein 1 (MFP1) or MFP2, key enzymes of the peroxisomal beta-oxidation. Degradation of 20-COOH-AA was partially impaired in MFP1, but not in MFP2 knockout hepatocytes. Taken together, peroxisomes and not mitochondria are the site of beta-oxidation of DC-PUFA, and MFP1 is involved in this process.     

Inhibition of glucagon-stimulated cAMP accumulation and fatty acid oxidation by E-series prostaglandins in isolated rat hepatocytes

E-series prostaglandins have been shown to inhibit hepatic glucagon-stimulated glycogenolysis without inhibiting glycogenolysis stimulated by cAMP analogs. In the present studies, prostaglandin E2 and 16,16-dimethylprostaglandin E2 inhibited glucagon-stimulated cAMP accumulation in isolated rat hepatocytes by 25% and 46%, respectively, without affecting basal cAMP levels. Half-maximal inhibition of glucagon-stimulated cAMP accumulation occurred at approx. 10(-7) M 16,16-dimethylprostaglandin E2. 16,16-Dimethylprostaglandin E2 inhibited glucagon-stimulated palmitate oxidation in intact hepatocytes without affecting basal rates of palmitate oxidation. 16,16-Dimethylprostaglandin E2 had no effect on palmitate oxidation in a liver homogenate system. These studies demonstrate that prostaglandin E antagonizes the effects of glucagon on hepatic metabolism by inhibiting glucagon-stimulated cAMP accumulation.     

Mitochondrial and peroxisomal beta oxidation of the branched chain fatty acid 2-methylpalmitate in rat liver

A number of isoprenoids (e.g. pristanic acid and the side chains of fat soluble-vitamins) is degraded or shortened via beta oxidation. We synthesized 2-methyl-palmitate and 2-methyl[1-14C] palmitate as a model substrate for the study of the beta oxidation of branched (isoprenoid) fatty acids in rat liver. 2-Methylpalmitate was well oxidized by isolated hepatocytes and its oxidation was stimulated after treatment of the animals with a peroxisome proliferator. Subcellular fractionation of rat liver demonstrated that 2-methylpalmitate is activated to its CoA ester in endoplasmic reticulum, mitochondria, and peroxisomes and that mitochondria and peroxisomes are capable of beta-oxidizing 2-methylpalmitate. At low unbound 2-methylpalmitate concentrations and in the presence of competing straight chain fatty acids, a condition encountered in vivo, peroxisomal 2-methyl-palmitate oxidation was 2- to 4-fold more active than mitochondrial oxidation. Treatment of rats with a peroxisome proliferator markedly stimulated mitochondrial but only slightly peroxisomal 2-methylpalmitate oxidation. The same treatment dramatically induced palmitoyl-CoA oxidase but did not change 2-methyl-palmitoyl-CoA oxidase activity. Our results indicate 1) that in untreated rats peroxisomes contribute for an important part to the oxidation of 2-methylpalmitate; 2) that treatment with a peroxisome proliferator stimulates mainly the mitochondrial component of 2-methylpalmitate oxidation; and 3) that palmitoyl-CoA and 2-methylpalmitoyl-CoA are oxidized by different peroxisomal oxidases.     

Induction of peroxisomal beta-oxidation enzymes in primary cultured rat hepatocytes by clofibric acid

Rat hepatocytes were cultured for 72 h with or without the addition of 0.5 mM clofibric acid. The activities of individual enzymes of the peroxisomal beta-oxidation pathway (acyl-CoA oxidase, enoyl-CoA hydratase-3-hydroxyacyl-CoA dehydrogenase bifunctional protein, and 3-ketoacyl-CoA thiolase) decreased in the control culture, but markedly increased synchronously in the clofibric acid-treated culture. The levels of mRNAs coding for these enzymes and the rates of synthesis of the enzymes were also elevated in the clofibric acid-treated culture, although no proportional relationship was observed between the time-dependent changes of these parameters. The increase in mRNAs was much higher than the increase in the rate of synthesis of the enzymes. The activity of catalase, its mRNA level and the rate of its synthesis were slightly affected. The effects of clofibric acid on the peroxisomal beta-oxidation enzymes and catalase in primary cultured hepatocytes were very similar to those observed in vivo. These results, therefore, suggest that primary culture of hepatocytes should provide a useful means for investigating the mechanism of induction of peroxisomal enzymes and the mechanism of action of peroxisome proliferators.     

Total and peroxisomal fatty acid oxidation by liver homogenates from autopsied Reye's and control subjects

To determine whether the accumulation of liver triglyceride in Reye's syndrome could be due to a block in beta-oxidation of the fatty acids, the ability of Reye's and control liver homogenates from samples obtained at autopsy to oxidize fatty acids was examined. Total fatty acid oxidation as measured by oxidation of [1-14C]oleoyl CoA, which mostly represents mitochondrial activity, was comparable between the groups. Peroxisomal fatty acid oxidation was, likewise, similar despite the reported increase in the numbers and sizes of these organelles. This disparity could not be explained by an artifactual dilution of product by accumulated endogenous substrate. Inference is made that active peroxisomal beta-oxidation may contribute to the increased short chain fatty CoA content of liver which was reported earlier.     

Partitioning of polyunsaturated fatty acid oxidation between mitochondria and peroxisomes in isolated rat hepatocytes studied by HPLC separation of oxidation products

The extent of mitochondrial and peroxisomal contribution to beta-oxidation of 18-, 20- and 24-carbon n-3 and n-6 polyunsaturated fatty acids (PUFAs) in intact rat hepatocytes is not fully clear. In this study, we analyzed radiolabeled acid soluble oxidation products by HPLC to identify mitochondrial and peroxisomal oxidation of 24:5n-3, 18- and 20-carbon n-3 and n-6 PUFAs. Mitochondrial fatty acid oxidation produced high levels of ketone bodies, tricarboxylic acid cycle intermediates and CO(2), while peroxisomal beta-oxidation released acetate. Inhibition of mitochondrial fatty acid oxidation with 2-tetradecylglycidic acid (TDGA), high amounts of [14C]acetate from oxidation of 24:5n-3, 18- and 20-carbon PUFAs were observed. In the absence of TDGA, high amounts of [14C]-labeled mitochondrial oxidation products were formed from oxidation of 24:5n-3, 18- and 20-carbon PUFAs. With 18:1n-9, high amounts of mitochondrial oxidation products were formed in the absence of TDGA, and TDGA strongly suppressed the oxidation of this fatty acid. Data of this study indicated that a shift in the partitioning from mitochondrial to peroxisomal oxidation differed for each individual fatty acid and is a specific property of 24:5n-3, 18- and 20-carbon n-3 and n-6 PUFAs.[14C]22:6n-3 was detected with [3-14C]24:5n-3, but not with [1-14C]24:5n-3 as the substrate, while [14C]16:0 was detected with [1-14C]24:5n-3, but not with [3-14C]24:5n-3 as the substrate. Furthermore, the amounts of 14CO(2) were similar when cells were incubated with [3-14C]24:5n-3 versus [1-14C]24:5n-3. These findings indicated that the proportion of 24:5n-3 oxidized in mitochondria was high, and that 24:5n-3 and 24:6n-3 were mostly beta-oxidized only one cycle in peroxisomes.     

Peroxisomal fatty acid oxidation is selectively inhibited by phenothiazines in isolated hepatocytes

The production of hydrogen peroxide by isolated hepatocytes in response to lauric, palmitic and oleic acids, a measurement of peroxisomal fatty acid oxidation, is inhibited by phenothiazines under conditions in which ketone body production, a measurement of mitochondrial fatty acid oxidation, does not reveal inhibition of mitochondrial activity. This novel finding provides a pharmacological tool for the study of peroxisomal function in whole cells. The mechanism of this effect of phenothiazines, detected in hepatocytes from rats treated with a peroxisome proliferation inducing drug, is not yet known.     

Action of the antiepileptic drug, valproic acid, on fatty acid oxidation in isolated rat hepatocytes

Valproate at 0.1 to 5 mM strongly inhibited oxidation of 1-(14C)-palmitate in isolated rat hepatocytes. Valproate at the same concentrations markedly decreased ketogenesis from 1 mM oleate. Valproate in a dose up to 5 mM did not significantly affect cellular concentration of ATP but lowered beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios which paralleled its effect on ketogenesis. Moreover concomitant acetyl-CoA levels were drastically decreased by valproate. From this it may be concluded that inhibition of fatty acid oxidation by valproate results in reduced production of two carbons units and a drop of NADH/NAD+ ratio in rat hepatocyte. This suggests that valproate seriously interferes with beta-oxidation of physiological long-chain fatty acids.     

Overexpression of mitochondrial GPAT in rat hepatocytes leads to decreased fatty acid oxidation and increased glycerolipid biosynthesis

Glycerol-3-phosphate acyltransferase (GPAT) catalyses the first committed step in glycerolipid biosynthesis. The mitochondrial isoform (mtGPAT) is mainly expressed in liver, where it is highly regulated, indicating that mtGPAT may have a unique role in hepatic fatty acid metabolism. Because both mtGPAT and carnitine palmitoyl transferase-1 are located on the outer mitochondrial membrane, we hypothesized that mtGPAT directs fatty acyl-CoA away from beta-oxidation and toward glycerolipid synthesis. Adenoviral-mediated overexpression of murine mtGPAT in primary cultures of rat hepatocytes increased mtGPAT activity 2.7-fold with no compensatory effect on microsomal GPAT activity. MtGPAT overexpression resulted in a dramatic 80% reduction in fatty acid oxidation and a significant increase in hepatic diacylglycerol and phospholipid biosynthesis. Following lipid loading of the cells, intracellular triacylglycerol biosynthesis was also induced by mtGPAT overexpression. Changing an invariant aspartic acid residue to a glycine [D235G] in mtGPAT resulted in an inactive enzyme, which helps define the active site required for mammalian mtGPAT function. To determine if obesity increases hepatic mtGPAT activity, two models of rodent obesity were examined and shown to have >2-fold increased enzyme activity. Overall, these results support the concept that increased hepatic mtGPAT activity associated with obesity positively contributes to lipid disorders by reducing oxidative processes and promoting de novo glycerolipid synthesis.     

Myocardial fatty acid oxidation during ischemia and reperfusion

Inhibition of fatty acid oxidation is an early event in myocardial ischemia that most likely contributes to tissue injury by the accumulation of potentially toxic intermediates such as acylCoA and acylcarnitine. After reperfusion both myocardial oxygen consumption and fatty acid oxidation may rapidly recover to preischemic levels, even when contractile function remains depressed. The mechanisms underlying the apparent dissociation between contractile function and oxidative metabolism early during reperfusion are still controversial. In isolated rat hearts subjected to 60 min of no-flow ischemia myocardial oxygen consumption and oxidation of palmitate were lowered during reperfusion by 3 mM of NiCl₂ and by 6 µM of ruthenium red. The results provide indirect evidence for the hypothesis that intracellular calcium transport may be involved in the mechanisms responsible for the high oxidative metabolic rate early after reperfusion