C6-C10-dicarboxylic acids in liver and kidney tissue in normal, diabetic ketotic and clofibrate-treated rats

Long-chain dicarboxylic acids play a critical role in inducing peroxisomal β-oxidation and hepatic triacylglycerol accumulation

Recent studies provide evidence that peroxisomal β-oxidation negatively regulates mitochondrial fatty acid oxidation, and induction of peroxisomal β-oxidation causes hepatic lipid accumulation. However, whether there exists a triggering mechanism inducing peroxisomal β-oxidation is not clear. Long-chain dicarboxylic acids (LCDAs) are the product of mono fatty acids subjected to ω-oxidation, and both fatty acid ω-oxidation and peroxisomal β-oxidation are induced under ketogenic conditions, indicating there might be a crosstalk between. Here, we revealed that administration of LCDAs strongly induces peroxisomal fatty acid β-oxidation and causes hepatic steatosis in mice through the metabolites acetyl-CoA and hydrogen peroxide. Under ketogenic conditions, upregulation of fatty acid ω-oxidation resulted in increased generation of LCDAs and induction of peroxisomal β-oxidation, which causes hepatic accumulation of lipid droplets in animals. Inhibition of fatty acid ω-oxidation reduced LCDA formation and significantly lowered peroxisomal β-oxidation and improved hepatic steatosis. Our results suggest that endogenous LCDAs act as triggering molecules inducing peroxisomal β-oxidation and hepatic triacylglycerol deposition. Targeting fatty acid ω-oxidation might be an effective pathway in treating fatty liver and related metabolic diseases through regulating peroxisomal β-oxidation.

The biochemistry and physiology of long-chain dicarboxylic acid metabolism

Mitochondrial β-oxidation is the most prominent pathway for fatty acid oxidation but alternative oxidative metabolism exists. Fatty acid ω-oxidation is one of these pathways and forms dicarboxylic acids as products. These dicarboxylic acids are metabolized through peroxisomal β-oxidation representing an alternative pathway, which could potentially limit the toxic effects of fatty acid accumulation. Although dicarboxylic acid metabolism is highly active in liver and kidney, its role in physiology has not been explored in depth. In this review, we summarize the biochemical mechanism of the formation and degradation of dicarboxylic acids through ω- and β-oxidation, respectively. We will discuss the role of dicarboxylic acids in different (patho)physiological states with a particular focus on the role of the intermediates and products generated through peroxisomal β-oxidation. This review is expected to increase the understanding of dicarboxylic acid metabolism and spark future research.

The Mystery of Extramitochondrial Proteins Lysine Succinylation

Lysine succinylation is a post-translational modification which alters protein function in both physiological and pathological processes. Mindful that it requires succinyl-CoA, a metabolite formed within the mitochondrial matrix that cannot permeate the inner mitochondrial membrane, the question arises as to how there can be succinylation of proteins outside mitochondria. The present mini-review examines pathways participating in peroxisomal fatty acid oxidation that lead to succinyl-CoA production, potentially supporting succinylation of extramitochondrial proteins. Furthermore, the influence of the mitochondrial status on cytosolic NAD⁺ availability affecting the activity of cytosolic SIRT5 iso1 and iso4—in turn regulating cytosolic protein lysine succinylations—is presented. Finally, the discovery that glia in the adult human brain lack subunits of both alpha-ketoglutarate dehydrogenase complex and succinate-CoA ligase—thus being unable to produce succinyl-CoA in the matrix—and yet exhibit robust pancellular lysine succinylation, is highlighted.

Metabolomic differences in heart failure patients with and without major depression

Metabolomics is an emerging technology that allows researchers to characterize hundreds of small molecules that comprise the metabolome. We sought to determine metabolic differences in depressed and nondepressed participants. The sample consisted of a depressed group of patients with heart failure enrolled in an NIMH-supported clinical trial of sertraline versus placebo in depressed heart failure patients, and a nondepressed comparator group of heart failure patients. Plasma was obtained from blood samples provided by participants at baseline, and samples were profiled on GC-MS and LC-MS metabolomics platforms for biochemical content. A number of biochemicals were significantly different between groups, with depressed participants showing higher concentrations of several amino acids and dicarboxylic fatty acids. These results are consistent with prior findings where changes in neurotransmitter systems and fatty acid metabolism were shown to associate with the depressed state. It is unclear what role heart failure may have played in these differing concentrations.

Untargeted metabolomic profiling as an evaluative tool of fenofibrate-induced toxicology in Fischer 344 male rats

Peroxisome proliferator-activated receptor-alpha (PPARalpha) agonists such as fenofibrate are used to treat dyslipidemia. Although fenofibrate is considered safe in humans, it is known to cause hepatocarcinogenesis in rodents. To evaluate untargeted metabolic profiling as a tool for gaining insight into the underlying pharmacology and hepatotoxicology, Fischer 344 male rats were dosed with 300 mg/kg/day of fenofibrate for 14 days and the urine and plasma were analyzed on days 2 and 14. A combination of liquid and gas chromatography mass spectrometry returned the profiles of 486 plasma and 932 urinary metabolites. Aside from known pharmacological effects, such as accelerated fatty acid beta-oxidation and reduced plasma cholesterol, new observations on the drug's impact on cellular metabolism were generated. Reductions in TCA cycle intermediates and biochemical evidence of lactic acidosis demonstrated that energy metabolism homeostasis was altered. Perturbation of the glutathione biosynthesis and elevation of oxidative stress markers were observed. Furthermore, tryptophan metabolism was up-regulated, resulting in accumulation of tryptophan metabolites associated with reactive oxygen species generation, suggesting the possibility of oxidative stress as a mechanism of nongenotoxic carcinogenesis. Finally, several metabolites related to liver function, kidney function, cell damage, and cell proliferation were altered by fenofibrate-induced toxicity at this dose.

The Identification of a Succinyl-CoA Thioesterase Suggests a Novel Pathway for Succinate Production in Peroxisomes

Dicarboxylic acids are formed by omega-oxidation of fatty acids in the endoplasmic reticulum and degraded as the CoA ester via beta-oxidation in peroxisomes. Both synthesis and degradation of dicarboxylic acids occur mainly in kidney and liver, and the chain-shortened dicarboxylic acids are excreted in the urine as the free acids, implying that acyl-CoA thioesterases (ACOTs), which hydrolyze CoA esters to the free acid and CoASH, are needed for the release of the free acids. Recent studies show that peroxisomes contain several acyl-CoA thioesterases with different functions. We have now expressed a peroxisomal acyl-CoA thioesterase with a previously unknown function, ACOT4, which we show is active on dicarboxylyl-CoA esters. We also expressed ACOT8, another peroxisomal acyl-CoA thioesterase that was previously shown to hydrolyze a large variety of CoA esters. Acot4 and Acot8 are both strongly expressed in kidney and liver and are also target genes for the peroxisome proliferator-activated receptor alpha. Enzyme activity measurements with expressed ACOT4 and ACOT8 show that both enzymes hydrolyze CoA esters of dicarboxylic acids with high activity but with strikingly different specificities. Whereas ACOT4 mainly hydrolyzes succinyl-CoA, ACOT8 preferentially hydrolyzes longer dicarboxylyl-CoA esters (glutaryl-CoA, adipyl-CoA, suberyl-CoA, sebacyl-CoA, and dodecanedioyl-CoA). The identification of a highly specific succinyl-CoA thioesterase in peroxisomes strongly suggests that peroxisomal beta-oxidation of dicarboxylic acids leads to formation of succinate, at least under certain conditions, and that ACOT4 and ACOT8 are responsible for the termination of beta-oxidation of dicarboxylic acids of medium-chain length with the concomitant release of the corresponding free acids.

Cytochrome P450 changes in rats with streptozocin-induced diabetes

It is known that the metabolism of some drugs is altered in diabetic patients and in rats with experimental diabetes induced by chemical agents, such as streptozocin. The induction and/or suppression of hepatic cytochrome P450 isozymes seen in diabetes seem to contribute to this alteration. Both metabolic and hormonal disturbances following insulin deficiency in diabetic rats are responsible for these changes. Marked changes in hepatic P450 isozymes in diabetic rats include increases in the isozymes induced by ketones and lipids, including fatty acids, and decreases in the isozymes regulated by growth hormone and testosterone. Suppressed secretion of thyroid hormones also participates in the mechanism causing these changes. Analysis of cytochrome P450 isozymes in diabetic rats is helpful in elucidating the impaired metabolism of some endogenous substrates catalyzed by the cytochrome P450, such as steroid hormones and fatty acids, in diabetes. The results of these analyses also provide insight into the prescription of drugs for diabetic patients.

Changes in amounts of cytochrome P450 isozymes and levels of catalytic activities in hepatic and renal microsomes of rats with streptozocin-induced diabetes

Hepatic microsomal cytochrome P450s, which are involved in the metabolism of drugs, hormones, prostaglandins and fatty acids, change when animals develop diabetes. We studied changes in cytochrome P450 isozymes in both hepatic and renal microsomes of rats with diabetes caused by streptozocin, and compared the results with changes in catalytic activities in the microsomes. In hepatic microsomes of diabetic rats, the amount of cytochrome P450 2E1, an acetone-inducible isozyme, was two and a half times that of control rats, and that of P450 4A2, a major renal isozyme, was three times that in the controls. The amounts of cytochrome P450s 2A1, 2C6, 2C7, 3A2 and 4A3 increased in hepatic microsomes of diabetic rats, and P450 2C11 decreased. Treatment with insulin restored these to the levels in the controls. The catalytic activities of aniline hydroxylation, 7-ethoxycoumarin O-dealkylation, testosterone 2 beta, 6 beta, 7 alpha, and 16 beta-hydroxylation, and omega-, (omega-1)-hydroxylation of lauric acid were high in the hepatic microsomes of diabetic rats, and testosterone 2 alpha and 16 alpha-hydroxylation activities were low. In renal microsomes of diabetic rats, cytochrome P450s 2E1, 4A2 and K-4 were induced, and omega- and (omega-1)-hydroxylation activities were high. These changes were reversed by insulin treatment. The induction and suppression of cytochrome P450 isozymes in diabetic rats were consistent with the changes in the catalytic activities. In both hepatic and renal microsomes, P450s 2E1 and 4A2 were induced, altered metabolism of ketones and fatty acids in diabetes may contribute to these changes.

Metabolic pathways in mammalian peroxisomes

This article summarizes our current knowledge of the metabolic pathways present in mammalian peroxisomes. Emphasis is placed on those aspects that are not covered by other articles in this issue: peroxisomal enzyme content and topology; the peroxisomal beta-oxidation system; substrates of peroxisomal beta-oxidation such as very-long-chain fatty acids, branched fatty acids, dicarboxylic fatty acids, prostaglandins and xenobiotics; the role of peroxisomes in the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen products such as hydrogen peroxide, superoxide anions and epoxides.

A probable defect in the beta-oxidation of lipids in rats fed alcohol for 6 months

Studies on fatty acid oxidation were made in rats fed for 6 months on a liquid diet containing 15% total calories as ethanol. After 6 months, a marked diminution was observed in the in vivo production of 14CO2 from labeled palmitate and octanoate in the ethanol-fed animals compared to their pair-fed isocaloric controls not receiving alcohol. Similar results were obtained in 14CO2 formation from 14C-fatty acids using liver mitochondria from these ethanol-fed rats after 6 months. The effect on octanoate beta-oxidation was larger than that for palmitate. Addition of purified acyl CoA dehydrogenase complexes and additional electron transfer flavoprotein complex to the mitochondria suggested that the ethanol-fed animals could have been deficient in the medium-chain acyl CoA-dehydrogenase complex.

Urinary C6-C12 dicarboxylic acylcarnitines in Reye's syndrome

C6-C12 dicarboxylic acylcarnitines have been identified for the first time in urine from a 2-year-old girl presenting with Reye's syndrome. The acylcarnitines were extracted by ion-exchange chromatography and analysed, both underivatised and as methyl esters using high-resolution fast-atom-bombardment mass spectrometry and B/E-linked scanning. The acylcarnitines were quantified by capillary gas chromatography of the acids extracted after hydrolysis of the acylcarnitine esters. Dodecandioylcarnitine was present in the highest concentration (35.9 mmol/mol creatinine) which exceeded the urinary free dodecandioic acid concentration. The adipic, suberic and sebacic acylcarnitine concentrations were less than 10% of the respective free acid concentrations. It is possible that beta-oxidation of dicarboxylic acids is partially inhibited in Reye's syndrome leading to accumulation of precursor dodecandioyl CoA which is metabolised to dodecandioylcarnitine. The accumulation of these metabolic intermediates may be significant in the pathogenesis of Reye's syndrome.