Peroxisomes, lipid metabolism, and human disease

In the past few years, much has been learned about the metabolic functions of peroxisomes. These studies have shown that peroxisomes play a major role in lipid metabolism, including fatty acid β-oxidation, etherphospholipid biosynthesis, and phytanic acid α-oxidation. This article describes the current state of knowledge concerning the role of peroxisomes in these processes, especially in relation to various peroxisomal disorders in which there is an impairment in peroxisomal lipid metabolism.     

Metabolic interactions between peroxisomes and mitochondria with a special focus on acylcarnitine metabolism

Carnitine plays an essential role in mitochondrial fatty acid β-oxidation as a part of a cycle that transfers long-chain fatty acids across the mitochondrial membrane and involves two carnitine palmitoyltransferases (CPT1 and CPT2). Two distinct carnitine acyltransferases, carnitine octanoyltransferase (COT) and carnitine acetyltransferase (CAT), are peroxisomal enzymes, which indicates that carnitine is not only important for mitochondrial, but also for peroxisomal metabolism. It has been demonstrated that after peroxisomal metabolism, specific intermediates can be exported as acylcarnitines for subsequent and final mitochondrial metabolism. There is also evidence that peroxisomes are able to degrade fatty acids that are typically handled by mitochondria possibly after transport as acylcarnitines. Here we review the biochemistry and physiological functions of metabolite exchange between peroxisomes and mitochondria with a special focus on acylcarnitines.     

Lipid metabolism in peroxisomes: enzymology, functions and dysfunctions of the fatty acid alpha- and beta-oxidation systems in humans

Peroxisomes are subcellular organelles present in virtually all eukaryotic cells catalysing a number of indispensable functions in cellular metabolism. The importance of peroxisomes in man is stressed by the existence of an expanding group of genetic diseases in which there is an impairment in one or more peroxisomal functions. One of the major functions of peroxisomes concerns their role in lipid metabolism, which includes: (i) fatty acid betaoxidation; (ii) ether phospholipid synthesis; (iii) fatty acid alpha-oxidation; and (iv) isoprenoid biosynthesis. In this paper, we review the current state of knowledge concerning the peroxisomal fatty acid alpha- and beta-oxidation systems with particular emphasis on the enzymes involved and the various disorders of fatty acid oxidation in peroxisomes. We also pay attention to the fact that some of the metabolites that accumulate as the result of a defect in peroxisomal alpha- and/or beta-oxidation are activators of members of the family of nuclear receptors, including peroxisome-proliferator-activated receptor alpha.     

Biochemistry of peroxisomes in health and disease

The ubiquitous distribution of peroxisomes and the identification of a number of inherited diseases associated with peroxisomal dysfunction indicate that peroxisomes play an essential part in cellular metabolism. Some of the most important metabolic functions of peroxisomes include the synthesis of plasmalogens, bile acids, cholesterol and dolichol, and the oxidation of fatty acids (very long chain fatty acids > C22, branched chain fatty acids (e.g. phytanic acid), dicarboxylic acids, unsaturated fatty acids, prostaglandins, pipecolic acid and glutaric acid). Peroxisomes are also responsible for the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen species (e.g. O-2 and H2O2). Peroxisomal diseases result from the dysfunction of one or more peroxisomal metabolic functions, the majority of which manifest as neurological abnormalities. The quantitation of peroxisomal metabolic functions (e.g. levels of specific metabolites and/or enzyme activity) has bec ome the basis of clinical diagnosis of diseases associated with the organelle. The study of peroxisomal diseases has also contributed towards the further elucidation of a number of metabolic functions of peroxisomes. (Mol Cell Biochem 167:1-29, 1997)     

Peroxisomal fatty acid uptake mechanism in Saccharomyces cerevisiae

Peroxisomes play a major role in human cellular lipid metabolism, including fatty acid β-oxidation. The most frequent peroxisomal disorder is X-linked adrenoleukodystrophy, which is caused by mutations in ABCD1. The biochemical hallmark of X-linked adrenoleukodystrophy is the accumulation of very long chain fatty acids (VLCFAs) due to impaired peroxisomal β-oxidation. Although this suggests a role of ABCD1 in VLCFA import into peroxisomes, no direct experimental evidence is available to substantiate this. To unravel the mechanism of peroxisomal VLCFA transport, we use Saccharomyces cerevisiae as a model organism. Here we provide evidence that in this organism very long chain acyl-CoA esters are hydrolyzed by the Pxa1p-Pxa2p complex prior to the actual transport of their fatty acid moiety into the peroxisomes with the CoA presumably being released into the cytoplasm. The Pxa1p-Pxa2p complex functionally interacts with the acyl-CoA synthetases Faa2p and/or Fat1p on the inner surface of the peroxisomal membrane for subsequent re-esterification of the VLCFAs. Importantly, the Pxa1p-Pxa2p complex shares this molecular mechanism with HsABCD1 and HsABCD2.     

The emerging role of acyl-CoA thioesterases and acyltransferases in regulating peroxisomal lipid metabolism

The importance of peroxisomes in lipid metabolism is now well established and peroxisomes contain approximately 60 enzymes involved in these lipid metabolic pathways. Several acyl-CoA thioesterase enzymes (ACOTs) have been identified in peroxisomes that catalyze the hydrolysis of acyl-CoAs (short-, medium-, long- and very long-chain), bile acid-CoAs, and methyl branched-CoAs, to the free fatty acid and coenzyme A. A number of acyltransferase enzymes, which are structurally and functionally related to ACOTs, have also been identified in peroxisomes, which conjugate (or amidate) bile acid-CoAs and acyl-CoAs to amino acids, resulting in the production of amidated bile acids and fatty acids. The function of ACOTs is to act as auxiliary enzymes in the α- and β-oxidation of various lipids in peroxisomes. Human peroxisomes contain at least two ACOTs (ACOT4 and ACOT8) whereas mouse peroxisomes contain six ACOTs (ACOT3, 4, 5, 6, 8 and 12). Similarly, human peroxisomes contain one bile acid-CoA:amino acid N-acyltransferase (BAAT), whereas mouse peroxisomes contain three acyltransferases (BAAT and acyl-CoA:amino acid N-acyltransferases 1 and 2: ACNAT1 and ACNAT2). This review will focus on the human and mouse peroxisomal ACOT and acyltransferase enzymes identified to date and discuss their cellular localizations, emerging structural information and functions as auxiliary enzymes in peroxisomal metabolic pathways. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.     

过氧化物酶体脂肪酸β氧化

除线粒体外,过氧化物酶体也是真核细胞脂肪酸β氧化分解的重要部位．过氧化物酶体β氧化过程包括氧化、加水、脱氢和硫解4步反应,主要参与极长链、支链脂肪酸等的分解．近年关于过氧化物酶体β氧化的研究活跃,在代谢途径及功能等方面有了新的认识,尤其在对相关代谢酶的研究中取得了较大进展．本文就过氧化物酶体β氧化相关进展作一综述．     

The essential role of fungal peroxisomes in plant infection

Several filamentous fungi are ecologically and economically important plant pathogens that infect a broad variety of crops. They cause high annual yield losses and contaminate seeds and fruits with mycotoxins. Not only powerful infection structures and detrimental toxins, but also cell organelles, such as peroxisomes, play important roles in plant infection. In this review, we summarize recent research results that revealed novel peroxisomal functions of filamentous fungi and highlight the importance of peroxisomes for infection of host plants. Central for fungal virulence are two primary metabolic pathways, fatty acid β-oxidation and the glyoxylate cycle, both of which are required to produce energy, acetyl-CoA, and carbohydrates. These are ultimately needed for the synthesis of cell wall polymers and for turgor generation in infection structures. Most novel results stem from different routes of secondary metabolism and demonstrate that peroxisomes produce important precursors and house various enzymes needed for toxin production and melanization of appressoria. All these peroxisomal functions in fungal virulence might represent elegant targets for improved crop protection.     

Peroxisomal β-oxidation stimulates cholesterol biosynthesis in the liver in diabetic mice

Although diabetes normally causes an elevation of cholesterol biosynthesis and induces hypercholesterolemia in animals and human, the mechanism linking diabetes to the dysregulation of cholesterol biosynthesis in the liver is not fully understood. As liver peroxisomal β-oxidation is induced in the diabetic state and peroxisomal oxidation of fatty acids generates free acetate, we hypothesized that peroxisomal β-oxidation might play a role in liver cholesterol biosynthesis in diabetes. Here, we used erucic acid, a specific substrate for peroxisomal β-oxidation, and 10,12-tricosadiynoic acid, a specific inhibitor for peroxisomal β-oxidation, to specifically induce and suppress peroxisomal β-oxidation. Our results suggested that induction of peroxisomal β-oxidation increased liver cholesterol biosynthesis in streptozotocin-induced diabetic mice. We found that excessive oxidation of fatty acids by peroxisomes generated considerable free acetate in the liver, which was used as a precursor for cholesterol biosynthesis. In addition, we show that specific inhibition of peroxisomal β-oxidation decreased cholesterol biosynthesis by reducing acetate formation in the liver in diabetic mice, demonstrating a crosstalk between peroxisomal β-oxidation and cholesterol biosynthesis. Based on these results, we propose that induction of peroxisomal β-oxidation serves as a mechanism for a fatty acid-induced upregulation in cholesterol biosynthesis and also plays a role in diabetes-induced hypercholesterolemia.     

The Proteome of Human Liver Peroxisomes: Identification of Five New Peroxisomal Constituents by a Label-Free Quantitative Proteomics Survey

The peroxisome is a key organelle of low abundance that fulfils various functions essential for human cell metabolism. Severe genetic diseases in humans are caused by defects in peroxisome biogenesis or deficiencies in the function of single peroxisomal proteins. To improve our knowledge of this important cellular structure, we studied for the first time human liver peroxisomes by quantitative proteomics. Peroxisomes were isolated by differential and Nycodenz density gradient centrifugation. A label-free quantitative study of 314 proteins across the density gradient was accomplished using high resolution mass spectrometry. By pairing statistical data evaluation, cDNA cloning and in vivo colocalization studies, we report the association of five new proteins with human liver peroxisomes. Among these, isochorismatase domain containing 1 protein points to the existence of a new metabolic pathway and hydroxysteroid dehydrogenase like 2 protein is likely involved in the transport or β-oxidation of fatty acids in human peroxisomes. The detection of alcohol dehydrogenase 1A suggests the presence of an alternative alcohol-oxidizing system in hepatic peroxisomes. In addition, lactate dehydrogenase A and malate dehydrogenase 1 partially associate with human liver peroxisomes and enzyme activity profiles support the idea that NAD⁺ becomes regenerated during fatty acid β-oxidation by alternative shuttling processes in human peroxisomes involving lactate dehydrogenase and/or malate dehydrogenase. Taken together, our data represent a valuable resource for future studies of peroxisome biochemistry that will advance research of human peroxisomes in health and disease.     

Engineering the Monomer Composition of Polyhydroxyalkanoates Synthesized in Saccharomyces cerevisiae

Polyhydroxyalkanoates (PHAs) have received considerable interest as renewable-resource-based, biodegradable, and biocompatible plastics with a wide range of potential applications. We have engineered the synthesis of PHA polymers composed of monomers ranging from 4 to 14 carbon atoms in either the cytosol or the peroxisome of Saccharomyces cerevisiae by harnessing intermediates of fatty acid metabolism. Cytosolic PHA production was supported by establishing in the cytosol critical β-oxidation chemistries which are found natively in peroxisomes. This platform was utilized to supply medium-chain (C₆ to C₁₄) PHA precursors from both fatty acid degradation and synthesis to a cytosolically expressed medium-chain-length (mcl) polymerase from Pseudomonas oleovorans. Synthesis of short-chain-length PHAs (scl-PHAs) was established in the peroxisome of a wild-type yeast strain by targeting the Ralstonia eutropha scl polymerase to the peroxisome. This strain, harboring a peroxisomally targeted scl-PHA synthase, accumulated PHA up to approximately 7% of its cell dry weight. These results indicate (i) that S. cerevisiae expressing a cytosolic mcl-PHA polymerase or a peroxisomal scl-PHA synthase can use the 3-hydroxyacyl coenzyme A intermediates from fatty acid metabolism to synthesize PHAs and (ii) that fatty acid degradation is also possible in the cytosol as β-oxidation might not be confined only to the peroxisomes. Polymers of even-numbered, odd-numbered, or a combination of even- and odd-numbered monomers can be controlled by feeding the appropriate substrates. This ability should permit the rational design and synthesis of polymers with desired material properties.     

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.     

Construction of a series of intermediates in the β-oxidation pathway from THA to EPA via DHA in free acid form

β-Oxidation of most fatty acids occurs in the mitochondria. However, β-oxidation for ω-3 polyunsaturated fatty acids (PUFAs) is distinct from abundant fatty acids and occurs in the peroxisomes. Since little is known about peroxisomal β-oxidation, here we report the synthesis of proposed intermediates of ω-3 PUFA β-oxidation steps in free fatty acid form having a conjugated double bond, a β-hydroxyl group, a β-olefin and a β-carbonyl group. These fatty acids can serve as authentic samples for biological experiments.     

Metabolite transport across the peroxisomal membrane

In recent years, much progress has been made with respect to the unravelling of the functions of peroxisomes in metabolism, and it is now well established that peroxisomes are indispensable organelles, especially in higher eukaryotes. Peroxisomes catalyse a number of essential metabolic functions including fatty acid beta-oxidation, ether phospholipid biosynthesis, fatty acid alpha-oxidation and glyoxylate detoxification. The involvement of peroxisomes in these metabolic pathways necessitates the transport of metabolites in and out of peroxisomes. Recently, considerable progress has been made in the characterization of metabolite transport across the peroxisomal membrane. Peroxisomes posses several specialized transport systems to transport metabolites. This is exemplified by the identification of a specific transporter for adenine nucleotides and several half-ABC (ATP-binding cassette) transporters which may be present as hetero- and homo-dimers. The nature of the substrates handled by the different ABC transporters is less clear. In this review we will describe the current state of knowledge of the permeability properties of the peroxisomal membrane.     

Peroxisomes,lipid metabolism and lipotoxicity

Peroxisomes play an essential role in cellular lipid metabolism as exemplified by the existence of a number of genetic diseases in humans caused by the impaired function of one of the peroxisomal enzymes involved in lipid metabolism. Key pathways in which peroxisomes are involved include: (1.) fatty acid beta-oxidation; (2.) etherphospholipid biosynthesis, and (3.) fatty acid alpha-oxidation. In this paper we will describe these different pathways in some detail and will provide an overview of peroxisomal disorders of metabolism and in addition discuss the toxicity of the intermediates of peroxisomal metabolism as they accumulate in the different peroxisomal deficiencies.     

Bovine seminal ribonuclease: Non-hyperbolic kinetics in the second reaction step

Peroxisomes contain enzymes catalyzing the β-oxidation of fatty acids, which have been purified and partially characterized. Hypolipidemic drugs, including clofibrate, cause a marked proliferation of peroxisomes and a striking increase in the activity of their β-oxidation system. We have compared by sodium dodecyl sulfate—polyacrylamide gel electrophoresis the polypeptide patterns of normal and clofibrate-induced peroxisomes and the purified β-oxidation enzymes. The data allow a tentative identification of the β-oxidation enzymes among the peroxisomal polypeptides; these enzymes constitute only a small part of the protein of normal peroxisomes. A subset of peroxisomal polypeptides, including the β-oxidation enzymes, is preferentially increased by clofibrate.     

The peroxisome: an update on mysteries

Peroxisomes contribute to several crucial metabolic processes such as β-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, which render them indispensable to human health and development. Peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. In recent years, the interest in peroxisomes and their physiological functions has significantly increased. This review intends to highlight recent discoveries and trends in peroxisome research, and represents an update as well as a continuation of a former review article. Novel exciting findings on the biological functions, biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross-talk of peroxisomes with other subcellular compartments are addressed. Furthermore, recent findings on the role of peroxisomes in the brain are discussed.     

Indentification of β-oxidation enzymes among peroxisomal polypeptides: Increase in Coomassie blue-stainable protein after clofibrate treatment

Peroxisomes contain enzymes catalyzing the β-oxidation of fatty acids, which have been purified and partially characterized. Hypolipidemic drugs, including clofibrate, cause a marked proliferation of peroxisomes and a striking increase in the activity of their β-oxidation system. We have compared by sodium dodecyl sulfate—polyacrylamide gel electrophoresis the polypeptide patterns of normal and clofibrate-induced peroxisomes and the purified β-oxidation enzymes. The data allow a tentative identification of the β-oxidation enzymes among the peroxisomal polypeptides; these enzymes constitute only a small part of the protein of normal peroxisomes. A subset of peroxisomal polypeptides, including the β-oxidation enzymes, is preferentially increased by clofibrate.     

The peroxisome: still a mysterious organelle

More than half a century of research on peroxisomes has revealed unique features of this ubiquitous subcellular organelle, which have often been in disagreement with existing dogmas in cell biology. About 50 peroxisomal enzymes have so far been identified, which contribute to several crucial metabolic processes such as β-oxidation of fatty acids, biosynthesis of ether phospholipids and metabolism of reactive oxygen species, and render peroxisomes indispensable for human health and development. It became obvious that peroxisomes are highly dynamic organelles that rapidly assemble, multiply and degrade in response to metabolic needs. However, many aspects of peroxisome biology are still mysterious. This review addresses recent exciting discoveries on the biogenesis, formation and degradation of peroxisomes, on peroxisomal dynamics and division, as well as on the interaction and cross talk of peroxisomes with other subcellular compartments. Furthermore, recent advances on the role of peroxisomes in medicine and in the identification of novel peroxisomal proteins are discussed.