Defect In Peroxisome Biogenesis Research Articles

Lysosomal cholesterol accumulation is commonly found in most peroxisomal disorders and reversed by 2-hydroxypropyl-β-cyclodextrin.

Peroxisomal disorders (PDs) are a heterogenous group of diseases caused by defects in peroxisome biogenesis or functions. X-linked adrenoleukodystrophy is the most prevalent form of PDs and results from mutations in the ABCD1 gene, which encodes a transporter mediating the uptake of very long-chain fatty acids (VLCFAs). The curative approaches for PDs are very limited. Here, we investigated whether cholesterol accumulation in the lysosomes is a biochemical feature shared by a broad spectrum of PDs. We individually knocked down fifteen PD-associated genes in cultured cells and found ten induced cholesterol accumulation in the lysosome. 2-Hydroxypropyl-β-cyclodextrin (HPCD) effectively alleviated the cholesterol accumulation phenotype in PD-mimicking cells through reducing intracellular cholesterol content as well as promoting cholesterol redistribution to other cellular membranes. In ABCD1 knockdown cells, HPCD treatment lowered reactive oxygen species and VLCFA to normal levels. In Abcd1 knockout mice, HPCD injections reduced cholesterol and VLCFA sequestration in the brain and adrenal cortex. The plasma levels of adrenocortical hormones were increased and the behavioral abnormalities were greatly ameliorated upon HPCD administration. Together, our results suggest that defective cholesterol transport underlies most, if not all, PDs, and that HPCD can serve as a novel and effective strategy for the treatment of PDs.

Structure and function of the peroxisomal ubiquitin ligase complex.

Peroxisomes are membrane-bounded organelles that exist in most eukaryotic cells and are involved in the oxidation of fatty acids and the destruction of reactive oxygen species. Depending on the organism, they house additional metabolic reactions that range from glycolysis in parasitic protozoa to the production of ether lipids in animals and antibiotics in fungi. The importance of peroxisomes for human health is revealed by various disorders - notably the Zellweger spectrum - that are caused by defects in peroxisome biogenesis and are often fatal. Most peroxisomal metabolic enzymes reside in the lumen, but are synthesized in the cytosol and imported into the organelle by mobile receptors. The receptors accompany cargo all the way into the lumen and must return to the cytosol to start a new import cycle. Recycling requires receptor monoubiquitination by a membrane-embedded ubiquitin ligase complex composed of three RING finger (RF) domain-containing proteins: PEX2, PEX10, and PEX12. A recent cryo-electron microscopy (cryo-EM) structure of the complex reveals its function as a retro-translocation channel for peroxisomal import receptors. Each subunit of the complex contributes five transmembrane segments that assemble into an open channel. The N terminus of a receptor likely inserts into the pore from the lumenal side, and is then monoubiquitinated by one of the RFs to enable extraction into the cytosol. If recycling is compromised, receptors are polyubiquitinated by the concerted action of the other two RFs and ultimately degraded. The new data provide mechanistic insight into a crucial step of peroxisomal protein import.

Drosophila Lipase 3 Mediates the Metabolic Response to Starvation and Aging

The human LIPA gene encodes for the enzyme lysosomal acid lipase, which hydrolyzes cholesteryl ester and triacylglycerol. Lysosomal acid lipase deficiency results in Wolman disease and cholesteryl ester storage disease. The Drosophila genome encodes for two LIPA orthologs, Magro and Lipase 3. Magro is a gut lipase that hydrolyzes triacylglycerides, while Lipase 3 lacks characterization based on mutant phenotypes. We found previously that Lipase 3 transcription is highly induced in mutants with defects in peroxisome biogenesis, but the conditions that allow a similar induction in wildtypic flies are not known. Here we show that Lipase 3 is drastically upregulated in starved larvae and starved female flies, as well as in aged male flies. We generated a lipase 3 mutant that shows sex-specific starvation resistance and a trend to lifespan extension. Using lipidomics, we demonstrate that Lipase 3 mutants accumulate phosphatidylinositol, but neither triacylglycerol nor diacylglycerol. Our study suggests that, in contrast to its mammalian homolog LIPA, Lipase 3 is a putative phospholipase that is upregulated under extreme conditions like prolonged nutrient deprivation and aging.

A Floppy Infant with Facial Dysmorphism.

A Floppy Infant with Facial Dysmorphism.

PEX6: An Imaging Overlap Between Peroxisomal and Lysosomal Storage Diseases

Peroxisomal disorders are a group of expanding genetic diseases divided into two major categories: peroxisome biogenesis defects (Zellweger spectrum disorder), and single enzymatic defects. Disorders of Peroxisome Biogenesis occur when there are biallelic pathogenic variants in any of the 13 PEX genes, which code for the peroxins, proteins required for peroxisome biogenesis. This group of disorders includes two distinct phenotypes: Rhizomelic Chondrodysplasia Punctata Type-1 and Zellweger Spectrum Disorders (ZSD), of which Zellweger syndrome is the most severe, neonatal adrenoleukodystrophy is intermediate, and infantile Refsum is the mildest. The spectrum’s most frequent defects are observed in the proteins PEX1 and PEX6, and the most common clinical presentation is Zellweger spectrum, which is often associated with craniofacial dysmorphism with neurologic abnormalities. Typically, the neuroimaging pattern shows several malformative features, including a range of cortical gyral abnormalities such as microgyria and pachygyria, and impairment of the myelination. Nevertheless, we report two siblings with peroxisomal disorder, with unexpected leukodystrophy pattern of the brain mimicking lysosomal storage disease, with classical imaging features of Krabbe disease on brain magnetic resonance image. By whole exome sequencing, we identified two pathogenic variants in compound heterozygosity in PEX6: Chr6:42.933.455 C>T (c.2435G>A), and Chr6:42.935.188 C>T (c.1802G>A). Thus, a final diagnosis of peroxisome disorder was confirmed. The index cases highlight the importance of considering peroxisome disorders as a differential diagnosis for patients with imaging features that resemble Krabbe disease.

Serum very long-chain fatty acids (VLCFA) levels as predictive biomarkers of diseases severity and probability of survival in peroxisomal disorders.

Peroxisomal disorders (PD) are a heterogeneous group of rare diseases caused by a defect in peroxisome biogenesis or a disruption of a peroxisomal function at a single enzyme or at transporter level. The main biochemical markers for PD are very long-chain fatty acids (VLCFA). The aim of the study was to investigate the correlation of basic diagnostic parameter, i.e. VLCFA, with disease severity, determined through the survival time. We performed a retrospective study in patients with PD (n = 31; aged 1 week—21 years). Evaluation of VLCFA results from patients were as follows: 15 patients with classical Zellweger syndrome (ZS), 3 patients with mild outcome of ZS, 9 individuals with D-Bifunctional Protein Deficiency (DBP), and no specified results in the case of 4 patients. Patients with classical ZS had higher VLCFA levels, compared to individuals with mild form of ZS and also to patients with DBP; for C26:0/C22:0: 0.65±0.18; 0.11±0.09; 0.30±0.13 (P < 0.001) and for C26:0: 5.20±1.78; 0.76±0.46; 2.61±0.97[mg/mL] (P < 0.001) respectively. The only variable parameter, i.e. the one that determines the survival time of patients, was C26:0 (Chi2 = 19,311, P < 0.0001). Correlation coefficient between survival time and C26:0 level was statistically significant (r = -0.762), and the results showed that high levels of C26:0 were associated with short survival time.ConclusionVLCFA levels correlate with the severity of the clinical course of ZS, DBP and mild ZSD. The best predictive value for estimating the projected disease severity and survival time is a concentration of C26:0.

Mild Zellweger syndrome due to functionally confirmed novel PEX1 variants

Zellweger spectrum disorders (ZSD) constitute a group of rare autosomal recessive disorders characterized by a defect in peroxisome biogenesis due to mutations in one of 13 PEX genes. The broad clinical heterogeneity especially in late-onset presenting patients and a mild phenotype complicates and delays the diagnostic process. Here, we report a case of mild ZSD, due to novel PEX1 variants. The patient presented with an early hearing loss, bilateral cataracts, and leukodystrophy on magnetic resonance (MR) images. Normal results of serum very-long-chain fatty acids (VLCFA) and phytanic acid were found. Molecular diagnostics were performed to uncover the etiology of the clinical phenotype. Using whole exome sequencing, there have been found two variants in the PEX1 gene—c.3450T>A (p.Cys1150*) and c.1769T>C (p.Leu590Pro). VLCFA measurement in skin fibroblasts and C26:0-lysoPC in dried blood spot therefore was performed. Both results were in line with the diagnosis of ZSD. To conclude, normal results of routine serum VLCFA and branched-chain fatty acid measurement do not exclude mild forms of ZSD. The investigation of C26:0-lysoPC should be included in the diagnostic work-up in patients with cataract, hearing loss, and leukodystrophy on MR images suspected to suffer from ZSD.

A defect in the peroxisomal biogenesis in germ cells induces a spermatogenic arrest at the round spermatid stage in mice

Peroxisomes are involved in the degradation of very long-chain fatty acids (VLCFAs) by β-oxidation. Besides neurological defects, peroxisomal dysfunction can also lead to testicular abnormalities. However, underlying alterations in the testes due to a peroxisomal defect are not well characterized yet. To maintain all metabolic functions, peroxisomes require an import machinery for the transport of matrix proteins. One component of this translocation machinery is PEX13. Its inactivation leads to a peroxisomal biogenesis defect. We have established a germ cell-specific KO of Pex13 to study the function of peroxisomes during spermatogenesis in mice. Exon 2 of floxed Pex13 was specifically excised in germ cells prior to meiosis by using a transgenic mouse strain carrying a STRA8 inducible Cre recombinase. Germ cell differentiation was interrupted at the round spermatid stage in Pex13 KO mice with formation of multinucleated giant cells (MNCs) and loss of mature spermatids. Due to a different cellular content in the germinal epithelium of Pex13 KO testes compared to control, whole testes biopsies were used for the analyses. Thus, differences in lipid composition and gene expression are only shown for whole testicular tissue but cannot be limited to single cells. Gas chromatography revealed an increase of shorter fatty acids and a decrease of n-6 docosapentaenoic acid (C22:5n-6) and n-3 docosahexaenoic acid (C22:6n-3), the main components of sperm plasma membranes. Representative genes of the metabolite transport and peroxisomal β-oxidation were strongly down-regulated. In addition, structural components of the blood-testis barrier (BTB) were altered. To conclude, defects in the peroxisomal compartment interfere with normal spermatogenesis.

Allelic Expression Imbalance Promoting a Mutant PEX6 Allele Causes Zellweger Spectrum Disorder

Zellweger spectrum disorders (ZSDs) are autosomal-recessive disorders that are caused by defects in peroxisome biogenesis due to bi-allelic mutations in any of 13 different PEX genes. Here, we identified seven unrelated individuals affected with an apparent dominant ZSD in whom a heterozygous mutant PEX6 allele (c.2578C>T [p.Arg860Trp]) was overrepresented due to allelic expression imbalance (AEI). We demonstrated that AEI of PEX6 is a common phenomenon and is correlated with heterozygosity for a frequent variant in the 3' untranslated region (UTR) of the mutant allele, which disrupts the most distal of two polyadenylation sites. Asymptomatic parents, who were heterozygous for PEX c.2578C>T, did not show AEI and were homozygous for the 3' UTR variant. Overexpression models confirmed that the overrepresentation of the pathogenic PEX6 c.2578T variant compared to wild-type PEX6 c.2578C results in a peroxisome biogenesis defect and thus constitutes the cause of disease in the affected individuals. AEI promoting the overrepresentation of a mutant allele might also play a role in other autosomal-recessive disorders, in which only one heterozygous pathogenic variant is identified.

Management of pneumatosis intestinalis in children over the age of 6 months: a conservative approach

BackgroundPneumatosis intestinalis (PI) is an uncommon and poorly understood condition. Although it can be an incidental finding in asymptomatic individuals, it can also be secondary to life-threatening bowel ischaemia and...

Evaluation of C26:0-lysophosphatidylcholine and C26:0-carnitine as diagnostic markers for Zellweger spectrum disorders

IntroductionZellweger spectrum disorders (ZSD) are a group of genetic metabolic disorders caused by a defect in peroxisome biogenesis. This results in multiple metabolic abnormalities, including elevated very long-chain fatty acid (VLCFA) levels. Elevated levels of C26:0-lysophosphatidylcholine (C26:0-lysoPC) have been shown in dried blood spots (DBS) from ZSD patients. However, little is known about the sensitivity and specificity of this marker and C26:0-carnitine, another VLCFA-marker, in ZSD. We investigated C26:0-lysoPC and C26:0-carnitine as diagnostic markers for ZSD in DBS and fibroblasts.MethodsC26:0-lysoPC levels in 91 DBS from 37 different ZSD patients were determined and compared to the levels in 209 control DBS. C26:0-carnitine levels were measured in 41 DBS from 29 ZSD patients and 97 control DBS. We measured C26:0-lysoPC levels in fibroblasts from 24 ZSD patients and 61 control individuals.ResultsElevated C26:0-lysoPC levels (>72 nmol/L) were found in 86/91 ZSD DBS (n=33/37 patients) corresponding to a sensitivity of 89.2%. Median level was 567 nmol/l (range 28–3133 nmol/l). Consistently elevated C26:0-carnitine levels (>0.077 μmol/L) in DBS were found in 16 out of 29 ZSD patients corresponding to a sensitivity of 55.2%. C26:0-lysoPC levels were elevated in 21/24 ZSD fibroblast lines.DiscussionC26:0-lysoPC in DBS is a sensitive and useful marker for VLCFA accumulation in patients with a ZSD. C26:0-carnitine in DBS is elevated in some ZSD patients, but is less useful as a diagnostic marker. Implementation of C26:0-lysoPC measurement in the diagnostic work-up when suspecting a ZSD is advised. This marker has the potential to be used for newborn screening for ZSD.

Lifelong course of Zellweger spectrum disorders: A cohort study

Objective: Zellweger spectrum disorders (ZSD) are a group of genetic metabolic disorders caused by a defect in peroxisome biogenesis due to mutations in one of the PEX genes. Although originally seen as a rapidly lethal pediatric disease, an increasing number of patients with a relatively mild or aspecific phenotype were recognized over the past few years as a result of improved clinical and biochemical characterization. Consequently, a substantial percentage of ZSD patients reach adulthood nowadays. Since only limited data on the natural course of the disease in these relatively mild patients are available, this retrospective cohort study aims to gain insight in the natural course of these patients. Methods: Clinical and laboratory data were collected retrospectively from a large cohort of patients with a genetically confirmed ZSD. All patients were regularly followed up and examined as a part of standard patient care. Results: A total of 22 patients in this ZSD cohort reached adulthood. All patients, except one patient with PEX10 mutations, had vision and hearing impairment. Neurological symptoms comprised pyramidal tract dysfunction, cerebellar syndrome and peripheral neuropathy, in most cases eventually leading to a gait disorder. Two patients suffered from epileptic seizures. In multiple adult patients, a relatively rapid progression of mainly neurological symptoms is described after a long period of stabilization, accompanied by diffuse white matter abnormalities and general cerebral atrophy on MRI- imaging. Conclusion: Most patients with a ZSD at the mild end of the spectrum suffer from slowly progressive neurological symptoms, vision and hearing impairment. Rapid progression of neurological symptoms after a long period of stabilization can occur, whilst biochemical abnormalities have normalized over the years. The exact pathophysiology of this rapid deterioration remains to be elucidated.

Peroxisome biogenesis disorders.

Peroxisome biogenesis disorders (PBD) are a group of conditions caused by a partial or generalized defect in peroxisome biogenesis. They encompass two phenotypic groups: 1. the Zellweger spectrum disorders (ZSD) including severe, intermediate and milder forms [previously known respectively as Zellweger syndrome (ZS), Neonatal Adrenoleukodystrophy (NALD) and Infantile Refsum Disease (IRD)] and 2. Rhizomelic Chondrodysplasia Punctata type 1 (RCDP1), as well as the variant phenotypes now being described for both groups. PBD represent the paradigm for metabolic malformation syndromes. In a little over 50 years, an intense, concerted multidisciplinary effort has led to elucidating the fundamental biochemical, pathophysiological and molecular bases for these disorders effectively paving the way for therapeutic interventions and trials. Peroxisomes are membrane bound organelles derived from the endoplasmic reticulum, numbering up to several hundred per mammalian cell, mostly spherical in shape, up to 1μm in diameter. They are indispensable for normal life and highly conserved throughout evolution amongst most eukaryotes. Peroxisomes were first identified by biochemist Christian DeDuve in the 1960 s, as cytoplasmic particles containing hydrogen peroxide-generating oxidases [1]. Later, the neurologist Hans Zellweger reported a group of children with similar craniofacial dysmorphism and malformations in the brain, eye, liver, and kidney [2]. These cases, initially described under the eponym of “cerebro-hepato-renal-syndrome”, were subsequently termed Zellweger syndrome (ZS) [3]. ZS is considered today the prototype for ZSD. The reported absence of morphologically identifiable peroxisomes in hepatocytes and renal tubular cells of affected individuals using the peroxisomal matrix enzyme marker, catalase, [4] demonstrated a connection between ZS and peroxisome dysfunction. However, the etiological connection was not recognized until the identification of multiple peroxisomal enzymes and their deficiencies in affected patients. Similar findings were later demonstrated in the intermediate and milder ZSD phenotypes and RCDP1, which were also being reported at that time [5–7].

Clinical and Biochemical Pitfalls in the Diagnosis of Peroxisomal Disorders.

Peroxisomal disorders are a heterogeneous group of genetic metabolic disorders, caused by a defect in peroxisome biogenesis or a deficiency of a single peroxisomal enzyme. The peroxisomal disorders include the Zellweger spectrum disorders, the rhizomelic chondrodysplasia punctata spectrum disorders, X-linked adrenoleukodystrophy, and multiple single enzyme deficiencies. There are several core phenotypes caused by peroxisomal dysfunction that clinicians can recognize. The diagnosis is suggested by biochemical testing in blood and urine and confirmed by functional assays in cultured skin fibroblasts, followed by mutation analysis. This review describes the phenotype of the main peroxisomal disorders and possible pitfalls in (laboratory) diagnosis to aid clinicians in the recognition of this group of diseases.

Peroxisomes in cardiomyocytes and the peroxisome / peroxisome proliferator-activated receptor-loop.

It is well established that the heart is strongly dependent on fatty acid metabolism. In cardiomyocytes there are two distinct sites for the β-oxidisation of fatty acids: the mitochondrion and the peroxisome. Although the metabolism of these two organelles is believed to be tightly coupled, the nature of this relationship has not been fully investigated. Recent research has established the significant contribution of mitochondrial function to cardiac ATP production under normal and pathological conditions. In contrast, limited information is available on peroxisomal function in the heart. This is despite these organelles harbouring metabolic pathways that are potentially cardio-protective, and findings that patients with peroxisomal diseases, such as adult Refsum´s disease, can develop heart failure. In this article, we provide a comprehensive overview on the current knowledge of peroxisomes and the regulation of lipid metabolism by PPARs in cardiomyocytes. We also present new experimental evidence on the differential expression of peroxisome-related genes in the heart chambers and demonstrate that even a mild peroxisomal biogenesis defect (Pex11α-/-) can induce profound alterations in the cardiomyocyte´s peroxisomal compartment and related gene expression, including the concomitant deregulation of specific PPARs. The possible impact of peroxisomal dysfunction in the heart is discussed and a model for the modulation of myocardial metabolism via a peroxisome/PPAR-loop is proposed.

Peroxisomes are required for lipid metabolism and muscle function in Drosophila melanogaster.

Peroxisomes are ubiquitous organelles that perform lipid and reactive oxygen species metabolism. Defects in peroxisome biogenesis cause peroxisome biogenesis disorders (PBDs). The most severe PBD, Zellweger syndrome, is characterized in part by neuronal dysfunction, craniofacial malformations, and low muscle tone (hypotonia). These devastating diseases lack effective therapies and the development of animal models may reveal new drug targets. We have generated Drosophila mutants with impaired peroxisome biogenesis by disrupting the early peroxin gene pex3, which participates in budding of pre-peroxisomes from the ER and peroxisomal membrane protein localization. pex3 deletion mutants lack detectible peroxisomes and die before or during pupariation. At earlier stages of development, larvae lacking Pex3 display reduced size and impaired lipid metabolism. Selective loss of peroxisomes in muscles impairs muscle function and results in flightless animals. Although, hypotonia in PBD patients is thought to be a secondary effect of neuronal dysfunction, our results suggest that peroxisome loss directly affects muscle physiology, possibly by disrupting energy metabolism. Understanding the role of peroxisomes in Drosophila physiology, specifically in muscle cells may reveal novel aspects of PBD etiology.

Very-long-chain polyunsaturated fatty acids accumulate in phosphatidylcholine of fibroblasts from patients with Zellweger syndrome and acyl-CoA oxidase1 deficiency

Peroxisomes are subcellular organelles that function in multiple anabolic and catabolic processes, including β-oxidation of very-long-chain fatty acids (VLCFA) and biosynthesis of ether phospholipids. Peroxisomal disorders caused by defects in peroxisome biogenesis or peroxisomal β-oxidation manifest as severe neural disorders of the central nervous system. Abnormal peroxisomal metabolism is thought to be responsible for the clinical symptoms of these diseases, but their molecular pathogenesis remains to be elucidated. We performed lipidomic analysis to identify aberrant metabolites in fibroblasts from patients with Zellweger syndrome (ZS), acyl-CoA oxidase1 (AOx) deficiency, D-bifunctional protein (D-BP) and X-linked adrenoleukodystrophy (X-ALD), as well as in peroxisome-deficient Chinese hamster ovary cell mutants. In cells deficient in peroxisomal biogenesis, plasmenylethanolamine was remarkably reduced and phosphatidylethanolamine was increased. Marked accumulation of very-long-chain saturated fatty acid and monounsaturated fatty acids in phosphatidylcholine was observed in all mutant cells. Very-long-chain polyunsaturated fatty acid (VLC-PUFA) levels were significantly elevated, whilst phospholipids containing docosahexaenoic acid (DHA, C22:6n-3) were reduced in fibroblasts from patients with ZS, AOx deficiency, and D-BP deficiency, but not in fibroblasts from an X-ALD patient. Because patients with AOx deficiency suffer from more severe symptoms than those with X-ALD, accumulation of VLC-PUFA and/or reduction of DHA may be associated with the severity of peroxisomal diseases.

Stable overproducer of hepatitis B surface antigen in the methylotrophic yeast Hansenula polymorpha due to multiple integration of heterologous auxotrophic selective markers and defect in peroxisome biogenesis

Two methods of multicopy integrant selection in the methylotrophic yeast Hansenula polymorpha based on the use of heterologous yeast auxotrophic genes have been used to isolate effective overproducers of hepatitis B surface antigen (HBsAg). One selection marker was described earlier for this yeast, the Saccharomyces cerevisiae URA3 gene, whereas the second selection marker was developed by us, the Pichia pastoris ADE1 gene with shortened native promoter. Sequential use of both selection markers produced stable transformants containing up to 30 integration cassettes with HBsAg gene. Deletion of PEX3 gene coding for peroxine involved in the early step of peroxisome formation substantially increased the production of HBsAg in glucose medium as compared to the parental strain. Maximal production of HBsAg in Δpex3 strain was nearly 8-9% of the total cell protein.

Absent biochemical evidence at an early age delayed diagnosis in a patient with a clinically severe form of peroxisomal biogenesis disorder

The aim of the study is determining the level of very long chain fatty acids (VLCFA) is the screening method of choice in suspected peroxisomal disorders. Severe peroxisomal biogenesis defects in particular are usually recognized reliably. We report a patient presenting with typical facial stigmata, a treatment-resistant seizure disorder, and polymicrogyria, whose VLCFA levels were within normal limits until the age of 18 months. Only thereafter an elevation was found. Enzymatic and consecutively molecular-genetic analysis revealed a compound-heterozygous mutation in PEX 6. In conclusion, VLCFA levels within normal limits initially do not rule out global peroxisomal biogenesis defects and should be repeated subsequently. Persisting clinical suspicion justifies further enzymatic and molecular evaluation.

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.