Phytanoyl-CoA hydroxylase: recognition of 3-methyl-branched acyl-CoAs and requirement for GTP or ATP and Mg2+ in addition to its known hydroxylation cofactors

How Do Differences in Electronic Structure Affect the Use of Vanadium Intermediates as Mimics in Nonheme Iron Hydroxylases?

We study active-site models of nonheme iron hydroxylases and their vanadium-based mimics using density functional theory to determine if vanadyl is a faithful structural mimic. We identify crucial structural and energetic differences between ferryl and vanadyl isomers owing to the differences in their ground electronic states, i.e., high spin (HS) for Fe and low spin (LS) for V. For the succinate cofactor bound to the ferryl intermediate, we predict facile interconversion between monodentate and bidentate coordination isomers for ferryl species but difficult rearrangement for vanadyl mimics. We study isomerization of the oxo intermediate between axial and equatorial positions and find the ferryl potential energy surface to be characterized by a large barrier of ca. 10 kcal/mol that is completely absent for the vanadyl mimic. This analysis reveals even starker contrasts between Fe and V in hydroxylases than those observed for this metal substitution in nonheme halogenases. Analysis of the relative bond strengths of coordinating carboxylate ligands for Fe and V reveals that all of the ligands show stronger binding to V than Fe owing to the LS ground state of V in contrast to the HS ground state of Fe, highlighting the limitations of vanadyl mimics of native nonheme iron hydroxylases.

Targeting de novo lipogenesis and the Lands cycle induces ferroptosis in KRAS-mutant lung cancer

Mutant KRAS (KM), the most common oncogene in lung cancer (LC), regulates fatty acid (FA) metabolism. However, the role of FA in LC tumorigenesis is still not sufficiently characterized. Here, we show that KMLC has a specific lipid profile, with high triacylglycerides and phosphatidylcholines (PC). We demonstrate that FASN, the rate-limiting enzyme in FA synthesis, while being dispensable in EGFR-mutant or wild-type KRAS LC, is required for the viability of KMLC cells. Integrating lipidomic, transcriptomic and functional analyses, we demonstrate that FASN provides saturated and monounsaturated FA to the Lands cycle, the process remodeling oxidized phospholipids, such as PC. Accordingly, blocking either FASN or the Lands cycle in KMLC, promotes ferroptosis, a reactive oxygen species (ROS)- and iron-dependent cell death, characterized by the intracellular accumulation of oxidation-prone PC. Our work indicates that KM dictates a dependency on newly synthesized FA to escape ferroptosis, establishing a targetable vulnerability in KMLC.

Comparative metabolomics with Metaboseek reveals functions of a conserved fat metabolism pathway in C. elegans

Untargeted metabolomics via high-resolution mass spectrometry can reveal more than 100,000 molecular features in a single sample, many of which may represent unidentified metabolites, posing significant challenges to data analysis. We here introduce Metaboseek, an open-source analysis platform designed for untargeted comparative metabolomics and demonstrate its utility by uncovering biosynthetic functions of a conserved fat metabolism pathway, α-oxidation, using C. elegans as a model. Metaboseek integrates modules for molecular feature detection, statistics, molecular formula prediction, and fragmentation analysis, which uncovers more than 200 previously uncharacterized α-oxidation-dependent metabolites in an untargeted comparison of wildtype and α-oxidation-defective hacl-1 mutants. The identified metabolites support the predicted enzymatic function of HACL-1 and reveal that α-oxidation participates in metabolism of endogenous β-methyl-branched fatty acids and food-derived cyclopropane lipids. Our results showcase compound discovery and feature annotation at scale via untargeted comparative metabolomics applied to a conserved primary metabolic pathway and suggest a model for the metabolism of cyclopropane lipids.

Untargeted mass spectrometry-based metabolomics can reveal new biochemistry, but data analysis is challenging. Here, the authors develop Metaboseek, an open-source software that facilitates metabolite discovery, and apply it to characterize fatty acid alpha-oxidation in C. elegans.

The features of the fatty acid composition of Pyrus L. total lipids are determined by mountain ecosystem conditions

The composition of fatty acids (FAs) of total lipids of pericarp, seeds, and leaves of Pyrus caucasica Fed. and Pyrus communis L. growing in mountain ecosystems at different altitudes (300, 700 and 1200 m) was studied. It was found that the greatest differences in the relative content of FAs within a species, depending on the altitudes above sea level, were characteristic of the outer tissues of the pericarp (peel) and leaves, which were in direct contact with the external environment. Pericarp parenchyma to a lesser extent, and seeds practically did not differ in FA composition at different heights. At altitudes with increased UV radiation, conjugated octadecadienoates: rumenic acid (9,11-18:2) and 10,12-18:2 were registered in the pericarp and leaf of Purys L., the functions of which in plants were practically not studied. The wild P. caucasica at all growing altitudes was characterized by more very-long-chain FAs (VLCFAs) than the P. communis cultivar. At 700 m, most likely when exposed to fungal infections, the relative number of VLCFAs increased significantly, and new species of individual odd-chaine FAs appeared in their composition in both representatives. It was especially worth noting the appearance in peel and leaf melissic acid (30:0), which was rarely recorded in the plant. A characteristic feature of only P. communis at an altitude of 700 m was the large number of unsaturated individual VLCFAs. Based on the data obtained, a scheme of possible pathways for VLCFA biosynthesis in P. communis were proposed.

The presence of odd-chain fatty acids in Drosophila phospholipids

Most fatty acids in phospholipids and other lipid species carry an even number of carbon atoms. Also odd-chain fatty acids (OCFAs), such as C15:0 and C17:0, are widespread throughout the living organism. However, the qualitative and quantitative profiles of OCFAs-containing lipids in living organisms remain unclear. Here, we show that OCFAs are present in Drosophila phosphatidylcholine (PC) and phosphatidylethanolamine (PE) and that their level increases in accordance with progression of growth. Furthermore, we found that food-derived propionic acid/propanoic acid (C3:0) is utilized for production of OCFA-containing PC and PE. This study provides the basis for understanding in vivo function of OCFA-containing phospholipids in development and lipid homeostasis.

Slc25a17 Gene Trapped Mice: PMP34 Plays a Role in the Peroxisomal Degradation of Phytanic and Pristanic Acid

Mice lacking PMP34, a peroxisomal membrane transporter encoded by Slc25a17, did not manifest any obvious phenotype on a Swiss Webster genetic background, even with various treatments designed to unmask impaired peroxisomal functioning. Peroxisomal α- and β-oxidation rates in PMP34 deficient fibroblasts or liver slices were not or only modestly affected and in bile, no abnormal bile acid intermediates were detected. Peroxisomal content of cofactors like CoA, ATP, NAD+, thiamine-pyrophosphate and pyridoxal-phosphate, based on direct or indirect data, appeared normal as were tissue plasmalogen and very long chain fatty acid levels. However, upon dietary phytol administration, the knockout mice displayed hepatomegaly, liver inflammation, and an induction of peroxisomal enzymes. This phenotype was partially mediated by PPARα. Hepatic triacylglycerols and cholesterylesters were elevated and both phytanic acid and pristanic acid accumulated in the liver lipids, in females to higher extent than in males. In addition, pristanic acid degradation products were detected, as wells as the CoA-esters of all these branched fatty acids. Hence, PMP34 is important for the degradation of phytanic/pristanic acid and/or export of their metabolites. Whether this is caused by a shortage of peroxisomal CoA affecting the intraperoxisomal formation of pristanoyl-CoA (and perhaps of phytanoyl-CoA), or the SCPx-catalyzed thiolytic cleavage during pristanic acid β-oxidation, could not be proven in this model, but the phytol-derived acyl-CoA profile is compatible with the latter possibility. On the other hand, the normal functioning of other peroxisomal pathways, and especially bile acid formation, seems to exclude severe transport problems or a shortage of CoA, and other cofactors like FAD, NAD(P)+, TPP. Based on our findings, PMP34 deficiency in humans is unlikely to be a life threatening condition but could cause elevated phytanic/pristanic acid levels in older adults.

Phytol-induced pathology in 2-hydroxyacyl-CoA lyase (HACL1) deficient mice. Evidence for a second non-HACL1-related lyase

2-Hydroxyacyl-CoA lyase (HACL1) is a key enzyme of the peroxisomal α-oxidation of phytanic acid. To better understand its role in health and disease, a mouse model lacking HACL1 was investigated. Under normal conditions, these mice did not display a particular phenotype. However, upon dietary administration of phytol, phytanic acid accumulated in tissues, mainly in liver and serum of KO mice. As a consequence of phytanic acid (or a metabolite) toxicity, KO mice displayed a significant weight loss, absence of abdominal white adipose tissue, enlarged and mottled liver and reduced hepatic glycogen and triglycerides. In addition, hepatic PPARα was activated. The central nervous system of the phytol-treated mice was apparently not affected. In addition, 2OH-FA did not accumulate in the central nervous system of HACL1 deficient mice, likely due to the presence in the endoplasmic reticulum of an alternate HACL1-unrelated lyase. The latter may serve as a backup system in certain tissues and account for the formation of pristanic acid in the phytol-fed KO mice. As the degradation of pristanic acid is also impaired, both phytanoyl- and pristanoyl-CoA levels are increased in liver, and the ω-oxidized metabolites are excreted in urine. In conclusion, HACL1 deficiency is not associated with a severe phenotype, but in combination with phytanic acid intake, the normal situation in man, it might present with phytanic acid elevation and resemble a Refsum like disorder.

Differential activities of peroxisomes along the mouse intestinal epithelium

The presence of peroxisomes in mammalian intestine has been revealed formerly by catalase staining combined with electron microscopy. Despite the central role of intestine in lipid uptake and the established importance of peroxisomes in different lipid-related pathways, few data are available on the physiological role of peroxisomes in intestinal metabolism, more specifically, α-, β-oxidation, and etherlipid synthesis. Hence, the peroxisomal compartment was analyzed in more detail in mouse intestine. On the basis of immunohistochemistry, the organelles are mainly confined to the epithelial cells. The expression of the classical peroxisome marker catalase was highest in the proximal part of jejunum and decreased along the tract. PEX14 showed a similar profile, but was still substantial expressed in large intestinal epithelium. Immunoblotting of epithelial cells, isolated from the different segments, showed also such gradient for some enzymes, ie, catalase, ACOX1, and D-specific multifunctional protein 2, and for the ABCD1 transporter, being high in small and low or absent in large intestine. Other peroxisomal enzymes (PHYH, HACL1, and ACAA1), the ABCD2 and ABCD3 transporters, and peroxins PEX13 and PEX14, however, did not follow this pattern, displaying rather constant signals throughout the intestinal epithelium. The small intestine displayed the highest peroxisomal β-oxidation activity and is particularly active on dicarboxylic acids. Etherlipid synthesis was high in the large intestine, and colonic cells had the highest content of plasmalogens. Overall, these data suggest that peroxisomes exert different functions according to the intestinal segment.

Odd Chain Fatty Acids; New Insights of the Relationship Between the Gut Microbiota, Dietary Intake, Biosynthesis and Glucose Intolerance

Recent findings have shown an inverse association between circulating C15:0/C17:0 fatty acids with disease risk, therefore, their origin needs to be determined to understanding their role in these pathologies. Through combinations of both animal and human intervention studies, we comprehensively investigated all possible contributions of these fatty acids from the gut-microbiota, the diet, and novel endogenous biosynthesis. Investigations included an intestinal germ-free study and a C15:0/C17:0 diet dose response study. Endogenous production was assessed through: a stearic acid infusion, phytol supplementation, and a Hacl1^−/− mouse model. Two human dietary intervention studies were used to translate the results. Finally, a study comparing baseline C15:0/C17:0 with the prognosis of glucose intolerance. We found that circulating C15:0/C17:0 levels were not influenced by the gut-microbiota. The dose response study showed C15:0 had a linear response, however C17:0 was not directly correlated. The phytol supplementation only decreased C17:0. Stearic acid infusion only increased C17:0. Hacl1^−/− only decreased C17:0. The glucose intolerance study showed only C17:0 correlated with prognosis. To summarise, circulating C15:0 and C17:0 are independently derived; C15:0 correlates directly with dietary intake, while C17:0 is substantially biosynthesized, therefore, they are not homologous in the aetiology of metabolic disease. Our findings emphasize the importance of the biosynthesis of C17:0 and recognizing its link with metabolic disease.

A cell-free organelle-based in vitro system for studying the peroxisomal protein import machinery

Here we describe a protocol to dissect the peroxisomal matrix protein import pathway using a cell-free in vitro system. The system relies on a postnuclear supernatant (PNS), which is prepared from rat/mouse liver, to act as a source of peroxisomes and cytosolic components. A typical in vitro assay comprises the following steps: (i) incubation of the PNS with an in vitro-synthesized ³⁵S-labeled reporter protein; (ii) treatment of the organelle suspension with a protease that degrades reporter proteins that have not associated with peroxisomes; and (iii) SDS-PAGE/autoradiography analysis. To study transport of proteins into peroxisomes, it is possible to use organelle-resident proteins that contain a peroxisomal targeting signal (PTS) as reporters in the assay. In addition, a receptor (PEX5L/S or PEX5L.PEX7) can be used to report the dynamics of shuttling proteins that mediate the import process. Thus, different but complementary perspectives on the mechanism of this pathway can be obtained. We also describe strategies to fortify the system with recombinant proteins to increase import yields and block specific parts of the machinery at a number of steps. The system recapitulates all the steps of the pathway, including mono-ubiquitination of PEX5L/S at the peroxisome membrane and its ATP-dependent export back into the cytosol by PEX1/PEX6. An in vitro import(/export) experiment can be completed in 24 h.

Revisiting the intraperoxisomal pathway of mammalian PEX7

Newly synthesized peroxisomal proteins containing a cleavable type 2 targeting signal (PTS2) are transported to the peroxisome by a cytosolic PEX5-PEX7 complex. There, the trimeric complex becomes inserted into the peroxisomal membrane docking/translocation machinery (DTM), a step that leads to the translocation of the cargo into the organelle matrix. Previous work suggests that PEX5 is retained at the DTM during all the steps occurring at the peroxisome but whether the same applies to PEX7 was unknown. By subjecting different pre-assembled trimeric PEX5-PEX7-PTS2 complexes to in vitro co-import/export assays we found that the export competence of peroxisomal PEX7 is largely determined by the PEX5 molecule that transported it to the peroxisome. This finding suggests that PEX7 is also retained at the DTM during the peroxisomal steps and implies that cargo proteins are released into the organelle matrix by DTM-embedded PEX7. The release step does not depend on PTS2 cleavage. Rather, our data suggest that insertion of the trimeric PEX5-PEX7-PTS2 protein complex into the DTM is probably accompanied by conformational alterations in PEX5 to allow release of the PTS2 protein into the organelle matrix.

A PEX7-centered perspective on the peroxisomal targeting signal type 2-mediated protein import pathway

Peroxisomal matrix proteins are synthesized on cytosolic ribosomes and transported to the organelle by shuttling receptors. Matrix proteins containing a type 1 signal are carried to the peroxisome by PEX5, whereas those harboring a type 2 signal are transported by a PEX5-PEX7 complex. The pathway followed by PEX5 during the protein transport cycle has been characterized in detail. In contrast, not much is known regarding PEX7. In this work, we show that PEX7 is targeted to the peroxisome in a PEX5- and cargo-dependent manner, where it becomes resistant to exogenously added proteases. Entry of PEX7 and its cargo into the peroxisome occurs upstream of the first cytosolic ATP-dependent step of the PEX5-mediated import pathway, i.e., before monoubiquitination of PEX5. PEX7 passing through the peroxisome becomes partially, if not completely, exposed to the peroxisome matrix milieu, suggesting that cargo release occurs at the trans side of the peroxisomal membrane. Finally, we found that export of peroxisomal PEX7 back into the cytosol requires export of PEX5 but, strikingly, the two export events are not strictly coupled, indicating that the two proteins leave the peroxisome separately.

Glyoxalate reductase/hydroxypyruvate reductase interacts with the sodium-dependent vitamin C transporter-1 to regulate cellular vitamin C homeostasis

The human sodium-dependent vitamin C transporter 1 (hSVCT1) contributes to cellular uptake of ascorbic acid (AA). Although different aspects of hSVCT1 cell biology have been extensively studied, nothing is currently known about the broader hSVCT1 interactome that modulates its role in cellular physiology. Here, we identify the enzyme human glyoxalate reductase/hydroxypyruvate reductase (hGR/HPR) as an hSVCT1 associated protein by yeast two-hybrid (Y2H) screening of a human liver cDNA library. The interaction between hSVCT1 and hGR/HPR was further confirmed by in vitro GST pull-down assay, in vivo coimmunoprecipitation and mammalian two-hybrid firefly luciferase assays. This interaction had functional significance as coexpression of hGR/HPR with hSVCT1 led to an increase in AA uptake. Reciprocally, siRNA-mediated knockdown of endogenous hGR/HPR led to an inhibition of AA uptake. Given that oxalate is a degradation product of vitamin C and hGR/HPR acts to limit cellular oxalate levels, this association physically couples two independent regulators of cellular oxalate production. Furthermore, confocal imaging of human liver HepG2 cells coexpressing GFP-hSVCT1 and hGR/HPR-mCherry demonstrated that these two proteins colocalize within a subpopulation of intracellular organelles. This provides a possible molecular basis for organellar AA transport and regulation of local glyoxylate/glycolate concentration in the vicinity of organelle membranes.

Fatty acid 2-hydroxylase mediates diffusional mobility of Raft-associated lipids, GLUT4 level, and lipogenesis in 3T3-L1 adipocytes

Straight chain fatty acid alpha-oxidation increases during differentiation of 3T3-L1 adipocytes, leading to a marked accumulation of odd chain length fatty acyl moieties. Potential roles of this pathway in adipocyte differentiation and lipogenesis are unknown. Mammalian fatty acid 2-hydroxylase (FA2H) was recently identified and suggested to catalyze the initial step of straight chain fatty acid alpha-oxidation. Accordingly, we examined whether FA2H modulates adipocyte differentiation and lipogenesis in mature adipocytes. FA2H level markedly increases during differentiation of 3T3-L1 adipocytes, and small interfering RNAs against FA2H inhibit the differentiation process. In mature adipocytes, depletion of FA2H inhibits basal and insulin-stimulated glucose uptake and lipogenesis, which are partially rescued by the enzymatic product of FA2H, 2-hydroxy palmitic acid. Expression of fatty-acid synthase and SCD1 was decreased in FA2H-depleted cells, and levels of GLUT4 and insulin receptor proteins were reduced. 2-Hydroxy fatty acids are enriched in cellular sphingolipids, which are components of membrane rafts. Accelerated diffusional mobility of raft-associated lipids was shown to enhance degradation of GLUT4 and insulin receptor in adipocytes. Consistent with this, depletion of FA2H appeared to increase raft lipid mobility as it significantly accelerated the rates of fluorescence recovery after photobleaching measurements of lipid rafts labeled with Alexa 488-conjugated cholera toxin subunit B. Moreover, the enhanced recovery rates were partially reversed by treatment with 2-hydroxy palmitic acid. In conclusion, our findings document the novel role of FA2H in adipocyte lipogenesis possibly by modulation of raft fluidity and level of GLUT4.

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

In humans, peroxisomes harbor a complex set of enzymes acting on various lipophilic carboxylic acids, organized in two basic pathways, alpha-oxidation and beta-oxidation; the latter pathway can also handle omega-oxidized compounds. Some oxidation products are crucial to human health (primary bile acids and polyunsaturated FAs), whereas other substrates have to be degraded in order to avoid neuropathology at a later age (very long-chain FAs and xenobiotic phytanic acid and pristanic acid). Whereas total absence of peroxisomes is lethal, single peroxisomal protein deficiencies can present with a mild or severe phenotype and are more informative to understand the pathogenic factors. The currently known single protein deficiencies equal about one-fourth of the number of proteins involved in peroxisomal FA metabolism. The biochemical properties of these proteins are highlighted, followed by an overview of the known diseases.

Mutations in the fatty acid 2-hydroxylase gene are associated with leukodystrophy with spastic paraparesis and dystonia

Myelination is a complex, developmentally regulated process whereby myelin proteins and lipids are coordinately expressed by myelinating glial cells. Homozygosity mapping in nine patients with childhood onset spasticity, dystonia, cognitive dysfunction, and periventricular white matter disease revealed inactivating mutations in the FA2H gene. FA2H encodes the enzyme fatty acid 2-hydroxylase that catalyzes the 2-hydroxylation of myelin galactolipids, galactosylceramide, and its sulfated form, sulfatide. To our knowledge, this is the first identified deficiency of a lipid component of myelin and the clinical phenotype underscores the importance of the 2-hydroxylation of galactolipids for myelin maturation. In patients with autosomal-recessive unclassified leukodystrophy or complex spastic paraparesis, sequence analysis of the FA2H gene is warranted.

Alpha-Oxidation

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched chain fatty acid, which is a constituent of the human diet. The presence of the 3-methyl group of phytanic acid prevents degradation by beta-oxidation. Instead, the terminal carboxyl group is first removed by alpha-oxidation. The mechanism of the alpha-oxidation pathway and the enzymes involved are described in this review.

FA2H-dependent fatty acid 2-hydroxylation in postnatal mouse brain

2-Hydroxy fatty acids are relatively minor species of membrane lipids found almost exclusively as N-acyl chains of sphingolipids. In mammals, 2-hydroxy sphingolipids are uniquely abundant in myelin galactosylceramide and sulfatide. Despite the well-documented abundance of 2-hydroxy galactolipids in the nervous system, the enzymatic process of the 2-hydroxylation is not fully understood. To fill this gap, we have identified a human fatty acid 2-hydroxylase gene (FA2H) that is highly expressed in brain. In this report, we test the hypothesis that FA2H is the major fatty acid 2-hydroxylase in mouse brain and that free 2-hydroxy fatty acids are formed as precursors of myelin 2-hydroxy galactolipids. The fatty acid compositions of galactolipids in neonatal mouse brain gradually changed during the course of myelination. The relative ratio of 2-hydroxy versus nonhydroxy galactolipids was very low at 2 days of age ( approximately 8% of total galactolipids) and increased 6- to 8-fold by 30 days of age. During this period, free 2-hydroxy fatty acid levels in mouse brain increased 5- to 9-fold, and their composition was reflected in the fatty acids in galactolipids, consistent with a precursor-product relationship. The changes in free 2-hydroxy fatty acid levels coincided with fatty acid 2-hydroxylase activity and with the upregulation of FA2H expression. Furthermore, mouse brain fatty acid 2-hydroxylase activity was inhibited by anti-FA2H antibodies. Together, these data provide evidence that FA2H is the major fatty acid 2-hydroxylase in brain and that 2-hydroxylation of free fatty acids is the first step in the synthesis of 2-hydroxy galactolipids.

Studies on the specificity of unprocessed and mature forms of phytanoyl-CoA 2-hydroxylase and mutation of the iron binding ligands

The mature form of phytanoyl-coenzyme A 2-hydroxylase (PAHX), a nonheme Fe(II)- and 2-oxoglutarate-dependent oxygenase, catalyzes the alpha-hydroxylation of phytanoyl-CoA within peroxisomes. Mutations in PAHX result in some forms of adult Refsum's disease. Unprocessed PAHX (pro-PAHX) contains an N-terminal peroxisomal targeting sequence that is cleaved to give mature PAHX (mat-PAHX). Previous studies have implied a difference in the substrate specificity of the unprocessed and mature forms of PAHX. We demonstrate that both forms are able to hydroxylate a range of CoA derivatives, but under the same assay conditions, the N-terminal hexa-His-tagged unprocessed form is less active than the nontagged mature form. Analyses of the assay conditions suggest a rationale for the lack of activity previously reported for some substrates (e.g. isovaleryl-CoA) for the (His)6pro-PAHX. Site-directed mutagenesis was used to support proposals for the identity of the iron binding ligands (His-175, Asp-177, His-264) of the 2-His-1-carboxylate motif of PAHX. Mutation of other histidine residues (His-213, His-220, His-259) suggested that these residues were not involved in Fe(II) binding.

Breakdown of 2-hydroxylated straight chain fatty acids via peroxisomal 2-hydroxyphytanoyl-CoA lyase: a revised pathway for the alpha-oxidation of straight chain fatty acids

2-Hydroxyfatty acids, constituents of brain cerebrosides and sulfatides, were previously reported to be degraded by an alpha-oxidation system, generating fatty acids shortened by one carbon atom. In the current study we used labeled and unlabeled 2-hydroxyoctadecanoic acid to reinvestigate the degradation of this class of lipids. Both in intact and broken cell systems formate was identified as a main reaction product. Furthermore, the generation of an n-1 aldehyde was demonstrated. In permeabilized rat hepatocytes and liver homogenates, studies on cofactor requirements revealed a dependence on ATP, CoA, Mg(2+), thiamine pyrophosphate, and NAD(+). Together with subcellular fractionation data and studies on recombinant enzymes, this led to the following picture. In a first step, the 2-hydroxyfatty acid is activated to an acyl-CoA; subsequently, the 2-hydroxy fatty acyl-CoA is cleaved by 2-hydroxyphytanoyl-CoA lyase, to formyl-CoA and an n-1 aldehyde. The severe inhibition of formate generation by oxythiamin treatment of intact fibroblasts indicates that cleavage through the thiamine pyrophosphate-dependent 2-hydroxyphytanoyl-CoA lyase is the main pathway for the degradation of 2-hydroxyfatty acids. The latter protein was initially characterized as an essential enzyme in the peroxisomal alpha-oxidation of 3-methyl-branched fatty acids such as phytanic acid. Our findings point to a new role for peroxisomes in mammals, i.e. the breakdown of 2-hydroxyfatty acids, at least the long chain 2-hydroxyfatty acids. Most likely, the more abundant very long chain 2-hydroxyfatty acids are degraded in a similar manner.

Sequential ordered fatty acid alpha oxidation and Delta9 desaturation are major determinants of lipid storage and utilization in differentiating adipocytes

Herein, we exploit the power of global lipidomics to identify the critical role of peroxisomal processing of fatty acids in adipocyte lipid storage and metabolism. Remarkably, 3T3-L1 differentiating adipocytes rapidly acquired the ability to alpha oxidize unbranched fatty acids, which is manifested in the accumulation of odd chain length unbranched fatty acids in all major lipid classes. Moreover, in differentiating adipocytes, unsaturated odd chain length fatty acids in TAG molecular species contained exclusively Delta9 olefinic linkages. Unsaturated fatty acids (e.g., oleic and palmitoleic acids) were not subject to alpha oxidation, resulting in the absence of Delta8 unsaturated odd chain length fatty acids. This highly selective substrate utilization resulted in the obligatory sequential ordering of alpha oxidation prior to Delta9 desaturation. On the basis of these results, a putative type 2 peroxisomal localization sequence was identified at the N-terminus of mouse stearoyl-CoA desaturase I (SCD I) comprised of (30)KVKTVPLHL(38). Kinetic analysis demonstrated that the rate of alpha oxidation of exogenously administered [9,10-(3)H]palmitic acid increased 4-fold during differentiation. Similarly, quantitative PCR demonstrated a 4-fold increase in phytanoyl-CoA alpha hydroxylase (PAHX) and fatty acyl-CoA oxidase (FACO) mRNA levels during differentiation. Collectively, these results underscore the role of peroxisomal fatty acid processing as an important determinant of the metabolic fate of fatty acids in the differentiating adipocyte.

Further studies on the substrate spectrum of phytanoyl-CoA hydroxylase: implications for Refsum disease?

Refsum disease is a peroxisomal disorder characterized by adult-onset retinitis pigmentosa, anosmia, sensory neuropathy, ataxia, and an accumulation of phytanic acid in plasma and tissues. Approximately 45% of cases are caused by mutations in phytanoyl-CoA hydroxylase (PAHX), the enzyme catalyzing the second step in the peroxisomal alpha-oxidation of 3-methyl-branched fatty acids. To study the substrate specificity of human PAHX, different 3-alkyl-branched substrates were synthesized and incubated with a recombinant polyhistidine-tagged protein. The enzyme showed activity not only toward racemic phytanoyl-CoA and the isomers of 3-methylhexadecanoyl-CoA, but also toward a variety of other mono-branched 3-methylacyl-CoA esters with a chain length down to seven carbon atoms. Furthermore, PAHX hydroxylated a 3-ethylacyl-CoA quite well, whereas a 3-propylacyl-CoA was a poor substrate. Hydroxylation of neither 2- or 4-methyl-branched acyl-CoA esters, nor long or very long straight-chain acyl-CoA esters could be detected. The results presented in this paper show that the substrate specificity of PAHX, with regard to the length of both the acyl-chain and the branch at position 3, is broader than expected. Hence, Refsum disease might be characterized by an accumulation of not only phytanic acid but also other 3-alkyl-branched fatty acids.

Regulation of sterol carrier protein gene expression by the forkhead transcription factor FOXO3a

The SCP gene encodes two proteins, sterol carrier protein X (SCPx) and SCP2, that are independently regulated by separate promoters. SCPx has been shown to be the thiolase involved in the breakdown of branched-chain fatty acids and in the biosynthesis of bile acids. The in vivo function of SCP2 however remains to be established. The transcriptional regulation of SCPx and SCP2 is unclear, but their promoter regions contain several putative regulatory domains. We show here that both SCPx and SCP2 are upregulated by the daf-16-like Forkhead transcription factor FOXO3a (also known as FKHRL1) on the level of promoter activity. It was recently described that Forkheads regulate protection against (oxidative) stress in both Caenorhabditis elegans and mammalian cells. We looked into a role for SCP2 in the cellular defense against oxidative damage and found that a fluorescent fatty acid analog bound to SCP2 is protected against H2O2/Cu2+-induced oxidative damage. We propose a model for the way in which SCP2 could protect fatty acids from peroxidation.

Alpha-oxidation of 3-methyl-substituted fatty acids and its thiamine dependence

3-Methyl-branched fatty acids, as phytanic acid, undergo peroxisomal alpha-oxidation in which they are shortened by 1 carbon atom. This process includes four steps: activation, 2-hydroxylation, thiamine pyrophosphate dependent cleavage and aldehyde dehydrogenation. The thiamine pyrophosphate dependence of the third step is unique in peroxisomal mammalian enzymology. Human pathology due to a deficient alpha-oxidation is mostly linked to mutations in the gene coding for the second enzyme of the sequence, phytanoyl-CoA hydroxylase.

Phytanic acid alpha-oxidation, new insights into an old problem: a review

Phytanic acid (3,7,10,14-tetramethylhexadecanoic acid) is a branched-chain fatty acid which is known to accumulate in a number of different genetic diseases including Refsum disease. Due to the presence of a methyl-group at the 3-position, phytanic acid and other 3-methyl fatty acids can not undergo beta-oxidation but are first subjected to fatty acid alpha-oxidation in which the terminal carboxyl-group is released as CO(2). The mechanism of alpha-oxidation has long remained obscure but has been resolved in recent years. Furthermore, peroxisomes have been found to play an indispensable role in fatty acid alpha-oxidation, and the complete alpha-oxidation machinery is probably localized in peroxisomes. This Review describes the current state of knowledge about fatty acid alpha-oxidation in mammals with particular emphasis on the mechanism involved and the enzymology of the pathway.

Stereoselective analysis of 2-hydroxysebacic acid in urine of patients with Zellweger syndrome and of premature infants fed with medium-chain triglycerides

The chiral metabolite 2-hydroxysebacic acid (2-HS) is considered to be an important diagnostic marker for peroxisomal disorders. The pathway of formation of 2-HS, excreted in increased amounts in patients with peroxisomal diseases, is not absolutely clear. Moreover, there is no information about the enantiomeric distribution of 2-HS in human urine. Here, we describe the stereodifferentiation of 2-HS in urine samples of nine patients with Zellweger syndrome (ZS), and for the first time in urine samples of premature infants fed a medium-chain triglyceride (MCT)-containing diet. Using enantioselective multidimensional gas chromatography-mass spectrometry, an increased excretion of 2R-HS was observed in all investigated ZS patients. 2-HS was also present in urine samples of premature infants fed MCT. Analogously to the ZS patients, a dominant 2R-HS excretion in the urine samples of the premature infants was identified. The formation of 2-HS is expected to result from the same or similar pathways as described for ZS patients. Additionally, we determined the absolute configuration of urinary 3-hydroxysebacic acid (3-HS) in the cases investigated. The enantioselective analysis provides further information for the diagnosis and treatment of patients with impaired peroxisomal fatty acid oxidation. Further insight into the metabolic origin and the biochemical pathway leading to these urinary metabolites is provided.

Utilization of sterol carrier protein-2 by phytanoyl-CoA 2-hydroxylase in the peroxisomal alpha oxidation of phytanic acid

Since it possesses a 3-methyl group, phytanic acid is degraded by a peroxisomal alpha-oxidation pathway, the first step of which is catalyzed by phytanoyl-CoA 2-hydroxylase (PAHX). Mutations in human PAHX cause phytanic acid accumulations leading to Adult Refsum's Disease (ARD), which is also observed in a sterol carrier protein 2 (SCP-2)-deficient mouse model. Phytanoyl-CoA is efficiently 2-hydroxylated by PAHX in vitro in the presence of mature SCP-2. Other straight-chain fatty acyl-CoA esters were also 2-hydroxylated and the products isolated and characterized. Use of SCP-2 increases discrimination between straight-chain (e.g., hexadecanoyl-CoA) and branched-chain (e.g., phytanoyl-CoA) substrates by PAHX. The results explain the phytanic acid accumulation in the SCP-2-deficient mouse model and suggest that some of the common symptoms of ARD and other peroxisomal diseases may arise in part due to defects in SCP-2 function caused by increased phytanic acid levels.

Biotransformation of 1,1,1,3,3-Pentafluoropropane (HFC-245fa)

1,1,1,3,3-Pentafluoropropane (HFC-245fa) is being developed as a CFC substitute. 1,1,1,3,3-Pentafluoropropane has a low potential for toxicity: the only remarkable toxic effect seen in rats after inhalation exposure to 1,1,1,3,3-pentafluoropropane in concentrations of up to 50,000 ppm for 90 days was an increased incidence of diffuse myocarditis. To elucidate the possible role of biotransformation in 1,1,1,3,3-pentafluoropropane-induced cardiotoxicity, the biotransformation of 1,1,1,3,3-pentafluoropropane was investigated in rats after inhalation exposure and in rat and human liver microsomes. Male and female rats were exposed by inhalation to 50 000, 10 000, and 2000 ppm 1,1,1,3,3-pentafluoropropane for 6 h, urine was collected for 72 h, and metabolites excreted were identified by 19F NMR spectroscopy and quantified by GC/MS. Trifluoroacetic acid and inorganic fluoride were identified as major urinary metabolites of 1,1,1,3,3-pentafluoropropane; 3,3,3-trifluoropropanoic acid and 1,1,1,3,3-pentafluoropropane-2-ol were minor metabolites. The extent of 1,1,1,3,3-pentafluoropropane biotransformation after inhalation was dependent on exposure concentrations. Neither 3,3,3-trifluoropropanoic acid nor 3,3,3-trifluoropyruvic acid were metabolized to trifluoroacetic acid in vitro or in rats. In rat and human liver microsomes, 1,1,1,3,3-pentafluoropropane was biotransformed by a cytochrome P450-dependent reaction to trifluoroacetic acid and 3,3,3-trifluoropropanoic acid. Rates of trifluoroacetic acid formation were 99.2 +/- 20.5 pmol (mg of protein)(-)(1) min(-)(1) and of 3,3,3-trifluoropropanoic acid formation were 17.5 +/- 4.0 pmol (mg of protein)(-)(1) min(-)(1) in liver microsomes from male rats. In human liver microsomes, rates of trifluoroacetic acid formation ranged from 0 to 11.6 pmol (mg of protein)(-)(1) min(-)(1), and rates of 3,3,3-trifluoropropanoic acid formation ranged from 0.7 to 7.6 pmol (mg of protein)(-)(1) min(-)(1). The results show that 1,1,1,3,3-pentafluoropropane is metabolized at low rates in vivo and in vitro. The toxic effects of 1,1,1,3,3-pentafluoropropane may be associated with the formation of the minor metabolite 3,3,3-trifluoropropanoic acid, which is highly toxic in rats.

Studies on phytanoyl-CoA 2-hydroxylase and synthesis of phytanoyl-coenzyme A

Phytanoyl-CoA 2-hydroxylase (PAHX), an iron(II) and 2-oxoglutarate-dependent oxygenase, catalyses an essential step in the mammalian metabolism of beta-methylated fatty acids. Phytanoyl-CoA was synthesised and used to develop in vitro assays for PAHX. The product of the reaction was confirmed as 2-hydroxyphytanoyl-CoA by NMR and mass spectrometric analyses. In accord with in vivo analyses, hydroxylation of both 3R and 3S epimers of the substrate was catalysed by PAHX. Both pro- and mature- forms of PAHX were fully active.