Pervasive inflammatory activation in patients with deficiency in very-long-chain acyl-coA dehydrogenase (VLCADD)

OBJECTIVES

Very-long-chain acyl-CoA dehydrogenase deficiency (VLCADD) is a disorder of fatty acid oxidation. Symptoms are managed by dietary supplementation with medium-chain fatty acids that bypass the metabolic block. However, patients remain vulnerable to hospitalisations because of rhabdomyolysis, suggesting pathologic processes other than energy deficit. Since rhabdomyolysis is a self-destructive process that can signal inflammatory/immune cascades, we tested the hypothesis that inflammation is a physiologic dimension of VLCADD.

METHODS

All subjects (n = 18) underwent informed consent/assent. Plasma cytokine and cytometry analyses were performed. A prospective case analysis was carried out on a patient with recurrent hospitalisation. Health data were extracted from patient medical records.

RESULTS

Patients showed systemic upregulation of nine inflammatory mediators during symptomatic and asymptomatic periods. There was also overall abundance of immune cells with high intracellular expression of IFNγ, IL-6, MIP-1β (CCL4) and TNFα, and the transcription factors p65-NFκB and STAT1 linked to inflammatory pathways. A case analysis of a patient exhibited already elevated plasma cytokine levels during diagnosis in early infancy, evolving into sustained high systemic levels during recurrent rhabdomyolysis-related hospitalisations. There were corresponding activated leukocytes, with higher intracellular stores of inflammatory molecules in monocytes compared to T cells. Exposure of monocytes to long-chain free fatty acids recapitulated the cytokine signature of patients.

CONCLUSION

Pervasive plasma cytokine upregulation and pre-activated immune cells indicate chronic inflammatory state in VLCADD. Thus, there is rationale for practical implementation of clinical assessment of inflammation and/or translational testing, or adoption, of anti-inflammatory intervention(s) for personalised disease management.

Evaluation of mitochondrial bioenergetics, dynamics, endoplasmic reticulum-mitochondria crosstalk, and reactive oxygen species in fibroblasts from patients with complex I deficiency

Mitochondrial complex I (CI) deficiency is the most frequent cause of oxidative phosphorylation (OXPHOS) disorders in humans. In order to benchmark the effects of CI deficiency on mitochondrial bioenergetics and dynamics, respiratory chain (RC) and endoplasmic reticulum (ER)-mitochondria communication, and superoxide production, fibroblasts from patients with mutations in the ND6, NDUFV1 or ACAD9 genes were analyzed. Fatty acid metabolism, basal and maximal respiration, mitochondrial membrane potential, and ATP levels were decreased. Changes in proteins involved in mitochondrial dynamics were detected in various combinations in each cell line, while variable changes in RC components were observed. ACAD9 deficient cells exhibited an increase in RC complex subunits and DDIT3, an ER stress marker. The level of proteins involved in ER-mitochondria communication was decreased in ND6 and ACAD9 deficient cells. |ΔΨ| and cell viability were further decreased in all cell lines. These findings suggest that disruption of mitochondrial bioenergetics and dynamics, ER-mitochondria crosstalk, and increased superoxide contribute to the pathophysiology in patients with ACAD9 deficiency. Furthermore, treatment of ACAD9 deficient cells with JP4-039, a novel mitochondria-targeted reactive oxygen species, electron and radical scavenger, decreased superoxide level and increased basal and maximal respiratory rate, identifying a potential therapeutic intervention opportunity in CI deficiency.

Elaidate, an 18-carbon trans-monoenoic fatty acid, but not physiological fatty acids increases intracellular Zn(2+) in human macrophages

Artificial trans fatty acids promote atherosclerosis by blocking macrophage clearance of cell debris. Classical fatty-acid response mechanisms include TLR4-NF-κB activation, and Erk1/2 phosphorylation, but these may not indicate long-term mechanisms. Indeed, nuclear NF-κB was increased by 60 min treatment by 30 μM of the 18 carbon trans unsaturated fatty acid elaidic acid (elaidate), the physiological cis-unsaturated fatty acid oleic acid (oleate), and the 18 or 16 carbon saturated fatty acids stearic and palmitic acid (stearate or palmitate). However, except for stearate, effects on related pathways were minimal at 44 h. To determine longer term effects of trans fatty acids, we compared mRNA expression profiles of (trans) elaidate to (cis) oleate, 30 μM, at 44 h in human macrophages. We found that elaidate changed Zn(2+) -homeostasis gene mRNAs markedly. This might be important because Zn(2+) is a major regulator of macrophage activity. Messenger RNAs of seven Zn(2+) -binding metallothioneins decreased 2-4-fold; the zinc importer SLC39A10 increased twofold, in elaidate relative to oleate-treated cells. Results were followed by quantitative PCR comparing cis, trans, and saturated fatty acid effects on Zn(2+) -homeostasis gene mRNAs. This confirmed that elaidate uniquely decreased metallothionein expression and increased SLC39A10 at 44 h. Further, intracellular Zn(2+) was measured using N-(carboxymethyl)-N-[2-[2-[2(carboxymethyl) amino]-5-(2,7,-difluoro-6-hydroxy-3-oxo-3H-xanthen-9-yl)-phenoxy]-ethoxy]-4-methoxyphenyl]glycine, acetoxymethyl ester (FluoZin-3-AM). This showed that, at 44 h, only cells treated with elaidate had increased Zn(2+) . The durable effect of elaidate on Zn(2+) activation is a novel and specific effect of trans fatty acids on peripheral macrophage metabolism.

Elaidate, an 18-carbon trans-monoenoic fatty acid, inhibits β-oxidation in human peripheral blood macrophages

Consumption of trans-unsaturated fatty acids promotes atherosclerosis, but whether degradation of fats in macrophages is altered by trans-unsaturated fatty acids is unknown. We compared the metabolism of oleate (C18:1Δ9-10 cis; (Z)-octadec-9-enoate), elaidate (C18:Δ9-10 trans; (E)-octadec-9-enoate), and stearate (C18:0, octadecanoate) in adherent peripheral human macrophages. Metabolism was followed by measurement of acylcarnitines in cell supernatants by MS/MS, determination of cellular fatty acid content by GC/MS, and assessment of β-oxidation rates using radiolabeled fatty acids. Cells incubated for 44 h in 100 µM elaidate accumulated more unsaturated fatty acids, including both longer- and shorter-chain, and had reduced C18:0 relative to those incubated with oleate or stearate. Both C12:1 and C18:1 acylcarnitines accumulated in supernatants of macrophages exposed to trans fats. These results suggested β-oxidation inhibition one reaction proximal to the trans bond. Comparison of [1-(14)C]oleate to [1-(14)C]elaidate catabolism showed that elaidate completed the first round of fatty acid β-oxidation at rates comparable to oleate. Yet, in competitive β-oxidation assays with [9,10-(3)H]oleate, tritium release rate decreased when unlabeled oleate was replaced by the same quantity of elaidate. These data show specific inhibition of monoenoic fat catabolism by elaidate that is not shared by other atherogenic fats.

Metabolomic profiling of amino acids and β-cell function relative to insulin sensitivity in youth

CONTEXT

In longitudinal studies of adults, elevated amino acid (AA) concentrations predicted future type 2 diabetes mellitus (T2DM).

OBJECTIVE

The aim of the present investigation was to examine whether increased plasma AA concentrations are associated with impaired β-cell function relative to insulin sensitivity [i.e. disposition index (DI)], a predictor of T2DM development.

DESIGN, SETTING, AND PARTICIPANTS

Metabolomic analysis for fasting plasma AAs was performed by tandem mass spectrometry in 139 normal-weight and obese adolescents with and without dysglycemia. First-phase insulin secretion was evaluated by a hyperglycemic (∼225 mg/dl) clamp and insulin sensitivity by a hyperinsulinemic-euglycemic clamp. DI was calculated as the product of first-phase insulin and insulin sensitivity.

RESULTS

DI was positively associated with branched-chain AAs (leucine/isoleucine and valine; r = 0.27 and 0.29, P = 0.001), neutrally transported AAs (phenylalanine and methionine; r = 0.30 and 0.35, P < 0.001), basic AAs (histidine and arginine; r = 0.28 and 0.23, P ≤ 0.007), serine (r = 0.35, P < 0.001), glycine (r = 0.26, P = 0.002), and branched-chain AAs-derived intermediates C3, C4, and C5 acylcarnitine (range r = 0.18-0.19, P ≤ 0.04).

CONCLUSION

In youth, increased plasma AA concentrations are not associated with a heightened metabolic risk profile for T2DM; rather, they are positively associated with β-cell function relative to insulin sensitivity. These contrasting observations between adults and youth may be a reflection of developmental differences along the lifespan dependent on the combined impact of the aging process together with the impact of progressive obesity.

Metabolomic profiling of fatty acid and amino acid metabolism in youth with obesity and type 2 diabetes: evidence for enhanced mitochondrial oxidation

OBJECTIVE

We compared acylcarnitine (AcylCN) species, common amino acid and fat oxidation (FOX) byproducts, and plasma amino acids in normal weight (NW; n = 39), obese (OB; n = 64), and type 2 diabetic (n = 17) adolescents.

RESEARCH DESIGN AND METHODS

Fasting plasma was analyzed by tandem mass spectrometry, body composition by dual energy X-ray absorptiometry and computed tomography, and total-body lipolysis and substrate oxidation by [(2)H(5)]glycerol and indirect calorimetry, respectively. In vivo insulin sensitivity (IS) was assessed with a 3-h hyperinsulinemic-euglycemic clamp.

RESULTS

Long-chain AcylCNs (C18:2-CN to C14:0-CN) were similar among the three groups. Medium- to short-chain AcylCNs (except C8 and C10) were significantly lower in type 2 diabetes compared with NW, and when compared with OB, C2-, C6-, and C10-CN were lower. Amino acid concentrations were lower in type 2 diabetes compared with NW. Fasting lipolysis and FOX were higher in OB and type 2 diabetes compared with NW, and the negative association of FOX to C10:1 disappeared after controlling for adiposity, Tanner stage, and sex. IS was lower in OB and type 2 diabetes with positive associations between IS and arginine, histidine, and serine after adjusting for adiposity, Tanner stage, and sex.

CONCLUSIONS

These metabolomics results, together with the increased rates of in vivo FOX, are not supportive of defective fatty acid or amino acid metabolism in obesity and type 2 diabetes in youth. Such observations are consistent with early adaptive metabolic plasticity in youth, which over time-with continued obesity and aging-may become dysfunctional, as observed in adults.

Increased levels of plasma acylcarnitines in obesity and type 2 diabetes and identification of a marker of glucolipotoxicity

Dysregulation of fatty acid oxidation (FAO) is recognized as important in the pathophysiology of obesity and insulin resistance (IR). However, demonstrating FAO defects in vivo in humans has entailed complex and invasive methodologies. Recently, the identification of genetic blocks in FAO has been vastly simplified by using tandem mass spectrometry (MS/MS) of dried bloodspots to specify acylcarnitine (AcylCN) alterations characteristic for each disorder. This technology has recently been applied to examine FAO alterations in human and animal models of obesity and type 2 diabetes mellitus (T2DM). This study focused on characterizing AcylCN profiles in human plasma from individuals with obesity and T2DM during fasting and insulin-stimulated conditions. Following an overnight fast, plasma was obtained from lean (n = 12), obese nondiabetic (n = 14), and T2DM (n = 10) participants and analyzed for AcylCN using MS/MS. Plasma samples were also obtained at the end of a 4-h insulin-stimulated euglycemic clamp. In obesity and T2DM, long-chain AcylCNs were similarly significantly increased in the fasted state; free-CN levels were also elevated. Additionally, T2DM subjects of comparable BMI had increased short- and medium-chain AcylCNs, both saturated and hydroxy, as well as increased C(4)-dicarboxylcarnitine (C(4)DC-CN) that correlated with an index of poor glycemic control (HbA(1c); r = 0.74; P < 0.0001). Insulin infusion reduced all species of plasma AcylCN but this reduction was blunted in T2DM. Plasma long-chain AcylCN species are increased in obesity and T2DM, suggesting that more fatty acids can enter mitochondria. In T2DM, many shorter species accumulate, suggesting that they have a generalized complex oxidation defect.

Evidence for physical association of mitochondrial fatty acid oxidation and oxidative phosphorylation complexes

Fatty acid β-oxidation (FAO) and oxidative phosphorylation (OXPHOS) are key pathways involved in cellular energetics. Reducing equivalents from FAO enter OXPHOS at the level of complexes I and III. Genetic disorders of FAO and OXPHOS are among the most frequent inborn errors of metabolism. Patients with deficiencies of either FAO or OXPHOS often show clinical and/or biochemical findings indicative of a disorder of the other pathway. In this study, the physical and functional interactions between these pathways were examined. Extracts of isolated rat liver mitochondria were subjected to blue native polyacrylamide gel electrophoresis (BNGE) to separate OXPHOS complexes and supercomplexes followed by Western blotting using antisera to various FAO enzymes. Extracts were also subjected to sucrose density centrifugation and fractions analyzed by BNGE or enzymatic assays. Several FAO enzymes co-migrated with OXPHOS supercomplexes in different patterns in the gels. When palmitoyl-CoA was added to the sucrose gradient fractions containing OXPHOS supercomplexes in the presence of potassium cyanide, cytochrome c was reduced. Cytochrome c reduction was completely blocked by myxothiazol (a complex III inhibitor) and 3-mercaptopropionate (an inhibitor of the first step of FAO), but was only partially inhibited by rotenone (a complex I inhibitor). Although palmitoyl-CoA and octanoyl-CoA provided reducing equivalents to OXPHOS-containing supercomplex fractions, no accumulation of their intermediates was detected. In contrast, short branched acyl-CoA substrates were not metabolized by OXPHOS-containing supercomplex fractions. These data provide evidence of a multifunctional FAO complex within mitochondria that is physically associated with OXPHOS supercomplexes and promotes metabolic channeling.

Participation of two members of the very long-chain acyl-CoA synthetase family in bile acid synthesis and recycling

Bile acids are synthesized de novo in the liver from cholesterol and conjugated to glycine or taurine via a complex series of reactions involving multiple organelles. Bile acids secreted into the small intestine are efficiently reabsorbed and reutilized. Activation by thioesterification to CoA is required at two points in bile acid metabolism. First, 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid, the 27-carbon precursor of cholic acid, must be activated to its CoA derivative before side chain cleavage via peroxisomal beta-oxidation. Second, reutilization of cholate and other C24 bile acids requires reactivation prior to re-conjugation. We reported previously that homolog 2 of very long-chain acyl-CoA synthetase (VLCS) can activate cholate (Steinberg, S. J., Mihalik, S. J., Kim, D. G., Cuebas, D. A., and Watkins, P. A. (2000) J. Biol. Chem. 275, 15605-15608). We now show that this enzyme also activates chenodeoxycholate, the secondary bile acids deoxycholate and lithocholate, and 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid. In contrast, VLCS activated 3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoate, but did not utilize any of the C24 bile acids as substrates. We hypothesize that the primary function of homolog 2 is in the reactivation and recycling of C24 bile acids, whereas VLCS participates in the de novo synthesis pathway. Results of in situ hybridization, topographic orientation, and inhibition studies are consistent with the proposed roles of these enzymes in bile acid metabolism.

The human L-pipecolic acid oxidase is similar to bacterial monomeric sarcosine oxidases rather than D-amino acid oxidases

L-Pipecolic acid oxidase activity is deficient in patients with peroxisome biogenesis disorders (PBDs). Because its role, if any, in these disorders is unknown, the authors cloned the human gene to order to further study its functions. BLAST search of the translated sequence showed greatest homology to Bacillus sp. NS-129 monomeric sarcosine oxidase. The purified enzyme could use either L-pipecolic acid or sarcosine as a substrate. No homology was found to the peroxisomal D-amino acid oxidases. A further comparison of L-pipecolic acid oxidase to the two D-amino acid oxidases in peroxisomes showed that the proteins differed in many ways. First, both D-amino acid oxidase and L-pipecolic acid oxidase showed no enzyme activity in liver from Zellweger syndrome patients; D-aspartate oxidase activity was unchanged from control levels. Although all were targeted to peroxisomes, their targeting signals differed. No L-pipecolic acid oxidase was found in brain or other tissues outside of liver and kidney. The D-amino acid oxidases were similarly and more widely distributed. Finally, although D-amino acid degradation is limited to peroxisomes in mammals, L-pipecolic acid can be oxidized in either mitochondria or peroxisomes, or both.

The human liver-specific homolog of very long-chain acyl-CoA synthetase is cholate:CoA ligase

Unconjugated bile acids must be activated to their CoA thioesters before conjugation to taurine or glycine can occur. A human homolog of very long-chain acyl-CoA synthetase, hVLCS-H2, has two requisite properties of a bile acid:CoA ligase, liver specificity and an endoplasmic reticulum subcellular localization. We investigated the ability of this enzyme to activate the primary bile acid, cholic acid, to its CoA derivative. When expressed in COS-1 cells, hVLCS-H2 exhibited cholate:CoA ligase (choloyl-CoA synthetase) activity with both non-isotopic and radioactive assays. Other long- and very long-chain acyl-CoA synthetases were incapable of activating cholate. Endogenous choloyl-CoA synthetase activity was also detected in liver-derived HepG2 cells but not in kidney-derived COS-1 cells. Our results are consistent with a role for hVLCS-H2 in the re-activation and re-conjugation of bile acids entering liver from the enterohepatic circulation rather than in de novo bile acid synthesis.

L-Pipecolic acid oxidase, a human enzyme essential for the degradation of L-pipecolic acid, is most similar to the monomeric sarcosine oxidases

L-Pipecolic acid oxidase activity is deficient in patients with peroxisome biogenesis disorders (PBDs). Because its role, if any, in these disorders is unknown, we cloned the associated human gene and expressed its protein product. The cDNA was cloned with the use of a reverse genetics approach based on the amino acid sequence obtained from purified L-pipecolic acid oxidase from monkey. The complete cDNA, obtained by conventional library screening and 5' rapid amplification of cDNA ends, encompassed an open reading frame of 1170 bases, translating to a 390-residue protein. The translated protein terminated with the sequence AHL, a peroxisomal targeting signal 1. Indirect immunofluorescence studies showed that the protein product was expressed in human fibroblasts in a punctate pattern that co-localized with the peroxisomal enzyme catalase. A BLAST search with the amino acid sequence showed 31% identity and 53% similarity with Bacillus sp. NS-129 monomeric sarcosine oxidase, as well as similarity to all sarcosine oxidases and dehydrogenases. No similarity was found to the peroxisomal D-amino acid oxidases. The recombinant enzyme oxidized both L-pipecolic acid and sarcosine. However, PBD patients who lack the enzyme activity accumulate only L-pipecolic acid, suggesting that in humans in vivo, this enzyme is involved mainly in the degradation of L-pipecolic acid.

Human very-long-chain acyl-CoA synthetase: cloning, topography, and relevance to branched-chain fatty acid metabolism

Very-long-chain acyl-CoA synthetases (VLCS) activate very-long-chain fatty acids (VLCFA) containing 22 or more carbons to their CoA derivatives. We cloned the human ortholog (hVLCS) of the gene encoding the rat liver enzyme (rVLCS). Both hVLCS and rVLCS contain 620 amino acids, are expressed primarily in liver and kidney, and have a potential peroxisome targeting signal 1 (-LKL) at their carboxy termini. When expressed in COS-1 cells, hVLCS activated the VLCFA lignoceric acid (C24:0), a long-chain fatty acid (C16:0), and two branched-chain fatty acids, phytanic acid and pristanic acid. Immunofluorescence and immunoblot studies localized hVLCS to both peroxisomes and endoplasmic reticulum. In peroxisomes of HepG2 cells, hVLCS was topographically oriented facing the matrix and not the cytoplasm. This orientation, coupled with the observation that hVLCS activates branched-chain fatty acids, suggests that hVLCS could play a role in the intraperoxisomal reactivation of pristanic acid produced via alpha-oxidation of phytanic acid.

Characterization of phytanoyl-Coenzyme A hydroxylase in human liver and activity measurements in patients with peroxisomal disorders

Phytanoyl-Coenzyme A hydroxylase is a newly recognized peroxisomal enzyme which catalyses the first step in the alpha-oxidation of phytanoyl-Coenzyme A. Since measurement of this enzyme activity in human liver homogenate is of great importance especially in relation to inherited diseases in which this enzyme activity is deficient, we have studied its characteristics in human liver. The results described in this paper show that optimal activity measurements require preformed phytanoyl-Coenzyme A plus 2-oxoglutarate, Fe2+ and ascorbate. The conditions developed can be used to determine phytanoyl-Coenzyme A hydroxylase activity in human liver homogenates which is of utmost importance not only for the diagnosis of patients, but also for the purification of the enzyme from various sources.

Identification of PAHX, a Refsum disease gene

Refsum disease is an autosomal recessive disorder characterized by retinitis pigmentosa, peripheral polyneuropathy, cerebellar ataxia and increased cerebrospinal fluid protein. Biochemically, the disorder is defined by two related properties: pronounced accumulation of phytanic acid and selective loss of the peroxisomal dioxygenase required for alpha-hydroxylation of phytanoyl-CoA2. Decreased phytanic-acid oxidation is also observed in human cells lacking PEX7, the receptor for the type-2 peroxisomal targetting signal (PTS2; refs 3,4), suggesting that the enzyme defective in Refsum disease is targetted to peroxisomes by a PTS2. We initially identified the human PAHX and mouse Pahx genes as expressed sequence tags (ESTs) capable of encoding PTS2 proteins. Human PAHX is targetted to peroxisomes, requires the PTS2 receptor for peroxisomal localization, interacts with the PTS2 receptor in the yeast two-hybrid assay and has intrinsic phytanoyl-CoA alpha-hydroxylase activity that requires the dioxygenase cofactor iron and cosubstrate 2-oxoglutarate. Radiation hybrid data place PAHX on chromosome 10 between the markers D10S249 and D10S466, a region previously implicated in Refsum disease by homozygosity mapping. We find that both Refsum disease patients examined are homozygous for inactivating mutations in PAHX, demonstrating that mutations in PAHX can cause Refsum disease.

Cloning and functional expression of a mammalian gene for a peroxisomal sarcosine oxidase

Sarcosine oxidation in mammals occurs via a mitochondrial dehydrogenase closely linked to the electron transport chain. An additional H2O2-producing sarcosine oxidase has now been purified from rabbit kidney. A corresponding cDNA was cloned from rabbit liver and the gene designated sox. This rabbit sox gene encodes a protein of 390 amino acids and a molecular mass of 44 kDa identical to the molecular mass estimated for the purified enzyme. Sequence analysis revealed an N-terminal ADP-betaalphabeta-binding fold, a motif highly conserved in tightly bound flavoproteins, and a C-terminal peroxisomal targeting signal 1. Sarcosine oxidase from rabbit liver exhibits high sequence homology (25-28% identity) to monomeric bacterial sarcosine oxidases. Both purified sarcosine oxidase and a recombinant fusion protein synthesized in Escherichia coli contain a covalently bound flavin, metabolize sarcosine, L-pipecolic acid, and L-proline, and cross-react with antibodies raised against L-pipecolic acid oxidase from monkey liver. Subcellular fractionation demonstrated that sarcosine oxidase is a peroxisomal enzyme in rabbit kidney. Transfection of human fibroblast cell lines and CV-1 cells (monkey kidney epithelial cells) with the sox cDNA resulted in a peroxisomal localization of sarcosine oxidase and revealed that the import into the peroxisomes is mediated by the peroxisomal targeting signal 1 pathway.

Phytanoyl-CoA hydroxylase is present in human liver, located in peroxisomes, and deficient in Zellweger syndrome: direct, unequivocal evidence for the new, revised pathway of phytanic acid alpha-oxidation in humans

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid which accumulates in a number of inherited diseases in human. Because beta-oxidation is blocked by the methyl group at C-3, phytanic acid first undergoes decarboxylation via an alpha-oxidation mechanism. The structure and subcellular localization of the phytanic acid alpha-oxidation pathway have remained enigmatic through the years, although they have generally been assumed to involve phytanic acid and not its CoA-ester. This view has recently been challenged by the findings that in rat liver phytanic acid first has to be activated to its CoA-ester before alpha-oxidation and by the discovery of a new enzyme, phytanoyl-CoA hydroxylase, which converts phytanoyl-CoA to 2-hydroxyphytanoyl-CoA. We now show that this newly discovered enzyme is also present in human liver. Furthermore, we show that this enzyme is located in peroxisomes and deficient in liver from Zellweger patients who lack morphologically distinguishable peroxisomes, which provides an explanation for the long-known deficient oxidation of phytanic acid in these patients. These results suggest that phytanic acid alpha-oxidation is peroxisomal and that it utilizes the coenzyme A derivative as substrate, thus giving further support in favour of the new, revised pathway of phytanic acid alpha-oxidation.

Phytanic acid activation in rat liver peroxisomes is catalyzed by long-chain acyl-CoA synthetase

In Refsum disease, disorders of peroxisome biogenesis, and rhizomelic chondrodysplasia punctata, pathological accumulation of phytanic acid results from impaired alpha-oxidation of this branched-chain fatty acid. Previous studies from this laboratory indicated that activation of phytanic acid to its CoA derivative precedes its alpha-oxidation in peroxisomes. It was reported that this reaction is catalyzed by a unique phytanoyl-CoA synthetase in human peroxisomes. We wanted to determine whether phytanic acid activation in rats required long-chain acyl-CoA synthetase (LCS), very long-chain acyl-CoA synthetase (VLCS), or a different enzyme. To test directly whether LCS could activate phytanic acid, rat liver cDNA encoding this enzyme was transcribed and translated in vitro. The expressed enzyme had both LCS activity (assayed with palmitic acid, C16: 0) and phytanoyl-CoA synthetase activity; VLCS activity (assayed with lignoceric acid, C24: 0) was not detectable. The ratio of phytanoyl-CoA synthetized activity to palmitoyl-CoA synthetase activity for LCS synthetized in vitro (approximately 205) was higher than that observed in peroxisomes isolated from rat liver (5-10%), suggesting that the expressed enzyme contained sufficient phytanoyl-Coa synthetase activity to account for all activity observed in intact peroxisomes. Further experiments were carried out to verify that phytanic acid was activated by LCS in rat liver peroxisomes. Attempts to separate LCS from phytanoyl-CoA synthetase by chromatography on several matrices were unsuccessful. Preparative isoelectric focusing revealed that phytanoyl-CoA synthetase and LCS had indistinguishable isoelectric points. Phytanoyl-CoA synthetase activity was inhibited by unlabeled palmitic acid but not by lignoceric acid. Heat treatment inactivated both phytanoyl-CoA and palmitoyl-CoA synthetase activities at similar rates. 5,8,11,14-Eicosatetraynoic acid inhibited activation of phytanic acid and palmitic acid in a parallel dose-dependent manner, whereas activation of lignoceric acid was not affected. These data support our conclusion that rat liver LCS, an enzyme known to be present in peroxisomal membranes, has phytanoyl-CoA synthetase activity.

Characterization of a novel component of the peroxisomal protein import apparatus using fluorescent peroxisomal proteins

Fluorescent peroxisomal probes were developed by fusing green fluorescent protein (GFP) to the matrix peroxisomal targeting signals PTS1 and PTS2, as well as to an integral peroxisomal membrane protein (IPMP). These proteins were used to identify and characterize novel peroxisome assembly (pas) mutants in the yeast Pichia pastoris. Mutant cells lacking the PAS10 gene mislocalized both PTS1-GFP and PTS2-GFP to the cytoplasm but did incorporate IPMP-GFP into peroxisome membranes. Similar distributions were observed for endogenous peroxisomal matrix and membrane proteins. While peroxisomes from translocation-competent pas mutants sediment in sucrose gradients at the density of normal peroxisomes, >98% of peroxisomes from pas10 cells migrated to a much lower density and had an extremely low ratio of matrix:membrane protein. These data indicate that Pas10p plays an important role in protein translocation across the peroxisome membrane. Consistent with this hypothesis, we find that Pas10p is an integral protein of the peroxisome membrane. In addition, Pas10p contains a cytoplasmically-oriented C3HC4 zinc binding domain that is essential for its biological activity.

Phytanic acid alpha-oxidation in rat liver peroxisomes. Production of alpha-hydroxyphytanoyl-CoA and formate is enhanced by dioxygenase cofactors

Patients with generalized peroxisomal disorders, rhizomelic chondrodysplasia punctata, and Refsum disease are all unable to alpha-oxidize 3,7,11,15-tetramethylhexadecanoic (phytanic) acid. The exact cause of the oxidation defect in these patients is not well characterized, in part because there is only limited knowledge of the biochemical pathway. In 1969, the alpha-oxidation of phytanic acid was reported [Tsai, S.-C., Avigan, J. & Steinberg, D. (1969) Studies on the alpha-oxidation of phytanic acid by rat liver mitochondria, J. Biol. Chem. 244, 2682-2692] to involve the formation of an alpha-hydroxyphytanic acid intermediate prior to removal of the alpha carbon. Subsequently, most researchers have had difficulty detecting this intermediate. In the present study, cofactors known to form hydroxy intermediates by both monooxygenase and dioxygenase reaction mechanisms were incubated with purified rat liver peroxisomes and either [2,3-3H]phytanic acid or [1-14C]phytanic acid. Reaction products were separated by reverse-phase HPLC. A single reaction product, identified as alpha-hydroxyphytanoyl-CoA rather than the free fatty acid, was detected when 2-oxoglutarate/Fe+2/ascorbate, cofactors associated with a dioxygenase reaction mechanism, were present. Concomitant with alpha-hydroxyphytanoyl-CoA production, there was an increased accumulation of formate and CO2. This increase in alpha-oxidation products is evidence that alpha-hydroxyphytanoyl-CoA is a true pathway intermediate and that the entire pathway functions in peroxisomes. In contrast, alpha-hydroxyphytanoyl-CoA was not formed in any quantity in mitochondria. These studies suggest that the alpha-hydroxylation step of phytanic acid oxidation, which has been shown to be defective in Refsum disease, is located in peroxisomes.

Phytanic acid must be activated to phytanoyl-CoA prior to its alpha-oxidation in rat liver peroxisomes

alpha-Oxidation of the branched-chain fatty acid, phytanic acid, is defective in patients with Refsum's disease, the disorders of peroxisome biogenesis (e.g., Zellweger syndrome), and in rhizomelic chondrodysplasia punctata. 3H-Release from [2,3-3H]phytanic acid, which is impaired in cultured skin fibroblasts from these patients, was investigated in rat liver peroxisomes. Cofactors necessary for optimal 3H-release, ATP, Mg2+, and coenzyme A, were also necessary for optimal acyl-CoA synthetase activity, suggesting that the substrate for 3H-release might be phytanoyl-CoA. 5,8,11,14-Eicosatetraynoic acid (ETYA), an inhibitor of long-chain acyl-CoA synthetase activity, blocked phytanoyl-CoA synthesis as well as 3H-release from [2,3-3H]phytanic acid in a dose-dependent manner. However, this inhibitor had little effect on 3H-release from [2,3-3H]phytanoyl-CoA. Tetradecylglycidic acid (TDGA) inhibited 3H-release from [2,3-3H]phytanic acid in peroxisomal but not in mitochondrial fractions from rat liver. This agent inhibited 3H-release from [2,3-3H]phytanic acid and [2,3-3H]phytanoyl-CoA equally. In contrast to ETYA, which appeared to decrease 3H-release as a consequence of synthetase inhibition, TDGA appeared to act directly on the enzyme catalyzing 3H-release. This enzyme was partially purified from rat liver. The purified enzyme, which did not possess phytanoyl-CoA synthetase activity, catalyzed tritium release from [2,3-3H]phytanoyl-CoA. This enzyme catalyzed 3H-release from [2,3-3H]phytanic acid only if a source of phytanoyl-CoA synthetase was present. We conclude that in rat liver peroxisomes, phytanic acid must be activated to its coenzyme A derivative prior to subsequent alpha-oxidation.

Purification and characterization of peroxisomal L-pipecolic acid oxidase from monkey liver

L-Pipecolic acid oxidase has been purified to near homogeneity from Rhesus monkey liver. The protein, a yellow monomer, has a molecular weight of 46,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a pI of 8.9. It contains a covalently bound flavin with absorption maxima at 457 and 383 nm and a shoulder at 480 nm. The purified enzyme is most reactive toward L-pipecolic acid, with lesser reactivities toward L-proline and sarcosine. The enzyme has no significant reactivity toward the D-enantiomer of pipecolic acid or toward any other amino acid tested. Benzoic acid is a competitive inhibitor of the enzyme with a Ki of 750 microM. The Km of the purified enzyme is 3.7 mM for L-pipecolic acid. With less purified preparations, the reaction product is alpha-aminodipic acid. The purified enzyme, however, produces an intermediate which reacts with ortho-aminobenzaldehyde to form an alpha-aminoadipic acid semialdehyde adduct. Thus, the formation of alpha-aminoadipic acid requires at least two enzymes.

The 22-kD peroxisomal integral membrane protein in Zellweger syndrome--presence, abundance, and association with a peroxisomal thiolase precursor protein

The primary genetic defect of Zellweger syndrome may be related to defective synthesis or impaired import of peroxisomal proteins. We analyzed the presence and measured the abundance of the 22-kD peroxisomal integral membrane protein (PMP) in patients with Zellweger syndrome. We determined the intracellular localization of the 22-kD PMP and compared it with the localization of a peroxisomal 44-kD thiolase precursor protein. The 22-kD PMP was quantified by immunoblot analyses in liver tissue (n = 7 patients). Immunoblot signals were evaluated using transmission photometry. The intracellular localization of the 22-kD PMP and the peroxisomal 44-kD thiolase precursor protein were determined by immunoblot analyses on fibroblast subcellular fractions prepared by Nycodenz (n = 5 patients) or sucrose density gradient centrifugation (n = 2 patients). The 22-kD PMP was present and associated with membrane fractions in all patients. Its abundance varied in patients as compared with normal human liver controls. The 22-kD PMP was located in subcellular membrane fractions having a lower density than normal peroxisomes or mitochondria. Using two different gradient techniques, the 22-kD PMP and the peroxisomal 44-kD thiolase precursor protein were found in the same low-density gradient fractions. These results suggest that in Zellweger syndrome peroxisome-like elements containing both the 22-kD PMP and a 44-kD thiolase precursor protein are formed. Globally defective synthesis or import of peroxisomal proteins is therefore unlikely to be the primary genetic defect in the patients we studied.

Species variation in organellar location and activity of L-pipecolic acid oxidation in mammals

The oxidation of L-pipecolic acid to alpha-aminoadipic acid was studied in eight species of mammals using an assay system more sensitive than those previously employed. After percoll-gradient fractionation, activity was localized to the mitochondrial-enriched fractions in tissues from rabbit, guinea pig, pig, dog, and sheep, with guinea pig kidney cortex showing greatest specific activity. These results contrast with the peroxisomal oxidation of L-pipecolic acid observed in macaques and man (Mihalik and Rhead 1989; Mihalik et al. 1989). Rats and mice had undetectable levels of both peroxisomal and mitochondrial L-pipecolic acid oxidation. In the rat, peroxisomal oxidation activity was not induced by feeding with either clofibrate or clofibrate and L-pipecolic acid. Thus, among mammals, both the ability to oxidize L-pipecolic acid and the organellar location of this oxidation is species dependent.

Mitochondrial oxidation of phytanic acid in human and monkey liver: implication that Refsum's disease is not a peroxisomal disorder

The subcellular site of oxidation of [1-14C]phytanic acid to 14CO2 was investigated in human and monkey liver. In both species, this activity was associated with fractions enriched in mitochondria. Fractions enriched in peroxisomes had no detectable phytanic acid oxidase activity. The mitochondrial inhibitors antimycin A and rotenone significantly decreased 14CO2 production in mitochondria-rich fractions from human and monkey liver. These inhibitors also blocked phytanic acid oxidation in cultured human skin fibroblasts. These data suggest that alpha-oxidation of phytanic acid is a mitochondrial rather than a peroxisomal process in primates.

Peroxisomal L-pipecolic acid oxidation is deficient in liver from Zellweger syndrome patients

L-Pipecolic acid, a cyclic imino acid produced during the degradation of lysine, accumulates in body fluids of infants with the generalized peroxisomal disorders, including Zellweger syndrome, neonatal adrenoleukodystrophy, and infantile Refsum disease. Peroxisome-enriched fractions from normal human liver oxidized L-[3H]pipecolic acid to alpha-[3H]aminoadipic acid (AAA). When human liver organelles were separated on a Percoll gradient, L-[3H]pipecolic acid oxidation activity (as measured by [3H]AAA formation) most closely segregated with the peroxisomal marker, catalase, and was not associated with the mitochondria. L-Pipecolic acid oxidation was not inhibited by antimycin A and rotenone and produced H2O2, consistent with its involving a peroxisomal oxidase. We measured L-pipecolic acid oxidation in liver specimens from patients with peroxisomal disorders. While liver homogenates from adult (n = 5) and infant (n = 10) controls formed 47.1 +/- 6.6 and 48.3 +/- 10.0 pmol AAA/mg protein/h, respectively, Zellweger syndrome livers (n = 8) formed only 1.7 +/- 0.3 pmol AAA/mg protein/h. L-pipecolic acid oxidation in normal infant livers was low at birth and increased with age, but Zellweger syndrome livers showed little activity at any age. Thus, the high circulating levels of L-pipecolic acid in Zellweger syndrome probably result from defective peroxisomal oxidation of L-pipecolic acid to AAA.

L-pipecolic acid oxidation in the rabbit and cynomolgus monkey. Evidence for differing organellar locations and cofactor requirements in each species

L-Pipecolic acid oxidation was studied in the rabbit and cynomolgus monkey. Tissue homogenates from both species incubated with L-[2,3,4,5,6-3H]pipecolic acid produced a single radioactive product identified as alpha-aminoadipic acid. In the rabbit, L-pipecolic acid oxidation was greatest in kidney cortex with progressively lesser specific activities in liver, heart, and brain. When rabbit kidney cortex was fractionated by differential centrifugation or on Percoll gradients, activity paralleled that of the mitochondrial marker, glutamate dehydrogenase. In sonicated mitochondria, 92% of the activity was in the soluble fraction. Activity was inhibited by both rotenone and antimycin A and was maximal when FAD, phenazine ethosulfate, and glycerol were included in the assay; Km,app was 0.74 +/- 0.16 mM. Nipecotic acid, piperidine, and cis-2,4-piperidine dicarboxylic acid did not inhibit L-pipecolic acid oxidation, while L-proline had a Ki greater than or equal to 10 mM. D-Alanine and kojic acid, substrate and inhibitor of D-amino acid oxidase, respectively, were also not inhibitory. When monkey kidney cortex was fractionated on Percoll gradients, L-pipecolic acid oxidation activity paralleled that of the peroxisomal marker, catalase. After organellar subfractionation, the activity was membrane-associated and maximal at pH 8.5; Km,app was 4.22 +/- 0.30 mM. L-Pipecolic acid oxidation produced hydrogen peroxide, suggesting involvement of an oxidase in alpha-aminoadipic acid formation. Antimycin A did not inhibit the reaction. No specific cofactor requirements were identified and phenazine ethosulfate inhibited the reaction. D-Pipecolic acid, L-proline, and the other compounds cited above did not significantly inhibit the activity.

Adrenoleukodystrophy and other peroxisomal disorders that affect the nervous system, including new observations on L-pipecolic acid oxidase in primates

Impaired nervous system function in childhood is encountered in 10 separate disorders of the peroxisome. Peroxisomal disorders are subdivided into three major groups. In group 1 there is failure to form the organelle and impairment of multiple peroxisomal functions. In group 3 peroxisome structure is intact and the defect involves a single enzyme. In group 2 (rhizomelic chondrodysplasia punctata) peroxisome structure is probably intact, but at least 2 peroxisomal enzymes fail to function normally. We present an overview of this newly recognized field which is so pertinent to brain development. In addition, we present recent and new data about the biochemical defect, genetics and therapy of X-linked adrenoleukodystrophy; about the normal metabolism of L-pipecolic acid in primates and its disturbance in the Zellweger syndrome, and about the prenatal diagnosis of peroxisomal disorders.