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

Gas-Phase Ion/Ion Reactions to Enable Radical-Directed Dissociation of Fatty Acid Ions: Application to Localization of Methyl Branching

Methyl branching on the carbon chains of fatty acids and fatty esters is among the structural variations encountered with fatty acids and fatty esters. Branching in fatty acid/ester chains is particularly prominent in bacterial species and, for example, in vernix caseosa and sebum. The distinction of branched chains from isomeric straight-chain species and the localization of branching can be challenging to determine by mass spectrometry (MS). Condensed-phase derivatization strategies, often used in conjunction with separations, are most commonly used to address the identification and characterization of branched fatty acids. In this work, a gas-phase ion/ion strategy is presented that obviates condensed-phase derivatization and introduces a radical site into fatty acid ions to facilitate radical-directed dissociation (RDD). The gas-phase approach is also directly amenable to fatty acid anions generated via collision-induced dissociation from lipid classes that contain fatty esters. Specifically, divalent magnesium complexes bound to two terpyridine ligands that each incorporate a ((2,2,6,6-tetramethyl-1-piperidine-1-yl)oxy) (TEMPO) moiety are used to charge-invert fatty acid anions. Following the facile loss of one of the ligands and the TEMPO group of the remaining ligand, a radical site is introduced into the complex. Subsequent collision-induced dissociation (CID) of the complex exhibits preferred cleavages that localize the site(s) of branching. The approach is illustrated with iso-, anteiso-, and isoprenoid branched-chain fatty acids and an intact glycerophospholipid and is applied to a mixture of branched- and straight-chain fatty acids derived from Bacillus subtilis.

Amelioration of Phytanic Acid-Induced Neurotoxicity by Nutraceuticals: Mechanistic Insights

Phytanic acid (PA) (3,7,11,15-tetramethylhexadecanoic acid) is a methyl-branched fatty acid that enters the body through food consumption, primarily through red meat, dairy products, and fatty marine foods. The metabolic byproduct of phytol is PA, which is then oxidized by the ruminal microbiota and some marine species. The first methyl group at the 3-position prevents the β-oxidation of branched-chain fatty acid (BCFA). Instead, α-oxidation of PA results in the production of pristanic acid (2,10,14-tetramethylpentadecanoic acid) with CO2. This fatty acid (FA) builds up in individuals with certain peroxisomal disorders and is historically linked to neurological impairment. It also causes oxidative stress in synaptosomes, as demonstrated by an increase in the production of reactive oxygen species (ROS), which is a sign of oxidative stress. This review concludes that the nutraceuticals (melatonin, piperine, quercetin, curcumin, resveratrol, epigallocatechin-3-gallate (EGCG), coenzyme Q10, ω-3 FA) can reduce oxidative stress and enhanced the activity of mitochondria. Furthermore, the use of nutraceuticals completely reversed the neurotoxic effects of PA on NO level and membrane potential. Additionally, the review further emphasizes the urgent need for more research into dairy-derived BCFAs and their impact on human health.

Disruption of mitochondrial bioenergetics and calcium homeostasis by phytanic acid in the heart: Potential relevance for the cardiomyopathy in Refsum disease

Refsum disease is an inherited peroxisomal disorder caused by severe deficiency of phytanoyl-CoA hydroxylase activity. Affected patients develop severe cardiomyopathy of poorly known pathogenesis that may lead to a fatal outcome. Since phytanic acid (Phyt) concentrations are highly increased in tissues of individuals with this disease, it is conceivable that this branched-chain fatty acid is cardiotoxic. The present study investigated whether Phyt (10-30 μM) could disturb important mitochondrial functions in rat heart mitochondria. We also determined the influence of Phyt (50-100 μM) on cell viability (MTT reduction) in cardiac cells (H9C2). Phyt markedly increased mitochondrial state 4 (resting) and decreased state 3 (ADP-stimulated) and uncoupled (CCCP-stimulated) respirations, besides reducing the respiratory control ratio, ATP synthesis and the activities of the respiratory chain complexes I-III, II, and II-III. This fatty acid also reduced mitochondrial membrane potential and induced swelling in mitochondria supplemented by exogenous Ca²⁺, which were prevented by cyclosporin A alone or combined with ADP, suggesting the involvement of the mitochondrial permeability transition (MPT) pore opening. Mitochondrial NAD(P)H content and Ca²⁺ retention capacity were also decreased by Phyt in the presence of Ca²⁺. Finally, Phyt significantly reduced cellular viability (MTT reduction) in cultured cardiomyocytes. The present data indicate that Phyt, at concentrations found in the plasma of patients with Refsum disease, disrupts by multiple mechanisms mitochondrial bioenergetics and Ca²⁺ homeostasis, which could presumably be involved in the cardiomyopathy of this disease.

Can individual fatty acids be used as functional biomarkers of dairy fat consumption in relation to cardiometabolic health? A narrative review

In epidemiological studies, dairy food consumption has been associated with minimal effect or decreased risk of some cardiometabolic diseases (CMD). However, current methods of dietary assessment do not provide objective and accurate measures of food intakes. Thus, the identification of valid and reliable biomarkers of dairy product intake is an important challenge to best determine the relationship between dairy consumption and health status. This review investigated potential biomarkers of dairy fat consumption, such as odd-chain, trans- and branched-chain fatty acids (FA), which may improve the assessment of full-fat dairy product consumption. Overall, the current use of serum/plasma FA as biomarkers of dairy fat consumption is mostly based on observational evidence, with a lack of well-controlled, dose-response intervention studies to accurately assess the strength of the relationship. Circulating odd-chain SFA and trans-palmitoleic acid are increasingly studied in relation to CMD risk and seem to be consistently associated with a reduced risk of type 2 diabetes in prospective cohort studies. However, associations with CVD are less clear. Overall, adding less studied FA such as vaccenic and phytanic acids to the current available evidence may provide a more complete assessment of dairy fat intake and minimise potential confounding from endogenous synthesis. Finally, the current evidence base on the direct effect of dairy fatty acids on established biomarkers of CMD risk (e.g. fasting lipid profiles and markers of glycaemic control) mostly derives from cross-sectional, animal and in vitro studies and should be strengthened by well-controlled human intervention studies.

NUDT7 Loss Promotes KrasG12D CRC Development

Studies have suggested that dysregulation of peroxisomal lipid metabolism might play an important role in colorectal cancer (CRC) development. Here, we found that Kras^G12D-driven CRC tumors demonstrate dysfunctional peroxisomal β-oxidation and identified Nudt7 (peroxisomal coenzyme A diphosphatase NUDT7) as one of responsible peroxisomal genes. In Kras^G12D-driven CRC tumors, the expression level of Nudt7 was significantly decreased. Treatment of azoxymethane/dextran sulfate sodium (AOM/DSS) into Nudt7 knockout (Nudt7^−/−) mice significantly induced lipid accumulation and the expression levels of CRC-related genes whereas xenografting of Nudt7-overexpressed LS-174T cells into mice significantly reduced lipid accumulation and the expression levels of CRC-related genes. Ingenuity pathway analysis of microarray using the colon of Nudt7^−/− and Nudt7^+/+ mice treated with AOM/DSS suggested Wnt signaling as one of activated signaling pathways in Nudt7^−/− colons. Upregulated levels of β-catenin were observed in the colons of Kras^G12D and AOM/DSS-treated Nudt7^−/− mice and downstream targets of β-catenin such as Myc, Ccdn1, and Nos2, were also significantly increased in the colon of Nudt7^−/− mice. We observed an increased level of palmitic acid in the colon of Nudt7^−/− mice and attachment of palmitic acid-conjugated chitosan patch into the colon of mice induced the expression levels of β-catenin and CRC-related genes. Overall, our data reveal a novel role for peroxisomal NUDT7 in Kras^G12D-driven CRC development.

Phytanic acid induced neurological alterations in rat brain synaptosomes and its attenuation by melatonin

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) (Phyt) is a saturated branched chain fatty acid which originates after the breakdown of chlorophyll molecule, phytol. It plays an important role in a variety of metabolic disorders with peroxisomal impairments. The aim of our investigation was to evaluate the adverse effects of Phyt on synaptic functions by using synaptosomal preparation of rat brain as an in vitro model and the possible protective role of melatonin against Phyt-induced neurotoxicity. Melatonin is an antioxidant, secreted by the pineal gland. Melatonin and its metabolites have neuroprotective effects on cellular stress, by reducing reactive oxygen species (ROS) and reactive nitrogen species (RNS). In the present investigation, synaptosomes prepared from rat brain were co-treated with melatonin (10μM) and Phyt (50μM) for 2h. Co-treatment of Phyt with melatonin significantly restored the altered levels of protein carbonyl (PC) contents and lipid peroxidation (LPO). It also replenished the Phyt-induced alterations on the levels of non-enzymatic antioxidant defence reduced glutathione (GSH), enzymatic antioxidants such as catalase (CAT) and superoxide dismutase (SOD) and synaptosomal integral enzymes such as AChE, Na⁺, K⁺-ATPase and MAO. We observed that Phyt induced oxidative stress in synaptosomes as indicated by an elevation in the generation of ROS and melatonin was able to inhibit the elevated ROS generation. Moreover, the neurotoxic effects elicited by Phyt on NO level and membrane potential were totally prevented by the treatment of melatonin. The results of our investigation emphasize the potential use of melatonin as a nutraceutical and mitigatory agent against Phyt-induced oxidative stress.

Milk fat biomarkers and cardiometabolic disease

PURPOSE OF REVIEW

Dairy is a major food group with potential impact on cardiometabolic health. Self-reported dairy intake has limitations that can partly be avoided by using biomarkers. This review aims to summarize the evidence of odd-chain saturated fatty acids (OCFAs), that is, pentadecanoic acid (C15 : 0) and heptadecanoic acid (17 : 0), as biomarkers of dairy fat intake. In addition, the associations of OCFA biomarkers with cardiometabolic disease will be overviewed.

RECENT FINDINGS

Adipose tissue 15 : 0 is the preferred biomarker but also circulating 15 : 0, and to a weaker extent 17 : 0, reflects both habitual and changes in dairy intake. Whereas results from studies assessing cardiovascular outcomes are inconsistent, OCFA biomarkers are overall associated with lower diabetes risk. Residual confounding should however be considered until interventional data and mechanisms are available. Although OCFA biomarkers mainly reflect dairy fat intake, recently proposed endogenous synthesis and metabolism do motivate further research.

SUMMARY

Taking into account the study population diet and limitations of OCFA biomarkers, both adipose and circulating levels of 15 : 0, in particular, are useful for estimating total dairy fat intake. OCFA biomarkers are overall not linked to cardiovascular disease risk, but a possible beneficial role of dairy foods in diabetes prevention warrant further study.

Phytanoyl-CoA Hydroxylase: A 2-Oxoglutarate-Dependent Dioxygenase Crucial for Fatty Acid Alpha-Oxidation in Humans

Phytanoyl-CoA hydroxylase belongs to the family of 2-oxoglutarate-dependent dioxygenases and plays a crucial role in the α-oxidation of fatty acids. The complete α-oxidation pathway involves five different enzymes localized in peroxisomes. Thus far, phytanoyl-CoA hydroxylase deficiency has remained the only genetically determined inborn error of metabolism affecting the α-oxidation pathway. In this chapter we describe the current state of knowledge on fatty acid α-oxidation with special emphasis on phytanoyl-CoA hydroxylase and its deficiency in Refsum disease.

A review of odd-chain fatty acid metabolism and the role of pentadecanoic Acid (c15:0) and heptadecanoic Acid (c17:0) in health and disease

The role of C17:0 and C15:0 in human health has recently been reinforced following a number of important biological and nutritional observations. Historically, odd chain saturated fatty acids (OCS-FAs) were used as internal standards in GC-MS methods of total fatty acids and LC-MS methods of intact lipids, as it was thought their concentrations were insignificant in humans. However, it has been thought that increased consumption of dairy products has an association with an increase in blood plasma OCS-FAs. However, there is currently no direct evidence but rather a casual association through epidemiology studies. Furthermore, a number of studies on cardiometabolic diseases have shown that plasma concentrations of OCS-FAs are associated with lower disease risk, although the mechanism responsible for this is debated. One possible mechanism for the endogenous production of OCS-FAs is α-oxidation, involving the activation, then hydroxylation of the α-carbon, followed by the removal of the terminal carboxyl group. Differentiation human adipocytes showed a distinct increase in the concentration of OCS-FAs, which was possibly caused through α-oxidation. Further evidence for an endogenous pathway, is in human plasma, where the ratio of C15:0 to C17:0 is approximately 1:2 which is contradictory to the expected levels of C15:0 to C17:0 roughly 2:1 as detected in dairy fat. We review the literature on the dietary consumption of OCS-FAs and their potential endogenous metabolism.

The functions of cardiolipin in cellular metabolism-potential modifiers of the Barth syndrome phenotype

The phospholipid cardiolipin (CL) plays a role in many cellular functions and signaling pathways both inside and outside of mitochondria. This review focuses on the role of CL in energy metabolism. Many reactions of electron transport and oxidative phosphorylation, the transport of metabolites required for these processes, and the stabilization of electron transport chain supercomplexes require CL. Recent studies indicate that CL is required for the synthesis of iron-sulfur (Fe-S) co-factors, which are essential for numerous metabolic pathways. Activation of carnitine shuttle enzymes that are required for fatty acid metabolism is CL dependent. The presence of substantial amounts of CL in the peroxisomal membrane suggests that CL may be required for peroxisomal functions. Understanding the role of CL in energy metabolism may identify physiological modifiers that exacerbate the loss of CL and underlie the variation in symptoms observed in Barth syndrome, a genetic disorder of CL metabolism.

Metabolism of γ-hydroxybutyrate in perfused rat livers

GHB (γ-hydroxybutyrate) is both a neurotransmitter and a drug of abuse (date-rape drug). We investigated the catabolism of this compound in perfused rat livers. Using a combination of metabolomics and mass isotopomer analysis, we showed that GHB is metabolized by multiple processes, in addition to its previously reported metabolism in the citric acid cycle via oxidation to succinate. A substrate cycle operates between GHB and γ-aminobutyrate via succinic semialdehyde. Also, GHB undergoes (i) β-oxidation to glycolyl-CoA+acetyl-CoA, (ii) two parallel processes which remove C-1 or C-4 of GHB and form 3-hydroxypropionate from C-2+C-3+C-4 or from C-1+C-2+C-3 of GHB, and (iii) degradation to acetyl-CoA via 4-phosphobutyryl-CoA. The present study illustrates the potential of the combination of metabolomics and mass isotopomer analysis for pathway discovery.

Stereospecificity of fatty acid 2-hydroxylase and differential functions of 2-hydroxy fatty acid enantiomers

FA 2-hydroxylase (FA2H) is an NAD(P)H-dependent enzyme that initiates FA α oxidation and is also responsible for the biosynthesis of 2-hydroxy FA (2-OH FA)-containing sphingolipids in mammalian cells. The 2-OH FA is chiral due to the asymmetric carbon bearing the hydroxyl group. Our current study performed stereochemistry investigation and showed that FA2H is stereospecific for the production of (R)-enantiomers. FA2H knockdown in adipocytes increases diffusional mobility of raft-associated lipids, leading to reduced GLUT4 protein level, glucose uptake, and lipogenesis. The effects caused by FA2H knockdown were reversed by treatment with exogenous (R)-2-hydroxy palmitic acid, but not with the (S)-enantiomer. Further analysis of sphingolipids demonstrated that the (R)-enantiomer is enriched in hexosylceramide whereas the (S)-enantiomer is preferentially incorporated into ceramide, suggesting that the observed differential effects are in part due to synthesis of sphingolipids containing different 2-OH FA enantiomers. These results may help clarify the mechanisms underlying the recently identified diseases associated with FA2H mutations in humans and may lead to potential pharmaceutical and dietary treatments. This study also provides critical information to help study functions of 2-OH FA enantiomers in FA α oxidation and possibly other sphingolipid-independent pathways.

Phytanic acid disturbs mitochondrial homeostasis in heart of young rats: a possible pathomechanism of cardiomyopathy in Refsum disease

Phytanic acid (Phyt) accumulates in tissues and biological fluids of patients affected by Refsum disease. Although cardiomyopathy is an important clinical manifestation of this disorder, the mechanisms of heart damage are poorly known. In the present study, we investigated the in vitro effects of Phyt on important parameters of oxidative stress in heart of young rats. Phyt significantly increased thiobarbituric acid-reactive substances levels (P < 0.001) and carbonyl formation (P < 0.01), indicating that this fatty acid induces lipid and protein oxidative damage, respectively. In contrast, Phyt did not alter sulfhydryl oxidation. Phyt also decreased glutathione (GSH) concentrations (P < 0.05), an important non-enzymatic antioxidant defense. Moreover, Phyt increased 2',7'-dichlorofluorescin oxidation (DCFH) (P < 0.01), reflecting increased reactive species generation. We also found that the induced lipid and protein oxidative damage, as well as the decreased GSH levels and increased DCFH oxidation provoked by this fatty acid were prevented or attenuated by the reactive oxygen species scavengers melatonin, trolox, and GSH, but not by the nitric oxide inhibitor N: (ω)-nitro-L: -arginine methyl ester, suggesting that reactive oxygen species were involved in these effects. Next, we verified that Phyt strongly inhibited NADH-cytochrome c oxidoreductase (complex I-III) activity (P < 0.001) in heart supernatants, and decreased membrane potential and the NAD(P)H pool in heart mitochondria, indicating that Phyt acts as a metabolic inhibitor and as an uncoupler of the electron transport chain. Therefore, it can be presumed that disturbance of cellular energy and redox homeostasis induced by Phyt may possibly contribute to the cardiomyopathy found in patients affected by Refsum disease.

Alkyl-glycerol rescues plasmalogen levels and pathology of ether-phospholipid deficient mice

A deficiency of plasmalogens, caused by impaired peroxisomal metabolism affects normal development and multiple organs in adulthood. Treatment options aimed at restoring plasmalogen levels may be relevant for the therapy of peroxisomal and non-peroxisomal disorders. In this study we determined the in vivo efficacy of an alkyl glycerol (AG), namely, 1-O-octadecyl-rac-glycerol, as a therapeutic agent for defects in plasmalogen synthesis. To achieve this, Pex7 knockout mice, a mouse model for Rhizomelic Chondrodysplasia Punctata type 1 characterized by the absence of plasmalogens, and WT mice were fed a control diet or a diet containing 2% alkyl-glycerol. Plasmalogen levels were measured in target organs and the biochemical data were correlated with the histological analysis of affected organs. Plasmalogen levels in all peripheral tissues of Pex7 KO mice fed the AG diet for 2 months normalized to the levels of AG fed WT mice. In nervous tissues of Pex7 KO mice fed the AG-diet, plasmalogen levels were significantly increased compared to control fed KO mice. Histological analysis of target organs revealed that the AG-diet was able to stop the progression of the pathology in testis, adipose tissue and the Harderian gland. Interestingly, the latter tissues are characterized by the presence of lipid droplets which were absent or reduced in size and number when ether-phospholipids are lacking, but which can be restored with the AAG treatment. Furthermore, nerve conduction in peripheral nerves was improved. When given prior to the occurrence of major pathological changes, the AG-diet prevented or ameliorated the pathology observed in Pex7 KO mice depending on the degree of plasmalogen restoration. This study provides evidence of the beneficial effects of treating a plasmalogen deficiency with alkyl-glycerol.

Chance and serendipity in science: two examples from my own career

The usual scientific paper follows a rather narrowly (but not ever rigidly) defined pattern. Both the author and the journal like to see a linear logical presentation of a "story." Seldom does the paper give the reader the "backstory." Where did the idea come from in the first place? How many false leads led down blind alleys? What happened by chance and what by logical planning? Was there an element of serendipity involved? Perhaps as we enter the paperless era and do not have to count words quite so religiously, it may be possible to encourage a more freewheeling scientific paper, but for now, we have to rely on the historians of science and/or those who "tell all" about their own research. "Reflections" seems an appropriate space for the latter. I have chosen two scenarios from my own career in which happy accidents played important roles but, unhappily, received little recognition in my published papers.

Analysis of a range of catabolic mutants provides evidence that phytanoyl-coenzyme A does not act as a substrate of the electron-transfer flavoprotein/electron-transfer flavoprotein:ubiquinone oxidoreductase complex in Arabidopsis during dark-induced senescence

The process of dark-induced senescence in plants is not fully understood, however, the functional involvement of an electron-transfer flavoprotein/electron-transfer flavoprotein:ubiquinone oxidoreductase (ETF/ETFQO), has been demonstrated. Recent studies have revealed that the enzymes isovaleryl-coenzyme A (CoA) dehydrogenase and 2-hydroxyglutarate dehydrogenase act as important electron donors to this complex. In addition both enzymes play a role in the breakdown of cellular carbon storage reserves with isovaleryl-CoA dehydrogenase being involved in degradation of the branched-chain amino acids, phytol, and lysine while 2-hydroxyglutarate dehydrogenase is exclusively involved in lysine degradation. Given that the chlorophyll breakdown intermediate phytanoyl-CoA accumulates dramatically both in knockout mutants of the ETF/ETFQO complex and of isovaleryl-CoA dehydrogenase following growth in extended dark periods we have investigated the direct importance of chlorophyll breakdown for the supply of carbon and electrons during this process. For this purpose we isolated three independent Arabidopsis (Arabidopsis thaliana) knockout mutants of phytanoyl-CoA 2-hydroxylase and grew them under the same extended darkness regime as previously used. Despite the fact that these mutants accumulated phytanoyl-CoA and also 2-hydroxyglutarate they exhibited no morphological changes in comparison to the other mutants previously characterized. These results are consistent with a single entry point of phytol breakdown into the ETF/ETFQO system and furthermore suggest that phytol is not primarily metabolized by this pathway. Furthermore analysis of isovaleryl-CoA dehydrogenase/2-hydroxyglutarate dehydrogenase double mutants generated here suggest that these two enzymes essentially account for the entire electron input via the ETF complex.

Phytanic acid metabolism in health and disease

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid which cannot be beta-oxidized due to the presence of the first methyl group at the 3-position. Instead, phytanic acid undergoes alpha-oxidation to produce pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) plus CO(2). Pristanic acid is a 2-methyl branched-chain fatty acid which can undergo beta-oxidation via sequential cycles of beta-oxidation in peroxisomes and mitochondria. The mechanism of alpha-oxidation has been resolved in recent years as reviewed in this paper, although some of the individual enzymatic steps remain to be identified. Furthermore, much has been learned in recent years about the permeability properties of the peroxisomal membrane with important consequences for the alpha-oxidation process. Finally, we present new data on the omega-oxidation of phytanic acid making use of a recently generated mouse model for Refsum disease in which the gene encoding phytanoyl-CoA 2-hydroxylase has been disrupted.

Fatty acid omega-oxidation as a rescue pathway for fatty acid oxidation disorders in humans

Fatty acids (FAs) can be degraded via different mechanisms including α-, β- and ω-oxidation. In humans, a range of different genetic diseases has been identified in which either mitochondrial FA β-oxidation, peroxisomal FA β-oxidation or FA α-oxidation is impaired. Treatment options for most of these disorders are limited. This has prompted us to study FA ω-oxidation as a rescue pathway for these disorders, based on the notion that if the ω-oxidation of specific FAs could be upregulated one could reduce the accumulation of these FAs and the subsequent detrimental effects in the different groups of disorders. In this minireview, we describe our current state of knowledge in this area with special emphasis on Refsum disease and X-linked adrenoleukodystrophy.

The enzymology of mitochondrial fatty acid beta-oxidation and its application to follow-up analysis of positive neonatal screening results

Oxidation of fatty acids in mitochondria is a key physiological process in higher eukaryotes including humans. The importance of the mitochondrial beta-oxidation system in humans is exemplified by the existence of a group of genetic diseases in man caused by an impairment in the mitochondrial oxidation of fatty acids. Identification of patients with a defect in mitochondrial beta-oxidation has long remained notoriously difficult, but the introduction of tandem-mass spectrometry in laboratories for genetic metabolic diseases has revolutionalized the field by allowing the rapid and sensitive analysis of acylcarnitines. Equally important is that much progress has been made with respect to the development of specific enzyme assays to identify the enzyme defect in patients subsequently followed by genetic analysis. In this review, we will describe the current state of knowledge in the field of fatty acid oxidation enzymology and its application to the follow-up analysis of positive neonatal screening results.

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.

Fatty acid 2-Hydroxylation in mammalian sphingolipid biology

2-Hydroxy fatty acids (hFA) are important components of a subset of mammalian sphingolipids. The presence of hFA in sphingolipids is best described in the nervous system, epidermis, and kidney. However, the literature also indicates that various hFA-sphingolipids are present in additional tissues and cell types, as well as in tumors. Biosynthesis of hFA-sphingolipids requires fatty acid 2-hydroyxlase, and degradation of hFA-sphingolipids depends, at least in part, on lysosomal acid ceramidase and the peroxisomal fatty acid alpha-oxidation pathway. Mutations in the fatty acid 2-hydroxylase gene, FA2H, have been associated with leukodystrophy and spastic paraparesis in humans, underscoring the importance of hFA-sphingolipids in the nervous system. In the epidermis, hFA-ceramides are essential for the permeability barrier function. Physiological function of hFA-sphingolipids in other organs remains largely unknown. Recent evidence indicates that hFA-sphingolipids have specific roles in cell signaling.

Biosynthesis of 2-hydroxy and iso-even fatty acids is connected to sphingolipid formation in myxobacteria

2-Hydroxy fatty acids can be found in several different organisms, including bacteria. In this study, we have studied the biosynthesis of 2-hydroxy fatty acids in the myxobacteria Myxococcus xanthus and Stigmatella aurantiaca, resulting in the identification of a family of stereospecific fatty acid alpha-hydroxylases. Although the stereospecificities of the hydroxylases differ between these two species, they share a common function in supporting fatty acid alpha-oxidation; that is, the oxidative shortening of fatty acids. Whereas in S. aurantiaca this process takes place during normal vegetative growth, in M. xanthus it takes place only under developmental conditions. We were also able to identify serine palmitoyltransferase encoding genes involved in sphingolipid biosynthesis as well as sphingolipids themselves in both types of myxobacteria, and were able to show that the alpha-hydroxylation reaction is in fact dependent on the presence of fatty acids bound to sphingolipids.

Refsum disease due to the splice-site mutation c.135-2A>G before exon 3 of the PHYH gene, diagnosed eight years after detection of retinitis pigmentosa

OBJECTIVES

If Refsum disease (RD) is not considered as a differential at onset of the initial manifestations the diagnosis of RD remains unrecognized for a long time as in the following case.

CASE REPORT

A 55-y old Caucasian female with hyperextensible joints developed progressive visual impairment due to retinitis pigmentosa and sensorimotor polyneuropathy of the lower limbs since age 32 y. Screening for causes of polyneuropathy at age 40 y revealed markedly elevated serum phytanic acid (PA) with a maximum value of 293.6 microg/ml (n:<6 microg/ml) why RD was diagnosed. Since age 48 y slowly progressive hypacusis was noted. RD was caused by the known transition A135G in exon 3 of the PHYH gene. Additionally, the polymorphism T153C in exon 3 of the PHYH gene was detected. Upon strict adherence to the Chelsea diet PA levels slightly decreased since onset of this therapy.

CONCLUSION

This case confirms that RD remains unrecognized for a long time if RD is not considered as a differential of retinitis pigmentosa as the initial manifestation of the disease. Early recognition of RD is important since there is the therapeutic option of starting a diet.

Alpha-methylacyl-CoA racemase--an 'obscure' metabolic enzyme takes centre stage

Branched-chain lipids are important components of the human diet and are used as drug molecules, e.g. ibuprofen. Owing to the presence of methyl groups on their carbon chains, they cannot be metabolized in mitochondria, and instead are processed and degraded in peroxisomes. Several different oxidative degradation pathways for these lipids are known, including alpha-oxidation, beta-oxidation, and omega-oxidation. Dietary branched-chain lipids (especially phytanic acid) have attracted much attention in recent years, due to their link with prostate, breast, colon and other cancers as well as their role in neurological disease. A central role in all the metabolic pathways is played by alpha-methylacyl-CoA racemase (AMACR), which regulates metabolism of these lipids and drugs. AMACR catalyses the chiral inversion of a diverse number of 2-methyl acids (as their CoA esters), and regulates the entry of branched-chain lipids into the peroxisomal and mitochondrial beta-oxidation pathways. This review brings together advances in the different disciplines, and considers new research in both the metabolism of branched-chain lipids and their role in cancer, with particular emphasis on the crucial role played by AMACR. These recent advances enable new preventative and treatment strategies for cancer.

Quantitative proteomic comparison of mouse peroxisomes from liver and kidney

The peroxisome plays a central role in the catabolic and anabolic pathways that contribute to the lipid homeostasis. Besides this main function, this organelle has gained functional diversity. Although several approaches have been used for peroxisomal proteome analysis, a quantitative protein expression analysis of peroxisomes from different tissues has not been elucidated yet. Here, we applied a 2-DE-based method on mouse liver and kidney peroxisomal enriched fractions to study the tissue-dependent protein expression. Ninety-one spots were identified from the 2-DE maps from pH 3.0-10.0 and 51 spots from the basic range corresponding to 31 peroxisomal proteins, 10 putative peroxisomal, 6 cytosolic, 17 mitochondrial and 1 protein from endoplasmic reticulum. Based on the identification and on the equivalent quality of both tissue preparations, the differences emerging from the comparison could be quantified. In liver, proteins involved in pathways such as alpha- and beta-oxidation, isoprenoid biosynthesis, amino acid metabolism and purine and pyrimidine metabolism were more abundant whereas in kidney, proteins from the straight-chain fatty acid beta-oxidation were highly expressed. These results indicate that tissue-specific functional classes of peroxisomal proteins could be relevant to study peroxisomal cellular responses or pathologies. Finally, a web-based peroxisomal proteomic database was built.

Biochemistry of mammalian peroxisomes revisited

In this review, we describe the current state of knowledge about the biochemistry of mammalian peroxisomes, especially human peroxisomes. The identification and characterization of yeast mutants defective either in the biogenesis of peroxisomes or in one of its metabolic functions, notably fatty acid beta-oxidation, combined with the recognition of a group of genetic diseases in man, wherein these processes are also defective, have provided new insights in all aspects of peroxisomes. As a result of these and other studies, the indispensable role of peroxisomes in multiple metabolic pathways has been clarified, and many of the enzymes involved in these pathways have been characterized, purified, and cloned. One aspect of peroxisomes, which has remained ill defined, is the transport of metabolites across the peroxisomal membrane. Although it is clear that mammalian peroxisomes under in vivo conditions are closed structures, which require the active presence of metabolite transporter proteins, much remains to be learned about the permeability properties of mammalian peroxisomes and the role of the four half ATP-binding cassette (ABC) transporters therein.

Brain pyruvate and 2-oxoglutarate dehydrogenase complexes are mitochondrial targets of the CoA ester of the Refsum disease marker phytanic acid

Pyruvate and 2-oxoglutarate dehydrogenase complexes are strongly inhibited by phytanoyl-CoA (IC(50) approximately 10(-6)-10(-7) M). Palmitoyl-CoA is 10-fold less potent. Phytanic or palmitic acids have no inhibitory effect up to 0.3 mM. At the substrate saturation, the acyl-CoA's affect the first and second enzymatic components of the 2-oxoglutarate dehydrogenase complex, while the third component is inhibited only at a low saturation with its substrate dihydrolipoamide. Thus, key regulatory branch points of mitochondrial metabolism are targets of a cellular derivative of phytanic acid. Decreased activity of the complexes might therefore contribute to neurological symptoms upon accumulation of phytanic acid in Refsum disease.

CYP4 isoform specificity in the omega-hydroxylation of phytanic acid, a potential route to elimination of the causative agent of Refsum's disease

The saturated C20 isoprenoid phytanic acid is physiologically derived from phytol released in the degradation of chlorophyll. The presence of a C-3 methyl group in this substrate blocks normal beta-oxidation, so phytanic acid degradation primarily occurs by initial peroxisomal alpha-oxidation to shift the register of the methyl group. However, individuals with Refsum's disease are genetically deficient in the required phytanoyl-CoA alpha-hydroxylase and suffer from neurological pathologies caused by the accumulation of phytanic acid. Recent work has shown that phytanic acid can also be catabolized by a pathway initiated by omega-hydroxylation of the hydrocarbon chain, followed by oxidation of the alcohol to the acid and conventional beta-oxidation. However, the enzymes responsible for the omega-hydroxylation of phytanic acid have not been identified. In this study, we have determined the activities of all of the rat and human CYP4A enzymes and two of the rat CYP4F enzymes, with respect to the omega-hydroxylation of phytanic acid. Furthermore, we have shown that the ability to omega-hydroxylate phytanic acid is elevated in microsomes from rats pretreated with clofibrate. The results support a possible role for CYP4 enzyme elevation in the elimination of phytanic acid in Refsum's disease patients.

Phytanic acid, AMACR and prostate cancer risk

The growing body of knowledge in cancer prevention demonstrates that for many cancers, risk must be defined in terms of both environmental and genetic factors. In prostate cancer, there is increasing evidence linking risk with polymorphisms in the alpha-methylacyl-CoA racemase (AMACR) gene and branched-chain fatty acids derived from specific sources of dietary fats. We are now at the point where we can begin to conceptualize possible inter-relationships between dietary and genetic risk as applied to prostate cancer, with the goal of generating testable hypotheses amenable to coordinated examinations. A greater understanding of such relationships should provide better ways to establish overall risk, to screen for the disease and perhaps to offer specific opportunities for prevention and treatment.

A study of the cytotoxicity of branched-chain phytanic acid with mitochondria and rat brain astrocytes

Phytanic acid, a saturated fatty acid of 20-carbon-atoms with isoprenoic structure, is formed from the phytol-side chain of chlorophyll in ruminants. Degradation of phytanic acid is blocked in Refsum disease by several enzymatic defects of peroxisomal degradation of branched-chain fatty acids. Refsum disease is an inherited neurological disorder progressively developing from early childhood to adultness. Clinical signs are attributed to toxicity of phytanic acid, which accumulates to unusually high levels in the tissue and serum of patients suffering from untreated Refsum disease. We report here that hippocampal astrocytes isolated from rat brain, which were exposed to phytanic acid (50 microM) die within a few hours. In situ depolarization of mitochondria and an increase of cytosolic Ca2+ precede cell death. Therefore, we also investigated the influence of phytanic acid on physiology of mitochondria isolated from rat brain. Mitochondria become functionally impaired by phytanic acid, as indicated by uncoupling (resting state), inhibition of the electron transport (state 3), stimulation of ROS-generation, decline of Ca2+ loading and severe release of cytochrome c. Thus, phytanic acid seems to initiate astrocyte cell death by activating the mitochondrial route of apoptosis.

Polyunsaturated fats attenuate the dietary phytol-induced increase in hepatic fatty acid oxidation in mice

The effects of dietary phytol and the type of dietary fat on hepatic fatty acid oxidation were examined in male ICR mice. Mice were fed diets containing 0 or 5 g/kg phytol and 100 g/kg palm, safflower, or fish oil for 21 d. Among the groups fed phytol-free diets, the activities and mRNA abundance of various enzymes involved in fatty acid oxidation were greater in mice fed fish oil than in those fed palm or safflower oil. Dietary phytol profoundly increased the activities and mRNA abundance of hepatic fatty acid oxidation enzymes in mice fed palm oil. However, safflower and fish oils, especially the latter, greatly attenuated the phytol-dependent increase in hepatic fatty acid oxidation. The hepatic concentration of phytanic acid, a metabolite of phytol that is the ligand and activator of retinoid X receptors and peroxisome proliferator-activated receptors, was higher in mice fed fish oil than safflower or palm oil, and in those administered safflower oil than palm oil. The hepatic mRNA abundance of sterol carrier protein-2, a lipid-binding protein involved in phytol metabolism, was inversely correlated with the hepatic concentration of phytanic acid. We demonstrated that polyunsaturated fats attenuate the enhancing effect of dietary phytol on hepatic fatty acid oxidation. Dietary fat-dependent changes in the hepatic phytanic acid concentration cannot account for this phenomenon.

Absorption, Distribution, Metabolism, and Elimination of 8-2 Fluorotelomer Alcohol in the Rat

The absorption, distribution, metabolism, and elimination of [3-14C] 8-2 fluorotelomer alcohol (8-2 FTOH, C7F1514CF2CH2CH2OH) following a single oral dose at 5 and 125 mg/kg in male and female rats have been determined. Following oral dosing, the maximum concentration of 8-2 FTOH in plasma occurred by 1 h postdose and cleared rapidly with a half-life of less than 5 h. The internal dose to 8-2 FTOH, as measured by area under the concentration-time curve to infinity, was similar for male and female rats and was observed to increase in a dose-dependent fashion. The majority of the 14C 8-2 FTOH (> 70%) was excreted in feces, and 37-55% was identified as parent. Less than 4% of the administered dose was excreted in urine, which contained low concentrations of perfluorooctanoate (approximately 1% of total 14C). Metabolites identified in bile were principally composed of glucuronide and glutathione conjugates, and perfluorohexanoate was identified in excreta and plasma, demonstrating the metabolism of the parent FTOH by sequential removal of multiple CF2 groups. At 7 days postdose, 4-7% of the administered radioactivity was present in tissues, and for the majority, 14C concentrations were greater than whole blood with the highest concentration in fat, liver, thyroid, and adrenals. Distribution and excretion of a single 125-mg/kg [3-14C] 8-2 FTOH dermal dose following a 6-h exposure in rats was also determined. The majority of the dermal dose either volatilized from the skin (37%) or was removed by washing (29%). Following a 6-h dermal exposure and a 7-day collection period, excretion of total radioactivity via urine (< 0.1%) and feces (< 0.2%) was minor, and radioactivity concentrations in most tissues were below the limit of detection. Systemic availability of 8-2 FTOH following dermal exposure was negligible.

Peroxisomal trans-2-enoyl-CoA reductase is involved in phytol degradation

Phytol is a naturally occurring precursor of phytanic acid. The last step in the conversion of phytol to phytanoyl-CoA is the reduction of phytenoyl-CoA mediated by an, as yet, unidentified enzyme. A candidate for this reaction is a previously described peroxisomal trans-2-enoyl-CoA reductase (TER). To investigate this, human TER was expressed in E. coli as an MBP-fusion protein. The purified recombinant protein was shown to have high reductase activity towards trans-phytenoyl-CoA, but not towards the peroxisomal beta-oxidation intermediates C24:1-CoA and pristenoyl-CoA. In conclusion, our results show that human TER is responsible for the reduction of phytenoyl-CoA to phytanoyl-CoA in peroxisomes.

Mechanism of toxicity of the branched‐chain fatty acid phytanic acid, a marker of Refsum disease, in astrocytes involves mitochondrial impairment

Phytanic acid is a saturated branched-chain fatty acid, which is formed by bacterial degradation of chlorophyll in the intestinal tract of ruminants. The methyl group in beta-position prevents degradation of phytanic acid by the beta-oxidation pathway. Therefore, degradation of phytanic acid is initiated by alpha-oxidation in peroxisomes. The inherited peroxisomal disorder Refsum disease is characterised by accumulation of phytanic acid. Unusually high concentrations of phytanic acid can be found in the plasma of Refsum disease patients, who suffer from neurodegeneration and muscle dystrophy. Phytanic acid has been suggested to be causally involved in the clinical symptoms. To elucidate the pathogenic mechanism, we investigated the influence of phytanic acid in rat hippocampal astrocytes by monitoring the cytosolic Ca(2+) concentration, the mitochondrial membrane potential (Deltapsi(m)), the generation of reactive oxygen species as well as the cellular ATP level. In response to phytanic acid (100 microM) cytosolic Ca(2+) was quickly increased. The phytanic acid-evoked Ca(2+) response was transient and involved activation of intracellular Ca(2+) stores. In isolated rat brain mitochondria, phytanic acid dissipated Deltapsi(m) in a reversible and dose-dependent manner. Moreover, phytanic acid released cytochrome c from mitochondria. Depending on the mitochondrial activity state, phytanic acid either stimulated or inhibited the electron flux within the respiratory chain. In addition, phytanic acid induced substantial generation of reactive oxygen species in isolated mitochondria as well as in intact cells. Phytanic acid caused cell death of astrocytes within a few hours of exposure. In conclusion, we suggest that phytanic acid initiates astrocyte cell death by activating the mitochondrial route of apoptosis.

Rotenone-like action of the branched-chain phytanic acid induces oxidative stress in mitochondria

Phytanic acid (Phyt) increase is associated with the hereditary neurodegenerative Refsum disease. To elucidate the still unclear toxicity of Phyt, mitochondria from brain and heart of adult rats were exposed to free Phyt. Phyt at low micromolar concentrations (maximally: 100 nmol/mg of protein) enhances superoxide (O(2)(.))(2) generation. Phyt induces O(2)(.) in state 3 (phosphorylating), as well as in state 4 (resting). Phyt stimulates O(2)(.) generation when the respiratory chain is fed with electrons derived from oxidation of glutamate/malate, pyruvate/malate, or succinate in the presence of rotenone. With succinate alone, Phyt suppresses O(2)(.) generation caused by reverse electron transport from succinate to complex I. The enhanced O(2)(.) generation by Phyt in state 4 is in contrast to the mild uncoupling concept. In this concept uncoupling by nonesterified fatty acids should abolish O(2)(.) generation. Stimulation of O(2)(.) generation by Phyt is paralleled by inhibition of the electron transport within the respiratory chain or electron leakage from the respiratory chain. The interference of Phyt with the electron transport was demonstrated by inhibition of state 3- and p-trifluoromethoxyphenylhydrazone (FCCP)-dependent respiration, inactivation of the NADH-ubiquinone oxidoreductase complex in permeabilized mitochondria, decrease in reduction of the synthetic electron acceptor 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide in state 4, and increase of the mitochondrial NAD(P)H level in FCCP-uncoupled mitochondria. Thus, we suggest that complex I is the main site of Phyt-stimulated O(2)(.) generation. Furthermore, inactivation of aconitase and oxidation of the mitochondrial glutathione pool show that enhanced O(2)(.) generation with chronic exposure to Phyt causes oxidative damage.

Archaeal C80 isoprenoid tetraacids responsible for naphthenate deposition in crude oil processing

The structure of a novel class of octaterpene tetracarboxylic acids which is responsible for naphthenate deposition in crude oil processing has been determined by NMR and mass spectroscopy.

Crystallization and preliminary X-ray crystallographic studies of fatty acid-CoA racemase from Mycobacterium tuberculosis H37Rv

Fatty acid-CoA racemase plays an important role in the beta-oxidation of branched-chain fatty acids and fatty-acid derivatives as it catalyzes the conversion of several (2R)-branched-chain fatty acid-CoAs to their (2S)-stereoisomers. Fatty acid-CoA racemase from Mycobacterium tuberculosis H37Rv has been purified to homogeneity and crystallized by the hanging-drop vapour-diffusion method with polyethylene glycol 4000 as precipitant. The crystals belong to the trigonal space group P3(1) or P3(2), with unit-cell parameters a = b = 109.56, c = 147.97 A. The asymmetric unit contains six monomers, corresponding to a VM value of 2.15 A3 Da(-1). A complete native data set has been collected at 2.7 A resolution using a synchrotron-radiation source.

Structure of human phytanoyl-CoA 2-hydroxylase identifies molecular mechanisms of Refsum disease

Refsum disease (RD), a neurological syndrome characterized by adult onset retinitis pigmentosa, anosmia, sensory neuropathy, and phytanic acidaemia, is caused by elevated levels of phytanic acid. Many cases of RD are associated with mutations in phytanoyl-CoA 2-hydroxylase (PAHX), an Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase that catalyzes the initial alpha-oxidation step in the degradation of phytenic acid in peroxisomes. We describe the x-ray crystallographic structure of PAHX to 2.5 A resolution complexed with Fe(II) and 2OG and predict the molecular consequences of mutations causing RD. Like other 2OG oxygenases, PAHX possesses a double-stranded beta-helix core, which supports three iron binding ligands (His(175), Asp(177), and His(264)); the 2-oxoacid group of 2OG binds to the Fe(II) in a bidentate manner. The manner in which PAHX binds to Fe(II) and 2OG together with the presence of a cysteine residue (Cys(191)) 6.7 A from the Fe(II) and two further histidine residues (His(155) and His(281)) at its active site distinguishes it from that of the other human 2OG oxygenase for which structures are available, factor inhibiting hypoxia-inducible factor. Of the 15 PAHX residues observed to be mutated in RD patients, 11 cluster in two distinct groups around the Fe(II) (Pro(173), His(175), Gln(176), Asp(177), and His(220)) and 2OG binding sites (Trp(193), Glu(197), Ile(199), Gly(204), Asn(269), and Arg(275)). PAHX may be the first of a new subfamily of coenzyme A-binding 2OG oxygenases.

The critical role of Arabidopsis electron-transfer flavoprotein:ubiquinone oxidoreductase during dark-induced starvation

In mammals, electron-transfer flavoprotein:ubiquinone oxidoreductase (ETFQO) and electron-transfer flavoprotein (ETF) are functionally associated, and ETF accepts electrons from at least nine mitochondrial matrix flavoprotein dehydrogenases and transfers them to ubiquinone in the inner mitochondrial membrane. In addition, the mammalian ETF/ETFQO system plays a key role in beta-oxidation of fatty acids and catabolism of amino acids and choline. By contrast, nothing is known of the function of ETF and ETFQO in plants. Sequence analysis of the unique Arabidopsis thaliana homologue of ETFQO revealed high similarity to the mammalian ETFQO protein. Moreover, green fluorescent protein cellular localization experiments suggested a mitochondrial location for this protein. RNA gel blot analysis revealed that Arabidopsis ETFQO transcripts accumulated in long-term dark-treated leaves. Analysis of three independent insertional mutants of Arabidopsis ETFQO revealed a dramatic reduction in their ability to withstand extended darkness, resulting in senescence and death within 10 d after transfer, whereas wild-type plants remained viable for at least 15 d. Metabolite profiling of dark-treated leaves of the wild type and mutants revealed a dramatic decline in sugar levels. In contrast with the wild type, the mutants demonstrated a significant accumulation of several amino acids, an intermediate of Leu catabolism, and, strikingly, high-level accumulation of phytanoyl-CoA. These data demonstrate the involvement of a mitochondrial protein, ETFQO, in the catabolism of Leu and potentially of other amino acids in higher plants and also imply a novel role for this protein in the chlorophyll degradation pathway activated during dark-induced senescence and sugar starvation.

Characterization of the final step in the conversion of phytol into phytanic acid

Phytol is a branched-chain fatty alcohol that is a naturally occurring precursor of phytanic acid, a fatty acid involved in the pathogenesis of Refsum disease. The conversion of phytol into phytanic acid is generally believed to take place via three enzymatic steps that involve 1) oxidation to its aldehyde, 2) further oxidation to phytenic acid, and 3) reduction of the double bond at the 2,3 position, yielding phytanic acid. Our recent investigations of this mechanism have elucidated the enzymatic steps leading to phytenic acid production, but the final step of the pathway has not been investigated so far. In this study, we describe the characterization of phytenic acid reduction in rat liver. NADPH-dependent conversion of phytenic acid into phytanic acid was detected, although at a slow rate. However, it was shown that phytenic acid can be activated to its CoA ester and that reduction of phytenoyl-CoA is much more efficient than that of phytenic acid. Furthermore, in rat hepatocytes cultured in the presence of phytol, phytenoyl-CoA could be detected, showing that it is a bona fide intermediate of phytol degradation. Subcellular fractionation experiments revealed that phytenoyl-CoA reductase activity is present in peroxisomes and mitochondria. With these findings, we have accomplished the full elucidation of the mechanism by which phytol is converted into phytanic acid.

Molecular genetics of the ichthyoses

The ichthyoses are a large, clinically, genetically, and etiologically heterogeneous group of disorders of cornification due to abnormal differentiation and desquamation of the epidermis. Although they differ in clinical features, inheritance, and structural and biochemical abnormalities of the epidermis, they often pose a diagnostic challenge. For each of the 12 ichthyoses and related disorders described here, the major disease genes have been identified and genotype-phenotype correlation have begun to emerge. The molecular findings reveal the functional importance and interactions of many different epidermal proteins and metabolic pathways, including major structural proteins (keratins, loricrin), enzymes involved in lipid metabolism (transglutaminase 1, lipoxygenases, fatty aldehyde dehydrogenase, steroid sulfatase, glucocerebrosidase, Delta8-Delta7 sterol isomerase, 3beta-hydroxysteroid dehydrogenase), and protein catabolism (LEKTI), peroxisomal transport and processing (Peroxin 7 receptor, Phytanoyl-CoA hydroxylase) and DNA repair (proteins of the transcription repair complex). This review highlights the spectacular advances in the molecular genetics and biology of heritable ichthyoses over the past decade. It illustrates the power of molecular diagnostics for refining disease classification, providing prenatal diagnosis, improving genetic counseling, and clinical management.

In brain mitochondria the branched-chain fatty acid phytanic acid impairs energy transduction and sensitizes for permeability transition

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) accumulates at high levels throughout the body in the adult form of Refsum disease, a peroxisomal genetic disorder. However, it is still unclear why increased levels of phytanic acid have cytotoxic effects. In the present study, we examined the influence of non-esterified phytanic acid on energy-related functions of mitochondria from adult rat brain. Phytanic acid at low concentrations (5-20 microM, i.e. 5-20 nmol/mg of mitochondrial protein) de-energized mitochondria, as indicated by depolarization, stimulation of non-phosphorylating oxygen uptake and inhibition of the reduction of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide. The unbranched homologue palmitic acid exerted much smaller effects. In addition, phytanic acid reduced state 3 respiration, which was partly due to inhibition of the ADP/ATP carrier. Phytanic acid decreased the rate of adenine nucleotide exchange and increased the degree of control, which the ADP/ATP carrier has on state 3 respiration. Important for functional consequences is the finding that mitochondria, which are preloaded with small amounts of Ca2+ (100 nmol/mg of protein), became highly sensitized to rapid permeability transition even when only low concentrations of phytanic acid (below 5 microM) were applied. In conclusion, the incorporation of phytanic acid into the inner mitochondrial membrane increases the membrane H+ conductance and disturbs the protein-linked functions in energy coupling. This is most probably essential for the short-term toxicity of phytanic acid. Thus in neural tissue, which becomes enriched with phytanic acid, the reduction in mitochondrial ATP supply and the facilitation of the opening of the permeability transition pore are two major mechanisms by which the branched-chain fatty acid phytanic acid induces the onset of degenerative processes.