Metabolism of phytanic acid and 3-methyl-adipic acid excretion in patients with adult Refsum disease

The biochemistry and physiology of long-chain dicarboxylic acid metabolism

Mitochondrial β-oxidation is the most prominent pathway for fatty acid oxidation but alternative oxidative metabolism exists. Fatty acid ω-oxidation is one of these pathways and forms dicarboxylic acids as products. These dicarboxylic acids are metabolized through peroxisomal β-oxidation representing an alternative pathway, which could potentially limit the toxic effects of fatty acid accumulation. Although dicarboxylic acid metabolism is highly active in liver and kidney, its role in physiology has not been explored in depth. In this review, we summarize the biochemical mechanism of the formation and degradation of dicarboxylic acids through ω- and β-oxidation, respectively. We will discuss the role of dicarboxylic acids in different (patho)physiological states with a particular focus on the role of the intermediates and products generated through peroxisomal β-oxidation. This review is expected to increase the understanding of dicarboxylic acid metabolism and spark future research.

The Biomedical Importance of the Missing Pathway for Farnesol and Geranylgeraniol Salvage

Isoprenoids are the output of the polymerization of five-carbon, branched isoprenic chains derived from isopentenyl pyrophosphate (IPP) and its isomer, dimethylallyl pyrophosphate (DMAPP). Isoprene units are consecutively condensed to form longer structures such as farnesyl and geranylgeranyl pyrophosphate (FPP and GGPP, respectively), necessary for the biosynthesis of several metabolites. Polyprenyl transferases and synthases use polyprenyl pyrophosphates as their natural substrates; however, it is known that free polyprenols, such as farnesol (FOH), and geranylgeraniol (GGOH) can be incorporated into prenylated proteins, ubiquinone, cholesterol, and dolichols. Furthermore, FOH and GGOH have been shown to block the effects of isoprenoid biosynthesis inhibitors such as fosmidomycin, bisphosphonates, or statins in several organisms. This phenomenon is the consequence of a short pathway, which was observed for the first time more than 25 years ago: the polyprenol salvage pathway, which works via the phosphorylation of FOH and GGOH. Biochemical studies in bacteria, animals, and plants suggest that this pathway can be carried out by two enzymes: a polyprenol kinase and a polyprenyl-phosphate kinase. However, to date, only a few genes have been unequivocally identified to encode these enzymes in photosynthetic organisms. Nevertheless, pieces of evidence for the importance of this pathway abound in studies related to infectious diseases, cancer, dyslipidemias, and nutrition, and to the mitigation of the secondary effects of several drugs. Furthermore, nowadays it is known that both FOH and GGOH can be incorporated via dietary sources that produce various biological effects. This review presents, in a simplified but comprehensive manner, the most important data on the FOH and GGOH salvage pathway, stressing its biomedical importance The main objective of this review is to bring to light the need to discover and characterize the kinases associated with the isoprenoid salvage pathway in animals and pathogens.

Tissue Proteome of 2-Hydroxyacyl-CoA Lyase Deficient Mice Reveals Peroxisome Proliferation and Activation of ω-Oxidation

Peroxisomal fatty acid α-oxidation is an essential pathway for the degradation of β-carbon methylated fatty acids such as phytanic acid. One enzyme in this pathway is 2-hydroxyacyl CoA lyase (HACL1), which is responsible for the cleavage of 2-hydroxyphytanoyl-CoA into pristanal and formyl-CoA. Hacl1 deficient mice do not present with a severe phenotype, unlike mice deficient in other α-oxidation enzymes such as phytanoyl-CoA hydroxylase deficiency (Refsum disease) in which neuropathy and ataxia are present. Tissues from wild-type and Hacl1^−/− mice fed a high phytol diet were obtained for proteomic and lipidomic analysis. There was no phenotype observed in these mice. Liver, brain, and kidney tissues underwent trypsin digestion for untargeted proteomic liquid chromatography-mass spectrometry analysis, while liver tissues also underwent fatty acid hydrolysis, extraction, and derivatisation for fatty acid gas chromatography-mass spectrometry analysis. The liver fatty acid profile demonstrated an accumulation of phytanic and 2-hydroxyphytanic acid in the Hacl1^−/− liver and significant decrease in heptadecanoic acid. The liver proteome showed a significant decrease in the abundance of Hacl1 and a significant increase in the abundance of proteins involved in PPAR signalling, peroxisome proliferation, and omega oxidation, particularly Cyp4a10 and Cyp4a14. In addition, the pathway associated with arachidonic acid metabolism was affected; Cyp2c55 was upregulated and Cyp4f14 and Cyp2b9 were downregulated. The kidney proteome revealed fewer significantly upregulated peroxisomal proteins and the brain proteome was not significantly different in Hacl1^−/− mice. This study demonstrates the powerful insight brought by proteomic and metabolomic profiling of Hacl1^−/− mice in better understanding disease mechanism in fatty acid α-oxidation disorders.

Fibroblast-specific genome-scale modelling predicts an imbalance in amino acid metabolism in Refsum disease

Refsum disease (RD) is an inborn error of metabolism that is characterised by a defect in peroxisomal α‐oxidation of the branched‐chain fatty acid phytanic acid. The disorder presents with late‐onset progressive retinitis pigmentosa and polyneuropathy and can be diagnosed biochemically by elevated levels of phytanate in plasma and tissues of patients. To date, no cure exists for RD, but phytanate levels in patients can be reduced by plasmapheresis and a strict diet. In this study, we reconstructed a fibroblast‐specific genome‐scale model based on the recently published, FAD‐curated model, based on Recon3D reconstruction. We used transcriptomics (available via GEO database with identifier GSE138379), metabolomics and proteomics (available via ProteomeXchange with identifier PXD015518) data, which we obtained from healthy controls and RD patient fibroblasts incubated with phytol, a precursor of phytanic acid. Our model correctly represents the metabolism of phytanate and displays fibroblast‐specific metabolic functions. Using this model, we investigated the metabolic phenotype of RD at the genome scale, and we studied the effect of phytanate on cell metabolism. We identified 53 metabolites that were predicted to discriminate between healthy and RD patients, several of which with a link to amino acid metabolism. Ultimately, these insights in metabolic changes may provide leads for pathophysiology and therapy.

Databases

Transcriptomics data are available via GEO database with identifier GSE138379, and proteomics data are available via ProteomeXchange with identifier PXD015518.

Pregnancy outcome in Refsum disease: Affected fetuses and children born to an affected mother

We describe the case of a young woman, from a consanguineous family, affected by adult Refsum disease (ARD, OMIM#266500). ARD is a rare peroxisomal autosomal recessive disease due to deficient alpha-oxidation of phytanic acid (PA), a branched-chain fatty acid. The accumulation of PA in organs is thought to be responsible for disease symptoms. The patient presented only bilateral shortening of metatarsals and has been treated with a low-PA diet. She is homoallelic for the c.135-2A > G mutation of PHYH, and she married her first cousin carrying the same mutation. She was pregnant seven times and had two homozygous girls. Due to a potential exacerbation of the disease during the third trimester of pregnancy, her weight and plasma PA levels were monitored. No specific events were noticed for the mother during the pregnancies and postpartum periods. This case also raised the question of potential exposure to PA (and its subsequent toxicity) of a homozygous fetus in a homozygous mother. Despite modestly elevated plasma concentrations of PA at birth (<30 μmol/L), the two affected girls did not present any specific sign of ARD and have so far developed normally. As only a few determinations of plasma PA levels in the mother could be performed during pregnancies, showing mild elevations (<350 μmol/L), it remains difficult to conclude as to a possible transplacental crossing of PA.

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

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

Environmental Enteric Dysfunction is Associated with Carnitine Deficiency and Altered Fatty Acid Oxidation

BACKGROUND

Environmental enteric dysfunction (EED), a condition characterized by small intestine inflammation and abnormal gut permeability, is widespread in children in developing countries and a major cause of growth failure. The pathophysiology of EED remains poorly understood.

METHODS

We measured serum metabolites using liquid chromatography-tandem mass spectrometry in 400 children, aged 12-59months, from rural Malawi. Gut permeability was assessed by the dual-sugar absorption test.

FINDINGS

80.7% of children had EED. Of 677 serum metabolites measured, 21 were negatively associated and 56 were positively associated with gut permeability, using a false discovery rate approach (q<0.05, p<0.0095). Increased gut permeability was associated with elevated acylcarnitines, deoxycarnitine, fatty acid β-oxidation intermediates, fatty acid ω-oxidation products, odd-chain fatty acids, trimethylamine-N-oxide, cystathionine, and homocitrulline, and with lower citrulline, ornithine, polyphenol metabolites, hippurate, tryptophan, and indolelactate.

INTERPRETATION

EED is a syndrome characterized by secondary carnitine deficiency, abnormal fatty acid oxidation, alterations in polyphenol and amino acid metabolites, and metabolic dysregulation of sulfur amino acids, tryptophan, and the urea cycle. Future studies are needed to corroborate the presence of secondary carnitine deficiency among children with EED and to understand how these metabolic derangements may negatively affect the growth and development of young children.

The Challenges of a Successful Pregnancy in a Patient with Adult Refsum's Disease due to Phytanoyl-CoA Hydroxylase Deficiency

We describe the management and outcomes of pregnancy in a 27-year-old woman with infantile-onset Adult Refsum's disease (ARD). She presented in infancy but was diagnosed with ARD at the age of 10 on basis of phytanic acidaemia and later confirmed to have the phytanoyl-CoA hydroxylase ((PHYH) c.164delT, p.L55fsX12) mutation. Despite repeated plasmapheresis sessions and strict dietary surveillance for 20 years, her phytanic acid levels persistently stayed above the ideal target level of 100 μmol/L but remained below 400 μmol/L. Initially the pregnancy was uncomplicated but in the third trimester of pregnancy the patient was admitted to the hospital with fluctuating hypertension, sinus tachycardia and breathlessness. The patient was compliant with diet during pregnancy and her phytanic levels were remained well controlled: 177 and 188 μmol/L in the first and second trimester, respectively. Peri-partum management required a coordinated team approach including a high-calorie and restricted diet to reduce the risk of acute metabolic decompensation. During the induced labour she required 10% dextrose infusions.Post-partum it took the mother a long time to recover from childbirth - her appetite was poor due to post-natal depression and her body weight decreased rapidly by 11 kg within 3 weeks after childbirth, resulting in a spike in phytanic acid to 366 μmol/L. Measures were taken to minimise the risk of acute neurological decompensation. The infant was unaffected and has made normal developmental progress in the subsequent 2 years.

Safety of long-term restrictive diets for peroxisomal disorders: vitamin and trace element status of patients treated for Adult Refsum Disease

BACKGROUND

Adult Refsum's Disease (ARD) is caused by defects in the pathway for alpha-oxidation of phytanic acid (PA). Treatment involves restricting the dietary intake of phytanic acid by reducing the intake of dairy-derived fat. The adequacy of micronutrient intake in patients with ARD is unknown.

METHODS

Patients established on the Chelsea low-PA diet had general diet macronutrients, vitamins and trace elements assessed using 7-day-weighed intakes and serial 24-h recalls. Intakes were compared with biochemical assessments of nutritional status for haematinics (ferritin), trace elements (copper, zinc, iron, selenium), water- (vitamin B6 , B12 and folate) and fat-soluble vitamins (A, D, E and K).

RESULTS

Eleven subjects (four women, seven men) were studied. Body mass index was 27 ± 5 kg/m(2) (range 19-38). All subjects had high sodium intakes (range 1873-4828 mg). Fat-soluble vitamin insufficiencies occurred in some individuals (vitamin A, n = 2; vitamin D, n = 6; vitamin E, n = 3; vitamin K, n = 10) but were not coincident. Vitamin B6 levels were normal or elevated (n = 6). Folate and 5-methyltetrahydrofolate concentrations were normal. Metabolic vitamin B12 insufficiency was suspected in four subjects based on elevated methylmalonic acid concentrations. Low copper and selenium intakes were noted in some subjects (n = 7, n = 2) but plasma levels were adequate. Iron, ferritin and zinc intakes and concentrations were normal.

CONCLUSION

Subjects with ARD can be safely managed on the Chelsea low PA without routine micronutrient supplementation. Sodium intake should be monitored and reduced. Periodic nutritional screening may be necessary for fat-soluble vitamins, vitamin B12 , copper or selenium.

Reactive nitrogen species mediate oxidative stress and astrogliosis provoked by in vivo administration of phytanic acid in cerebellum of adolescent rats: A potential contributing pathomechanism of cerebellar injury in peroxisomal disorders

Phytanic acid (Phyt) accumulates in various peroxisomal diseases including Refsum disease (RD) and Zellweger syndrome (ZS). Since the pathogenesis of the neurological symptoms and especially the cerebellar abnormalities in these disorders are poorly known, we investigated the effects of in vivo intracerebral administration of Phyt on a large spectrum of redox homeostasis parameters in the cerebellum of young rats. Malondialdehyde (MDA) levels, sulfhydryl oxidation, carbonyl content, nitrite and nitrate concentrations, 2',7'-dichlorofluorescein (DCFH) oxidation, total (tGS) and reduced glutathione (GSH) levels and the activities of important antioxidant enzymes were determined at different periods after Phyt administration. Immunohistochemical analysis was also carried out in the cerebellum. Phyt significantly increased MDA and nitric oxide (NO) production and decreased GSH levels, without altering tGS, DCFH oxidation, sulfhydryl oxidation, carbonyl content and the activities of glutathione peroxidase (GPx), superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G6PD). Furthermore, immunohistochemical analysis revealed that Phyt caused astrogliosis and protein nitrosative damage in the cerebellum. It was also observed that the NO synthase inhibitor Nω-Nitro-L-arginine methyl ester (L-NAME) prevented the increase of MDA and NO production as well as the decrease of GSH and the immunohistochemical alterations caused by Phyt, strongly suggesting that reactive nitrogen species (RNS) were involved in these effects. The present data provide in vivo solid evidence that Phyt disrupts redox homeostasis and causes astrogliosis in rat cerebellum probably mediated by RNS production. It is therefore presumed that disequilibrium of redox status may contribute at least in part to the cerebellum alterations characteristic of patients affected by RD and other disorders with Phyt accumulation.

Identification of small compound biomarkers of pituitary adenoma: a bilateral inferior petrosal sinus sampling study

Evaluation of the pathogenic mechanisms underlying Cushing disease (CD) is limited partly by the inaccessibility of the pituitary gland for biopsy. We used bilateral inferior petrosal sinus sampling (BIPSS), the gold standard in diagnosing pituitary sources of CD, to obtain central blood samples for in vivo metabolomic analysis of pathways involved in pituitary adenomas. We evaluated 16 samples from eight patients who underwent BIPSS to measure adrenocorticotropic hormone (ACTH) levels in the inferior petrosal sinus (IPS) bilaterally. Seven patients had CD with concordant BIPSS, surgical, and pathologic findings. Samples from the IPS contralateral to histologically proven lesions were used as controls. BIPSS of the eighth patient revealed no central pituitary ACTH source, and these samples were also included as controls. Plasma samples were profiled using a combination of three liquid chromatography tandem mass spectrometry methods, which assessed 259 metabolites. Following Bonferroni correction for multiple comparisons, three small compound biomarkers of CD (pyridoxate, deoxycholic acid, and 3-methyladipate) were identified to be significantly altered in pituitary adenomas. The pathway most significantly impacted in CD samples is one previously shown to be upregulated in other cancers. Exploiting the BIPSS technique, we showed a complete metabolite and lipid profile of pituitary adenomas in CD. These potential biomarkers of CD may elucidate tumor biology and suggest possible diagnostic molecular imaging probes as well as therapeutic targets in patients with recurrent disease after surgery.

Metabonomic variations associated with AOM-induced precancerous colorectal lesions and resveratrol treatment

Resveratrol (Res), 3,5,4'-trihydroxy-trans-stilbene, is an antioxidant found in the skin of red grapes and in several other plants. This phenolic compound has been recently reported to possess cancer chemopreventive activity that inhibits the process of carcinogenesis. However, the mechanisms underlying its anticancer effects remain largely unresolved. In this study, we investigated the chemoprotective effects of dietary Res in an azoxymethane (AOM) induced precancerous colorectal lesion model in male Wistar rats. The metabolic alterations in urine, sera, and colonic tissues of experimental rats perturbed by AOM intervention as well as the Res treatment were measured by a gas chromatography time-of-flight mass spectrometry (GC-TOFMS) analysis. Significant alterations of metabolites were observed in AOM group in urine, sera, and colonic tissues, which were attenuated by Res treatment and concurrent with the histopathological improvement with significantly decreased aberrant crypt foci (ACF) incidence. Representative metabolites include depleted glucose, β-hydroxybutyrate (ketone body), hypoxanthine, and elevated branched chain amino acids (isoleucine and valine) and tryptophan in colonic tissue, as well as elevated serum aminooxyacetate and urinary 4-hydroxyphenylacetate and xanthurenate. These metabolic changes suggest that the preventive effect of Res is associated with attenuation of impaired glucose and lipid metabolism and elevated protein breakdown in colonic tissues from AOM-exposed rats. It also appears that Res induced significant metabolic alterations independent of the AOM-induced metabolic changes. The significantly altered metabolites identified in Res-AOM group relative to AOM group include arachidonate, linoleate, glutamate, docosahexaenoate, palmitelaidate, 2-aminobutyrate, pyroglutamate, and threonate, all of which are involved in inflammation and oxidation processes. This suggests that Res exerts the chemopreventive effects on ACF formation by anti-inflammatory and antioxidant mechanisms in addition to amelioration of AOM-induced mitochondrial disruption.

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.

Adult Refsum disease: a form of tapetoretinal dystrophy accessible to therapy

Adult Refsum disease is characterized by an elevated plasma phytanic acid level and high concentrations of phytanic acid in a variety of tissues. Besides tapetoretinal degeneration, additional symptoms are anosmia, skeletal malformations, chronic polyneuropathy, cerebellar ataxia, sensorineural hearing loss, ichthyosis, and cardiac abnormalities. A diet low in phytanic acid ameliorates polyneuropathy and ataxia and slows or even stops the other manifestations. In order to be able to apply dietary therapy, as many patients as possible (even better if all of them are) have to be identified at an early stage. The ophthalmologist plays a crucial role in achieving this goal because of the early manifestation of the tapetoretinal degeneration.

Neurochemical evidence that phytanic acid induces oxidative damage and reduces the antioxidant defenses in cerebellum and cerebral cortex of rats

AIMS

In the present work we investigated the in vitro effects of phytanic acid (Phyt), that accumulates in Refsum disease and other peroxisomal diseases, on important parameters of oxidative stress in cerebellum and cerebral cortex from young rats.

MAIN METHODS

The parameters thiobarbituric acid-reactive substances levels (TBA-RS; lipid peroxidation), carbonyl formation and sulfhydryl oxidation (protein oxidative damage) and the concentrations of the most important nonenzymatic antioxidant defense reduced glutathione (GSH) were determined.

KEY FINDINGS

It was observed that Phyt significantly increased TBA-RS levels in both cerebral structures. This effect was prevented by the antioxidants alpha-tocopherol and melatonin, suggesting the involvement of free radicals. Phyt also provoked protein oxidative damage in both cerebellum and cerebral cortex, as determined by increased carbonyl content and sulfhydryl oxidation. Furthermore, Phyt significantly diminished the concentrations of GSH, while melatonin and alpha-tocopherol treatment totally blocked this effect. We also verified that Phyt does not behave as a direct acting oxidant, since Phyt did not oxidize commercial solutions of GSH and reduced cytochrome c to Phyt in a free cell medium.

SIGNIFICANCE

Our data indicate that oxidative stress is elicited in vitro by Phyt, a mechanism that may contribute at least in part to the pathophysiology of Refsum disease and other peroxisomal disorders where Phyt is accumulated.

Ataxia with loss of Purkinje cells in a mouse model for Refsum disease

Refsum disease is caused by a deficiency of phytanoyl-CoA hydroxylase (PHYH), the first enzyme of the peroxisomal alpha-oxidation system, resulting in the accumulation of the branched-chain fatty acid phytanic acid. The main clinical symptoms are polyneuropathy, cerebellar ataxia, and retinitis pigmentosa. To study the pathogenesis of Refsum disease, we generated and characterized a Phyh knockout mouse. We studied the pathological effects of phytanic acid accumulation in Phyh(-/-) mice fed a diet supplemented with phytol, the precursor of phytanic acid. Phytanic acid accumulation caused a reduction in body weight, hepatic steatosis, and testicular atrophy with loss of spermatogonia. Phenotype assessment using the SHIRPA protocol and subsequent automated gait analysis using the CatWalk system revealed unsteady gait with strongly reduced paw print area for both fore- and hindpaws and reduced base of support for the hindpaws. Histochemical analyses in the CNS showed astrocytosis and up-regulation of calcium-binding proteins. In addition, a loss of Purkinje cells in the cerebellum was observed. No demyelination was present in the CNS. Motor nerve conduction velocity measurements revealed a peripheral neuropathy. Our results show that, in the mouse, high phytanic acid levels cause a peripheral neuropathy and ataxia with loss of Purkinje cells. These findings provide important insights in the pathophysiology of Refsum disease.

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.

Dr Brian Gibberd (1931-2006): a pioneering clinician in Refsum's disease

Branched-chain fatty acids are common components of the human diet (phytanic acid) or are produced endogenously (bile acids), and are also used as medicines (ibuprofen). Owing to their branched-chain structure, they are metabolized in peroxisomes. In the case of phytanic acid, the presence of a 3-methyl group prevents beta-oxidation, and instead it undergoes one round of alpha-oxidation to allow further metabolism. Defects in this process give rise to neurological diseases and cancer. Dr Brian F. Gibberd was one of the first U.K. physicians to recognize the importance of these peroxisomal metabolic pathways in clinical medicine, and pioneered their study. This obituary recognizes his many achievements in neurology and especially in the treatment of peroxisomal disorders. The following four papers from this mini-symposium entitled 'Advances in peroxisomal alpha-, beta- and omega-oxidation' describe work done in this area as part of a collaborative study in which Dr Gibberd played a key role. This work was presented as part of the Cardiovascular Bioscience focused topic at the Life Sciences 2007 conference, and this mini-symposium was dedicated to Dr Gibberd and his important contributions to this field.

Peroxisomal disorders affecting phytanic acid α-oxidation: a review

Peroxisomes are involved in the synthesis and degradation of complex fatty acids. They contain enzymes involved in the α-, β- and ω-oxidation pathways for fatty acids. Investigation of these pathways and the diseases associated with mutations in enzymes involved in the degradation of phytanic acid have led to the clarification of the pathophysiology of Refsum's disease, rhizomelic chondrodysplasia and AMACR (α-methylacyl-CoA racemase) deficiency. This has highlighted the role of an Fe(II)- and 2-oxoglutarate-dependent oxygenases [PhyH (phytanoyl-CoA 2-hydroxylase), also known as PAHX], thiamin-dependent lyases (phytanoyl-CoA lyase) and CYP (cytochrome P450) family 4A in fatty acid metabolism. The differential regulation and biology of these pathways is suggesting novel ways to treat the neuro-ophthalmological sequelae of Refsum's disease. More recently, the discovery that AMACR and other peroxisomal β-oxidation pathway enzymes are highly expressed in prostate and renal cell cancers has prompted active investigation into the role of these oxidation pathways and the peroxisome in the progression of obesity- and insulin resistance-related cancers.

Peroxisomes, Refsum's disease and the α- and ω-oxidation of phytanic acid

In the present paper, we describe the current state of knowledge regarding the enzymology of the phytanic acid α-oxidation pathway. The product of phytanic acid α-oxidation, i.e. pristanic acid, undergoes three cycles of β-oxidation in peroxisomes after which the products, including 4,8-dimethylnonanoyl-CoA, propionyl-CoA and acetyl-CoA, are exported from the peroxisome via one of two routes, including (i) the carnitine-dependent route, mediated by CRAT (carnitine acetyltransferase) and CROT (carnitine O-octanoyltransferase), and (ii) the free acid route, mediated by one or more of the peroxisomal ACOTs (acyl-CoA thioesterases). We also describe our recent data on the ω-oxidation of phytanic acid, especially since pharmacological up-regulation of this pathway may form the basis of a new treatment strategy for ARD (adult Refsum's disease). In patients suffering from ARD, phytanic acid accumulates in tissues and body fluids due to a defect in the α-oxidation system.

Identification of the cytochrome P450 enzymes responsible for the ω-hydroxylation of phytanic acid

Patients suffering from Refsum disease have a defect in the alpha-oxidation pathway which results in the accumulation of phytanic acid in plasma and tissues. Our previous studies have shown that phytanic acid is also a substrate for the omega-oxidation pathway. With the use of specific inhibitors we now show that members of the cytochrome P450 (CYP450) family 4 class are responsible for phytanic acid omega-hydroxylation. Incubations with microsomes containing human recombinant CYP450s (Supersomes) revealed that multiple CYP450 enzymes of the family 4 class are able to omega-hydroxylate phytanic acid with the following order of efficiency: CYP4F3A>CYP4F3B>CYP4F2>CYP4A11.

Characterization of phytanic acid omega-hydroxylation in human liver microsomes

Phytanic acid is a 3-methyl branched-chain fatty acid which originates from dietary sources. Since the 3-methyl group blocks regular beta-oxidation, it is broken down by peroxisomal alpha-oxidation. Adult Refsum disease patients accumulate phytanic acid as a result of an impairment in peroxisomal alpha-oxidation, caused by the deficient activity of the enzyme phytanoyl-CoA hydroxylase in the majority of patients. In this paper, we studied an alternative degradation route for phytanic acid, namely omega-oxidation. During omega-oxidation a fatty acid is hydroxylated at its omega-end by a member of the cytochrome P450 multi-enzyme family. Subsequently, an alcohol dehydrogenase converts the formed hydroxyl group into an aldehyde, which is then converted into a carboxyl-group by an aldehyde dehydrogenase. In case of phytanic acid omega-hydroxylation would lead to the formation of phytanedioic acid, which can be degraded by beta-oxidation from the omega-end. Here, we show that phytanic acid indeed undergoes omega- and (omega-1)-hydroxylation in pooled human liver microsomes in an NADPH-dependent manner with a ratio of 15:1. Studies with imidazole antimycotics indicate that these reactions are catalyzed by one or more cytochrome P450 enzymes. Induction of the cytochrome P450 involved in phytanic acid omega-hydroxylation may increase the flux through the omega-oxidation pathway, causing increased clearance of phytanic acid in ARD patients. Hence, this alternative catabolic pathway is of potential therapeutic relevance.

The Refsum disease marker phytanic acid, a branched chain fatty acid, affects Ca2+ homeostasis and mitochondria, and reduces cell viability in rat hippocampal astrocytes

The saturated branched chain fatty acid, phytanic acid, a degradation product of chlorophyll, accumulates in Refsum disease, an inherited peroxisomal disorder with neurological clinical features. To elucidate the pathogenic mechanism, we investigated the influence of phytanic acid on cellular physiology of rat hippocampal astrocytes. Phytanic acid (100 microM) induced an immediate transient increase in cytosolic Ca2+ concentration, followed by a plateau. The peak of this biphasic Ca2+ response was largely independent of extracellular Ca2+, indicating activation of cellular Ca2+ stores by phytanic acid. Phytanic acid depolarized mitochondria without causing in situ swelling of mitochondria. The slow decrease of mitochondrial potential is not consistent with fast and simultaneous opening of the mitochondrial permeability transition pore. However, phytanic acid induced substantial generation of reactive oxygen species. Phytanic acid caused astroglia cell death after a few hours of exposure. We suggest that the cytotoxic effect of phytanic acid seems to be due to a combined action on Ca2+ regulation, mitochondrial depolarization, and increased ROS generation in brain cells.

Omega-hydroxylation of phytanic acid in rat liver microsomes: implications for Refsum disease

The 3-methyl-branched fatty acid phytanic acid is degraded by the peroxisomal alpha-oxidation route because the 3-methyl group blocks beta-oxidation. In adult Refsum disease (ARD), peroxisomal alpha-oxidation is defective, which is caused by mutations in the gene coding for phytanoyl-CoA hydroxylase in the majority of ARD patients. As a consequence, phytanic acid accumulates in tissues and body fluids. This study focuses on an alternative route of phytanic acid degradation, omega-oxidation. The first step in omega-oxidation is hydroxylation at the omega-end of the fatty acid, catalyzed by a member of the cytochrome P450 multienzyme family. To study this first step, the formation of hydroxylated intermediates was studied in rat liver microsomes incubated with phytanic acid and NADPH. Two hydroxylated metabolites of phytanic acid were formed, omega- and (omega-1)-hydroxyphytanic acid (ratio of formation, 5:1). The formation of omega-hydroxyphytanic acid was NADPH dependent and inhibited by imidazole derivatives. These results indicate that phytanic acid undergoes omega-hydroxylation in rat liver microsomes and that an isoform of cytochrome P450 catalyzes the first step of phytanic acid omega-oxidation.