Search

Phytanic acid is a methyl-branched fatty acid present in the human diet. Due to its structure, degradation by beta-oxidation is impossible. Instead, phytanic acid is oxidized by alpha-oxidation, yielding pristanic acid. Despite many efforts to elucidate the alpha-oxidation pathway, it remained unknown for more than 30 years. In recent years, the mechanism of alpha-oxidation as well as the enzymes involved in the process have been elucidated. The process was found to involve activation, followed by hydroxylase, lyase and dehydrogenase reactions. Part, if not all of the reactions were found to take place in peroxisomes. The final product of phytanic acid alpha-oxidation is pristanic acid. This fatty acid is degraded by peroxisomal beta-oxidation. After 3 steps of beta-oxidation in the peroxisome, the product is esterified to carnitine and shuttled to the mitochondrion for further oxidation. Several inborn errors with one or more deficiencies in the phytanic acid and pristanic degradation have been described. The clinical expressions of these disorders are heterogeneous, and vary between severe neonatal and often fatal symptoms and milder syndromes with late onset. Biochemically, these disorders are characterized by accumulation of phytanic and/or pristanic acid in tissues and body fluids. Several of the inborn errors involving phytanic acid and/or pristanic acid metabolism have been characterized on the molecular level.

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) is a branched-chain fatty acid present in various dietary products such as milk, cheese and fish. In patients with Refsum disease, accumulation of phytanic acid occurs due to a deficiency of phytanoyl-CoA hydroxylase, a peroxisomal enzyme containing a peroxisomal targeting signal 2. Recently, phytanoyl-CoA hydroxylase cDNA has been isolated and functional mutations have been identified. As it has been shown that phytanic acid activates the nuclear hormone receptors peroxisome proliferator-activated receptor (PPAR)alpha and all three retinoid X receptors (RXRs), the intracellular concentration of this fatty acid should be tightly regulated. When various cell lines were grown in the presence of phytanic acid, the activity of phytanoyl-CoA hydroxylase increased up to four times, depending on the particular cell type. In one cell line, HepG2, no induction of phytanoyl-CoA hydroxylase activity was observed. After addition of phytanic acid to COS-1 cells, an increase in phytanoyl-CoA hydroxylase activity was observed within 2 h, indicating a quick cell response. No stimulation of phytanoyl-CoA hydroxylase was observed when COS-1 cells were grown in the presence of clofibric acid, 9-cis-retinoic acid or both ligands together. This indicates that the activation of phytanoyl-CoA hydroxylase is not regulated via PPARalpha or RXR. However, stimulation of PPARalpha and all RXRs by clofibric acid and 9-cis-retinoic acid was observed in transient transfection assays. These results suggest that the induction of phytanoyl-CoA hydroxylase by phytanic acid does not proceed via one of the nuclear hormone receptors, RXR or PPARalpha.

We describe an 18-year-old patient with psychomotor retardation and abnormally short metatarsi and metacarpals but no other signs of classic Refsum disease. Molecular analysis of the phytanoyl-coenzyme A hydroxylase gene revealed a homozygous deletion causing a frameshift. Surprisingly, L-pipecolic acid was elevated in plasma, and microscopy of the liver showed a reduced number of peroxisomes per cell and a larger average peroxisome size. These abnormal peroxisomes lacked catalase as did peroxisomes in fibroblasts of this patient. Such generalized peroxisomal abnormalities are not present in classic Refsum disease.

Phytanic acid is broken down by alpha-oxidation in three steps finally yielding pristanic acid. The first step occurs in peroxisomes and is catalysed by phytanoyl-CoA hydroxylase. We have studied the second step in the alpha-oxidation pathway, which involves conversion of 2-hydroxyphytanoyl-CoA to pristanal catalysed by 2-hydroxyphytanoyl-CoA lyase. To this end, we have developed a stable isotope dilution gas chromatography-mass spectrometry assay allowing activity measurements in rat liver homogenates. Cell fractionation studies demonstrate that in rat liver 2-hydroxyphytanoyl-CoA lyase is localised in peroxisomes. This finding may have important implications for inherited diseases in man characterised by impaired phytanic acid alpha-oxidation.

The branched-chain fatty acid phytanic acid is a constituent of the diet, present in diary products, meat and fish. Degradation of this fatty acid in the human body is preceded by activation to phytanoyl-CoA and starts with one cycle of alpha-oxidation. Intermediates in this pathway are 2-hydroxy-phytanoyl-CoA and pristanal; the product is pristanic acid. After activation, pristanic acid is degraded by peroxisomal beta-oxidation. Several disorders have been described in which phytanic acid accumulates, in some cases in combination with pristanic acid. In classical Refsum disease, the enzyme that converts phytanoyl-CoA into 2-hydroxyphytanoyl-CoA--phytanoyl-CoA hydroxylase--is deficient, resulting in highly elevated levels of phytanic acid in blood and tissues. Also in rhizomelic chondrodysplasia punctata, phytanic acid accumulates, owing to a deficiency in the peroxisomal import of proteins with a peroxisomal targeting sequence type 2. In patients affected with generalized peroxisomal disorders, degradation of both phytanic acid and pristanic acid is impaired owing to absence of functional peroxisomes. In bifunctional protein deficiency, the disturbed oxidation of pristanic acid results in elevated levels of this fatty acid and a secondary elevation of phytanic acid. In addition, several variant peroxisomal disorders with unknown aetiology have been described in which phytanic acid and/or pristanic acid accumulate. This review describes the discovery of phytanic acid and pristanic acid and the initial attempts to elucidate the origins and fates of these fatty acids. The current knowledge on the alpha-oxidation and beta-oxidation of these branched-chain fatty acids is summarized. The disorders in which phytanic acid and/or pristanic acid accumulate are described and some remarks are made on the pathogenic mechanisms of elevated levels of phytanic acid and pristanic acid.

The alpha-oxidation of phytanic acid and the beta-oxidation of pristanitc acid were investigated in cultured fibroblasts from controls and patients affected with different peroxisomal disorders using deuterated substrates. Formation of [omega-2H6]4,8-dimethylnonanoylcarnitine ([omega-2H6]C11-carnitine) from [omega-2H6]phytanic acid and [omega-2H6]pristanic acid was used as marker for these processes. Analysis was performed by tandem mass spectrometry. In normal cells, formation of [omega-2H6]C11-carnitine from both [omega-2H6]phytanic acid and [omega-2H6]pristanic acid was observed. When peroxisome-deficient fibroblasts were incubated with these substrates, [omega-2H6]C11-carnitine was not detectable or, in two cases, very low, which results from deficiencies in both peroxisomal alpha- and beta-oxidation. In cells with an isolated beta-oxidation defect at the level of the peroxisomal bifunctional protein, formation of [omega-2H6]C11-carnitine could also not be detected. Cells with an isolated defect in the alpha-oxidation of phytanic acid, obtained from patients affected with Refsum disease (McKusick 266500) or rhizomelic chondrodysplasia punctata (McKusick 215100), did not form [omega-2H6]C11-carnitine from [omega-2H6]phytanic acid. The observed formation of [omega-2H6]C11-carnitine from [omega-2H6]pristanic acid in these cells is in accordance with a normal peroxisomal beta-oxidation in these disorders. This study shows that separate incubation of fibroblasts with [omega-2H6]phytanic acid and [omega-2H6]pristanic acid, followed by acylcarnitine analysis in the medium by tandem mass spectrometry, can be used for screening cell lines for deficiencies in the peroxisomal alpha- and beta-oxidation pathways. Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) and pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) are branched-chain fatty acids that are constituents of the human diet. As phytanic acid possesses a beta-methyl group, it cannot be degraded by beta-oxidation. Instead, phytanic acid is first degraded by alpha-oxidation, yielding pristanic acid, which is subsequently degraded by beta-oxidation (Figure 1). Phytanic acid alpha-oxidation is thought to occur partly, and pristanic acid beta-oxidation exclusively, in peroxisomes (see Wanders et al 1995 for review). Accumulation of phytanic acid and pristanic acid is found in blood and tissues of patients affected with generalized peroxisomal disorders. In this type of disorder, no morphologically distinguishable peroxisomes are present in tissues, resulting in accumulation of metabolites that are normally metabolized in these organelles (see Wanders et al 1995 for review). The group of generalized peroxisomal disorders consists of three diseases, differing in clinical presentation. Patients suffering from the most severe disease, Zellweger syndrome (McKusick 214100), have symptoms from birth on and usually do not live beyond their first year of life. Neonatal adrenoleukodystrophy (N-ALD, McKusick 202370) has a milder presentation, whereas infantile Refsum disease (IRD, McKusick 266510) is the mildest form among the generalized peroxisomal disorders. Not only in these generalized peroxisomal disorders, but also in some isolated peroxisomal beta-oxidation defects, elevated levels of phytanic acid and pristanic acid are found (ten Brink et al 1992a). The elevated phytanic acid levels are considered to be caused by product inhibition of alpha-oxidation by accumulating pristanic acid. This is reflected in a highly elevated pristanic acid to phytanic acid ratio in plasma from patients suffering from bifunctional protein deficiency or peroxisomal thiolase deficiency (ten Brink et al 1992a). Elevated phytanic acid concentrations are also found in plasma from patients affected with classical Refsum disease and rhizomelic chondrodysplasia punctata (RCDP). As pristanic acid beta-oxidation is not disturbed in these disorders, pristanic acid levels are normal (ten Brink et al 1992

We investigated the role of microsomal fatty aldehyde dehydrogenase (FALDH) in the conversion of pristanal into pristanic acid. Cultured skin fibroblasts from controls and patients with Sjögren-Larsson syndrome (SLS) who are genetically deficient in FALDH activity were incubated with [2,3-(3)H]phytanic acid. The release of aqueous-soluble radioactivity by the SLS cells was decreased to 25% of normal, consistent with an intact formation of pristanal but a deficiency of further oxidation. SLS cells also accumulated four-fold more radioactivity in N-alkyl-phosphatidyl ethanolamine, which arises from incorporation of free aldehyde into phosphatidyl ethanolamine. Recombinant human FALDH expressed in Chinese hamster ovary cells readily oxidized pristanal and cultured fibroblasts from SLS patients showed a severe deficiency in FALDH activity (13% of normal) when pristanal was used as substrate. Nevertheless, SLS patients did not accumulate phytanic acid in their plasma. We conclude that FALDH is involved in the oxidation of pristanal to pristanic acid and that this reaction is deficient in patients with SLS.

The relationship between peroxisomal and mitochondrial oxidation of the methyl branched fatty acids, phytanic acid and pristanic acid, was studied in normal and mutant human skin fibroblasts with established enzyme deficiencies. Tandem mass spectrometry was used for analysis of the acylcarnitine intermediates. In normal cells, 4,8-dimethylnonanoylcarnitine (C11:0) and 2,6-dimethylheptanoylcarnitine (C9:0) accumulated after incubation with either phytanic acid or pristanic acid. These intermediates were not observed when peroxisome-deficient cells from Zellweger patients were incubated with the same compounds, pointing to the involvement of peroxisomes in the formation of these acylcarnitine intermediates. Similar experiments with fibroblasts deficient in carnitine palmitoyltransferase I, carnitine-acylcarnitine translocase or carnitine palmitoyltransferase II revealed that mitochondrial carnitine palmitoyltransferase I is not required for the oxidation of phytanic acid or pristanic acid, whereas both carnitine-acylcarnitine translocase and carnitine palmitoyltransferase II are necessary. These studies demonstrate that both phytanic acid and pristanic acid are initially oxidized in peroxisomes to 4,8-dimethylnonanoyl-CoA, which is converted to the corresponding acylcarnitine (presumably by peroxisomal carnitine octanoyltransferase), and exported to the mitochondrion. After transport across the mitochondrial membrane and transfer of the acylgroup to coenzyme A, further oxidation to 2,6-dimethylheptanoyl-CoA occurs.

We studied the alpha-oxidation of phytanic acid in human fibroblasts of controls and patients affected with classical Refsum disease, rhizomelic chondrodysplasia punctata, generalized peroxisomal disorders and peroxisomal bifunctional protein deficiency. Cultured fibroblasts were incubated with phytanic acid, after which medium and cells were collected separately. 2-Hydroxyphytanic acid and pristanic acid were measured in the medium and cells by stable isotope dilution gas chromatography mass spectrometry. In controls, 2-hydroxyphytanic acid and pristanic acid could be detected in the medium after incubation with phytanic acid, proving that alpha-oxidation of phytanic acid via 2-hydroxyphytanoyl-CoA to pristanic acid was active and intermediates were excreted into the medium. In cells from patients with a defective alpha-oxidation (Refsum disease, rhizomelic chondrodysplasia punctata and generalized peroxisomal disorders) 2-hydroxyphytanic acid and pristanic acid were low or not detectable, showing that in these disorders the hydroxylation of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA is deficient. In cells with a peroxisomal beta-oxidation defect, 2-hydroxyphytanic acid and pristanic acid were formed in amounts comparable to those in the controls.

The degradation of the first intermediate in the alpha-oxidation of phytanic acid, 2-hydroxyphytanoyl-CoA, was investigated. Human liver homogenates were incubated with 2-hydroxyphytanoyl-CoA or 2-hydroxyphytanic acid, after which formation of 2-ketophytanic acid and pristanic acid were studied. 2-Hydroxyphytanic acid was converted into 2-ketophytanic acid and pristanic acid. When ATP, Mg2+, and coenzyme A were added to the incubation medium, higher amounts of pristanic acid were formed, whereas the formation of 2-ketophytanic acid strongly decreased. When 2-hydroxyphytanoyl-CoA was used as substrate, there was virtually no 2-ketophytanic acid formation. However, pristanic acid was formed in higher amounts than with 2-hydroxyphytanic acid as substrate. This reaction was stimulated by NAD+ and NADP+. Pristanic acid, and not pristanoyl-CoA was found to be the product of the reaction. These results suggest the existence of two pathways for decarboxylation of 2-hydroxyphytanic acid. The first one, starting from 2-hydroxyphytanic acid, involves the formation of 2-ketophytanic acid with only a small amount of pristanic acid being formed. The second pathway, which starts from 2-hydroxyphytanoyl-CoA, does not involve 2-ketophytanic acid and generates higher amounts of pristanic acid. The first pathway, which is peroxisomally localized, was found to be deficient in Zellweger syndrome, whereas the second pathway, localized in microsomes, was normally active. We conclude that the second pathway is predominant under in vivo conditions.

The structure and enzymology of the phytanic acid alpha-oxidation pathway have long remained an enigma. Recent studies have shown that phytanic acid first undergoes activation to its coenzyme A ester, followed by hydroxylation to 2-hydroxyphytanoyl-CoA. In this paper we have studied the mechanism of decarboxylation of 2-hydroxyphytanoyl-CoA in human liver. To this end, human liver homogenates were incubated with 2-hydroxyphytanoyl-CoA in the presence or absence of NAD+. Hereafter, the medium was analyzed for the presence of pristanal and pristanic acid by gas chromatography mass spectrometry. Our results show that pristanal is formed from 2-hydroxyphytanoyl-CoA. Pristanal is subsequently oxidized to pristanic acid in a NAD+ dependent reaction. These results finally resolve the mechanism of the phytanic acid alpha-oxidation process in human liver.

The aim of this study was to test whether formate is formed during alpha-oxidation of phytanic acid in humans. To a healthy volunteer, [1-13C]phytanic acid was given as an oral substrate in a dose of 15 mg/kg body weight, after which plasma, urine and breath air samples were collected during 35 h. The plasma concentrations of [1-13C]-phytanic acid, 2-hydroxy[1-13C]phytanic acid, pristanic acid and [13C]formate were analysed. The [1-13C]phytanic acid concentration increased within 5-7 h to 105 mumol/l, then decreased. Formation of 2-hydroxy[1-13C]phytanic acid increased during the first 11 h after which it decreased during the next 20 h. Pristanic acid increased slightly during the test. In breath air, 13CO2 enrichment was measured, showing a cumulative output of ca. 30% of the ingested dose after 35 h. In both urine and plasma, enrichment of [13C]formate, higher than that of 13CO2 was demonstrated. These findings show that formate is a decarboxylation product in the alpha-oxidation of phytanic acid in vivo.

The aim of this study was to test whether formate is formed during α-oxidation of phytanic acid in humans. To a healthy volunteer, [1-C]phytanic acid was given as an oral substrate in a dose of 15 mg/kg body weight, after which plasma, urine and breath air samples were collected during 35 h. The plasma concentrations of [1-C]phytanic acid, 2-hydroxy[1-C]phytanic acid, pristanic acid and [C]formate were analysed. The [1-C]phytanic acid concentration increased within 5-7 h to 105 μmol/l, then decreased. Formation of 2-hydroxy[1-C]phytanic acid increased during the first 11 h after which it decreased during the next 20 h. Pristanic acid increased slightly during the test. In breath air, CO enrichment was measured, showing a cumulative output of ca. 30% of the ingested dose after 35 h. In both urine and plasma, enrichment of [C]formate, higher than that of CO was demonstrated. These findings show that formate is a decarboxylation product in the α-oxidation of phytanic acid in vivo. [ABSTRACT FROM AUTHOR]