In vitro conversion of phytol to phytanic acid in rat liver: subcellular distribution of activity and chemical characterization of intermediates using a new bromination technique

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.

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.

Alpha-Oxidation

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

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.

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.

Potentiation of the teratogenic effects induced by coadministration of retinoic acid or phytanic acid/phytol with synthetic retinoid receptor ligands

Previous studies in our laboratory identified retinoid-induced defects that are mediated by RAR-RXR heterodimerization using interaction of synthetic ligands selective for the retinoid receptors RAR and RXR in mice (Elmazar et al. 1997, Toxicol Appl Pharmacol 146:21-28; Elmazar et al. 2001, Toxicol Appl Pharmacol 170:2-9; Nau and Elmazar 1999, Handbook of experimental pharmacology, vol 139, Retinoids, Springer-Verlag, pp 465-487). The present study was designed to investigate whether these RAR-RXR heterodimer-mediated defects can be also induced by interactions of natural and synthetic ligands for retinoid receptors. A non-teratogenic dose of the natural RXR agonist phytanic acid (100 mg/kg orally) or its precursor phytol (500 mg/kg orally) was coadministered with a synthetic RARalpha-agonist (Am580; 5 mg/kg orally) to NMRI mice on day 8.25 of gestation (GD8.25). Furthermore, a non-teratogenic dose of the synthetic RXR agonist LGD1069 (20 mg/kg orally) was also coadministered with the natural RAR agonist, all- trans-retinoic acid (atRA, 20 mg/kg orally) or its precursor retinol (ROH, 50 mg/kg orally) to NMRI mice on GD8.25. The teratogenic outcome was scored in day-18 fetuses. The incidence of Am580-induced resorptions, spina bifida aperta, micrognathia, anotia, kidney hypoplasia, dilated bladder, undescended testis, atresia ani, short and absent tail, fused ribs and fetal weight retardation were potentiated by coadministration of phytanic acid or its precursor phytol. Am580-induced exencephaly and cleft palate, which were not potentiated by coadministration with the synthetic RXR agonists, were also not potentiated by coadministration with either phytanic acid or its precursor phytol. LGD1069 potentiated atRA- and ROH-induced resorption, exencephaly, spina bifida, aperta, ear anotia and microtia, macroglossia, kidney hypoplasia, undescended testis, atresia ani, tail defects and fetal weight retardation, but not cleft palate. These results suggest that synergistic teratogenesis can be induced by coadministration of a natural RXR ligand (phytanic acid) with a synthetic RAR agonist (Am580). Thus, certain potentially useful therapeutic agents or nutritional factors such as phytanic acid should be tested for teratogenic risk by coadministration with other retinoid receptor agonists.

Natural occurrence of cancer-preventive geranylgeranoic acid in medicinal herbs

Geranylgeranoic acid (GGA; all-trans 3,7,11,15-tetramethyl-2,6,10,14-hexadecatetraenoic acid) has been shown to induce apoptosis in a human hepatoma-derived cell line, HuH-7. We aimed not only to confirm the apoptogenic properties of GGA and its derivatives, but also to search for natural GGA in medicinal herbs. GGA induced apoptosis in human hepatoma-derived cell lines, HuH-7, PLC/PRF-5, and mouse transformed hepatocyte-derived cell line, MLE-10, in a dose- and time-dependent manner, but failed to induce cell death in human hepatoblastoma-derived HepG-2 and mouse primary hepatocytes in the same condition. Besides GGA, 4,5-didehydro GGA, 14,15-dihydro GGA, and 2,3-dihydro GGA were also active to induce cell death in HuH-7 cells, while 4,5-didehydro-10,11, 14,15-tetrahydro GGA, 4,5,8,9-tetrahydro GGA, farnesoic acid, and geranylgeraniol were inert. By using liquid chromatography/mass spectrometry, we found natural GGA as a negative ion of m/z 303.4 in a Chinese herb, Schisandra chinensis, and Schisandra GGA was identified by derivatization with both mild methylation and catalytic hydrogenation. Some other GGAs hydrogenated in the different degrees, including phytanic acid (perhydro GGA), were also found in S. chinensis. GGA and phytanic acid were detected in 24 out of 25 herbs tested. The present study is the first report of natural GGA in medicinal herbs.

Human liver fatty aldehyde dehydrogenase: microsomal localization, purification, and biochemical characterization

To better understand the genetic disorder Sjogren-Larsson syndrome which is caused by a deficiency of fatty aldehyde dehydrogenase activity, we determined the subcellular localization of the enzyme and investigated its biochemical properties. Using density gradient centrifugation, we found that fatty aldehyde dehydrogenase activity was predominantly localized in the microsomal fraction in human liver. This fatty aldehyde dehydrogenase was solubilized from human liver microsomes and purified by chromatography on columns consisting of omega-aminohexyl-agarose and 5'-AMP-Sepharose 4B. The enzyme had an apparent subunit molecular weight of 54000, required NAD+ as cofactor, had optimal activity at pH 9.8, and was thermolabile at 47 degrees C. Fatty aldehyde dehydrogenase had high activity towards saturated and unsaturated aliphatic aldehydes ranging from 6 to 24 carbons in length, as well as dihydrophytal, a 20-carbon branched chain aldehyde. In contrast, acetaldehyde, propionaldehyde, crotonaldehyde, glutaraldehyde, benzaldehyde, and retinaldehyde were poor substrates. The enzyme was inhibited by disulfiram, iodoacetamide, alpha,p-dibromoacetophenone, and p-chloromercuribenzoate. These results indicate that microsomal fatty aldehyde dehydrogenase is a distinct human aldehyde dehydrogenase isozyme that acts on a variety of medium- and long-chain aliphatic substrates.

Characterization of the in vitro conversion of dolichol to dolichoate in bovine thyroid

The enzymic conversion of dolichol into dolichoic acid has been studied in bovine thyroid subcellular fractions using [1-3H]dolichol as a substrate. The presence of conversion activity could be demonstrated in both the mitochondrial- and supernatant fractions. Investigation of cofactor requirements revealed that NAD+ was essential for reaching optimal activity. From kinetic studies Km-values of 3.5-4 microM and 0.29 mM could be calculated for, respectively, dolichol and NAD+ using the mitochondrial fraction as an enzyme source. No inhibitory effects from ethanol or pyrazole were detected suggesting that alcohol dehydrogenase is not involved in the dolichol-->dolichoate conversion as observed in a bovine thyroid mitochondrial fraction. From inhibitor studies the conversion system behaves distinctly differently from the NADP(+)-depending microsomal oxidoreductase as well as from catalase. The conversion activity in the supernatant on the other hand must be ascribed, at least partially, to a side activity of alcohol dehydrogenase.

Phytanic acid alpha oxidation in rat liver: studies on alpha hydroxylation

1. The alpha-hydroxylation of [1-14C]phytanic acid was investigated in the postnuclear fraction of rat liver. 2. The reaction required ATP, Mg, Fe3+ and molecular oxygen. Fe3+ could be replaced by Fe2+. 3. The hydroxylase activity was optimal at pH 7.5 in phosphate buffer. 4. The activity increased with postnuclear protein (5-13 mg or protein), increased with the substrate concentration at low substrate concentration. 5. The amount of the hydroxyacid formed increased with time up to 10 min. 6. Coenzyme A (100 microM-2.5 mM) stimulated the activity. 7. The activity was further stimulated by NADP and NADPH slightly and by FAD and FMN strongly, all at 100 microM concentration. 8. While CO inhibited the reaction, phenobarbital inducible cytochrome P-450 did not appear to play a role in this reaction.