Refsum disease diagnostic marker phytanic acid alters the physical state of membrane proteins of liver mitochondria.

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid), a branched chain fatty acid accumulating in Refsum disease to high levels throughout the body, induces uncoupling of rat liver mitochondria similar to non-branched fatty acids (e.g. palmitic acid), but the contribution of the ADP/ATP carrier or the aspartate/glutamate carrier in phytanic acid-induced uncoupling is of minor importance. Possible deleterious effects of phytanic acid on membrane-linked energy coupling processes were studied by ESR spectroscopy using rat liver mitochondria and a membrane preparation labeled with the lipid-specific spin probe 5-doxylstearic acid (5-DSA) or the protein-specific spin probe MAL-TEMPO (4-maleimido-2,2,6, 6-tetramethyl-piperidine-1-oxyl). The effects of phytanic acid on phospholipid molecular dynamics and on the physical state of membrane proteins were quantified by estimation of the order parameter or the ratio of the amplitudes of the weakly to strongly immobilized MAL-TEMPO binding sites (W/S ratio), respectively. It was found, that phytanic acid (1) increased the mobility of phospholipid molecules (indicated by a decrease in the order parameter) and (2) altered the conformational state and/or the segmental mobility of membrane proteins (indicated by a drastic decrease in the W/S ratio). Unsaturated fatty acids with multiple cis-double bonds (e.g. linolenic or arachidonic acid), but not non-branched FFA (ranging from chain length C10:0 to C18:0), also decrease the W/S ratio. It is hypothesized that the interaction of phytanic acid with transmembrane proteins might stimulate the proton permeability through the mitochondrial inner membrane according to a mechanism, different to a protein-supported fatty acid cycling.     

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

Adult Refsum disease (ARD) is associated with defective alpha-oxidation of phytanic acid (PA). omega-Oxidation of PA to 3-methyl-adipic acid (3-MAA) occurs although its clinical significance is unclear. In a 40 day study of a new ARD patient, where the plasma half-life of PA was 22.4 days, omega-oxidation accounted for 30% initially and later all PA excretion. Plasma and adipose tissue PA and 3-MAA excretion were measured in a cross-sectional study of 11 patients. The capacity of the omega-oxidation pathway was 6.9 (2.8-19.4) mg [20.4 (8.3-57.4) micromol] PA/day. 3-MAA excretion correlated with plasma PA levels (r = 0.61; P = 0.03) but not adipose tissue PA content. omega-Oxidation during a 56 h fast was studied in five patients. 3-MAA excretion increased by 208 +/- 58% in parallel with the 158 (125-603)% rise in plasma PA. Plasma PA doubled every 29 h, while 3-MAA excretion followed second-order kinetics. Acute sequelae of ARD were noted in three patients (60%) after fasting. The omega-oxidation pathway can metabolise PA ingested by patients with ARD, but this activity is dependent on plasma PA concentration. omega-Oxidation forms a functional reserve capacity that enables patients with ARD undergoing acute stress to cope with limited increases in plasma PA levels.     

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.     

The subcellular localization of phytanic acid oxidase in rat liver

Peroxisomal disorders (Zellweger's syndrome, neonatal adrenoleukodystrophy, infantile Refsum's syndrome, rhizomelic chondrodysplasia) show a series of enzymatic defects related to peroxisomal dysfunctions. Accumulation of phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) has been found in several of these patients, caused by a defect in the alpha-oxidation mechanism of this acid. The fact that the alpha-oxidation of phytanic acid is defective in the peroxisomal disorders as well as in classical Refsum's disease makes it likely that this oxidation normally takes place in the peroxisomes. A series of experiments preformed to localize the phytanic acid oxidase in subcellular fractions of rat liver show, however, that the alpha-oxidation of phytanic acid is a mitochondrial process. Free phytanic acid is the substrate, and the only cofactors necessary are ATP and Mg2+.     

Location of double bonds in two unsaturated forms of phytanic acid from Refsum disease as determined by mass spectrometry

A monounsaturated and a triunsaturated form of phytanic acid (3,7,11,15-tetramethylhexacosanoate) were isolated from plasma lipids of a patient with Refsum disease. Both were converted to their methyl esters, oxidized to polyhydroxy acids by treatment with OsO4 and converted to their vicinal trimethylsilyl ethers. These derivatives were analyzed by gas chromatography-mass spectrometry using both electron impact ionization (at 21 and 70 eV) and chemical ionization conditions to obtain clear evidence to establish the structure of the monounsaturated form of phytanic acid as 3,7,11,15-tetramethylhexadec-15-monoenoic acid and that of the triunsaturated form of phytanic acid as 3,7,11,15-tetramethylhexadec-6,10,14-trienoic acid. The possible metabolic and dietary sources for these novel fatty acids are discussed.     

Phosphatidic acid metabolism in rat liver microsomes.

Rat liver microsomes contain phosphatidate phosphatases which split phosphatidic acid into inorganic phosphate and diacylglycerol and a system of phospholipases and lipases, which split phosphatidic acid into free fatty acids, glycerol and inorganic phosphate. In the presence of ATP,CoA and [1-14C]palmitate, part of the monoacyl-sn-glycerol 3-phosphate formed by phospholipase action is reesterified, yielding radioactive phosphatidic acid. The sum of di- and triacylglycerols formed from phosphatidic acid in the presence of ATP and CoA exceeded the amount of diacylglycerol formed in their absence. The yield of neutral lipids from sn-glycerol 3-phosphate and monoacyl-sn-glycerol 3-phosphate markedly exceeded that from phosphatidic acid. Comparison of the yields of di- and triacylglcerols from glycerol-labelled and fatty-acid-labelled phosphatidic acid was used to establish the extent of deacylation and reacylation. About 60% of the diacylglycerol was formed by direct dephosphorylation. The triacylglycerols, on the other hand, were formed almost exclusively from recycled phosphatidic acid.     

Association of folic acid with rat liver microsomes

Summary Shin et al. (Biochim Biophys Acta 444: 794–801, 1976) described the subcellular location of [³H]folic acid after injection into rats. The microsomal fraction of the liver contained relatively large amounts of tracer initially but lower amounts at later times. Because of the heterogeneous nature of the microsomal fraction of the liver we re-examined the nature of the folate binding fraction. The location of injected [³H]folic acid resembled that of the microsomes derived from the plasma membrane, where ultracentrifugal analysis was conducted in the presence and absence of cesium ions. The location of the folate did not resemble that of microsomes derived from the endoplasmic reticulum (ER). One of the marker enzymes of the ER was the vitamin K-dependent carboxylase. A simple method for reducing vitamin K is described.     

Phytanoyl-CoA hydroxylase from rat liver. Protein purification and cDNA cloning with implications for the subcellular localization of phytanic acid alpha-oxidation

Phytanoyl-CoA hydroxylase (PhyH) catalyzes the conversion of phytanoyl-CoA to 2-hydroxyphytanoyl-CoA, which is the first step in the phytanic acid alpha-oxidation pathway. Recently, several studies have shown that in humans, phytanic acid alpha-oxidation is localized in peroxisomes. In rat, however, the alpha-oxidation pathway has been reported to be mitochondrial. In order to clarify this differential subcellular distribution, we have studied the rat PhyH protein. We have purified PhyH from rat liver to apparent homogeneity as judged by SDS-PAGE. Sequence analysis of two PhyH peptide fragments allowed cloning of the rat PHYH cDNA encoding a 38. 6 kDa protein. The deduced amino acid sequence revealed strong homology to human PhyH including the presence of a peroxisome targeting signal type 2 (PTS2). Heterologous expression of rat PHYH in Saccharomyces cerevisiae yielded a 38.6 kDa protein whereas the PhyH purified from rat liver had a molecular mass of 35 kDa. This indicates that PhyH is probably processed in rat by proteolytic removal of a leader sequence containing the PTS2. This type of processing has been reported in several other peroxisomal proteins that contain a PTS2. Subcellular localization studies using equilibrium density centrifugation showed that PhyH is indeed a peroxisomal protein in rat. The finding that PhyH is peroxisomal in both rat and humans provides strong evidence against the concept of a differential subcellular localization of phytanic acid alpha-oxidation in rat and human.     

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.     

Metabolism of glycyrrhetic acid by rat liver microsomes: glycyrrhetinate dehydrogenase

Glycyrrhetic acid, derived from a main component of liquorice, was converted to 3-ketoglycyrrhetic acid reversibly by rat liver homogenates in the presence of NADPH or NADP⁺. Glycyrrhetic acid-oxidizing and 3-ketoglycyrrhetic acid-reducing activities were localized in microsomes among the subcellular fractions of rat liver. Glycyrrhetic acid-oxidizing activity and 3-ketoglycyrrhetic acid-reducing activities showed pH optima at 6.3 and 8.5, respectively, and required NADP⁺ or NAD⁺ and NADPH or NADH, respectively, indicating that these activities were due to glycyrrhetinate dehydrogenase. The dehydrogenase was not solubilized from the membranes by the treatment with 1 M NaCl or sonication, indicating that the enzyme is a membrane component. The dehydrogenase was solubilized with detergents such as Emalgen 913, Triton X-100 and sodium cholate, and then separated from 3β-hydroxysteroid dehydrogenase (5β-androstan-3β-ol-17-one-oxidizing activity) by butyl-Toyopearl 650 M column chromatography. Partially purified enzyme catalyzed the reversible reaction between glycyrrhetic acid and 3-ketoglycyrrhetic acid, but was inactive toward 3-epiglycyrrhetic acid and other steroids having the 3β-hydroxyl group. The enzyme required NADP⁺ and NADPH for the highest activities of oxidation and reduction, respectively, and NAD⁺ and NADH for considerable activities, similar to the results with microsomes. From these results the enzyme is defined as glycyrrhetinate dehydrogenase, being quite different from 3β-hydroxysteroid dehydrogenase of Ruminococcus sp. from human intestine, which is active for both glycyrrhetic acid and steroids having the 3β-hydroxyl group.     

Transfer of arachidonic acid between phospholipids in rat liver microsomes

Phosphatidylcholine and phosphatidylinositol labelled with radioactive oleic, arachidonic or linoleic acids in the 2-acyl position were prepared. Rat liver microsomes were incubated with either lysophosphatidylcholine or lyso-phosphatidylinositol and the opposite 2-acyl-labelled phospholipid, and were found to catalyse a transfer of fatty acids between the two phospholipids. This was shown to be a direct Co-enzyme A-mediated transfer that does not involve a free fatty acid intermediate (i.e. it is independent of phospholipase A₂ activity). Arachidonoyl transfer took place at about four times the rate of linoleoyl transfer; oleoyl transfer was not detectable. The role of direct arachidonoyl transfer to phosphatidylinositol in the controlled release of arachidonic acid for prostaglandin synthesis is discussed.     

Dihydroxyfumaric acid induced lipid peroxidation in rat liver microsomes.

Dihydroxyfumaric acid induced lipid peroxidation in rat liver microsomes. This reaction was heat-insensitive contrary to the mitochondrial peroxidation reported in the previous paper, and was enhanced by p-chloromercuribenzoate. Additions of Fe²⁺ and Fe³⁺ stimulated both the lipid peroxidation and the disappearance of dihydroxyfumaric acid. On the other hand, addition of Mn²⁺ or Cu²⁺, which stimulated the disappearance of dihydroxyfumaric acid, inhibited the lipid peroxidation. Hydroxyl radical scavengers, superoxide dismutase and catalase had no effect on this lipid peroxidation and dihydroxyfumaric acid disappearance. The cytochrome p-450 content decreased about 70 % in parallel with the lipid peroxidation.     

Evidence for two enzymatic pathways for omega-oxidation of docosanoic acid in rat liver microsomes

We studied the omega-oxidation of docosanoic acid (C22:0) in rat liver microsomes. C22:0 and 22-hydroxy-docosanoic acid (omega-hydroxy-C22:0) were used as substrates, and the reaction products were analyzed by electrospray ionization mass spectrometry. In the presence of NADPH, omega-oxidation of C22:0 produced not only the hydroxylated product, omega-hydroxy-C22:0, but also the dicarboxylic acid of C22:0, docosanedioic acid (C22:0-DCA). When rat liver microsomes were incubated with omega-hydroxy-C22:0 in the presence of either NAD+ or NADPH, C22:0-DCA was formed readily. Formation of C22:0-DCA from either C22:0 or omega-hydroxy-C22:0 with NADPH as cofactor was inhibited strongly by miconazole and disulfiram, whereas no inhibition was found with NAD+ as cofactor. Furthermore, omega-oxidation of C22:0 was reduced significantly when molecular oxygen was depleted. The high sensitivity toward the more specific cytochrome P450 inhibitors ketoconazole and 17-octadecynoic acid suggests that hydroxylation of C22:0 and omega-hydroxy-C22:0 may be catalyzed by one or more cytochrome P450 hydroxylases belonging to the CYP4A and/or CYP4F subfamily. This study demonstrates that C22:0 is a substrate for the omega-oxidation system in rat liver microsomes and that the product of the first hydroxylation step, omega-hydroxy-C22:0, may undergo further oxidation via two distinct pathways driven by NAD+ or NADPH.     

Oxidative decarboxylation of retinoic acid in microsomes of rat liver and kidney

Liver and kidney microsomes have been found to catalyze a rapid decarboxylation of retinoic acid in vitro. The reaction requires NADPH and Fe(2+), and is further stimulated by the presence of pyrophosphate. Thiamine pyrophosphate contained sufficient iron as an impurity to provide strong enhancement of the reaction in the absence of added iron. The decarboxylation could also be shown to occur nonenzymatically in the presence of ascorbate, Fe(2+), and boiled microsomes, but there was little autoxidation resulting in decarboxylation. The reaction was strongly inhibited by chelating agents, N,N'-diphenyl-p-phenylene diamine, phenazine methosulfate, and ferricyanide, and resembled lipid peroxidation in both its cofactor requirements and response to inhibitors. The product of the reaction appeared to lack only the C-15 of the original retinoic acid molecule. It was not retained by diethylaminoethyl cellulose, was more polar than retinoic acid upon silicic acid chromatography, had a lower UV absorption maximum (295 m micro ) than the starting product, and seemed to have an aldehyde group at C-14. The physiological significance of the decarboxylation remains to be assessed, but its rapidity makes it important to in vitro work on retinoic acid.     

The composition of rat liver microsomes. The structural proteins of rat liver microsomes

1. The structural-protein component of microsomal membranes was isolated by three separate methods. Analysis by polyacrylamide-gel electrophoresis indicated that the microsomal structural component is made up of a heterogeneous group of proteins. These proteins were further characterized by their phospholipid-binding capacity. The electrophoretic patterns of microsomal structural proteins were found to differ significantly from those of mitochondrial structural proteins. 2. The reticulosomal fraction was also characterized by electrophoresis with reference to total microsomal proteins, microsomal structural proteins and ribosomal proteins. The reticulosomes gave an electrophoretic pattern significantly different from those of the other three preparations examined. It is suggested that reticulosomes consist largely of enzymic proteins of the endoplasmic reticulum.