Studies on the Stereospecificity of Mitochondrial Oxidation of Phytanic Acid and of α-Hydroxyphytanic Acid

Novel aspects of health promoting compounds in meat

Meat is an integral part of the human diet. Besides essential amino acids and nutritive factors of high quality and availability, meat provides often overlooked components of importance for human health. These are amino acids and bioactive compounds that may be very important in i) preventing muscle wasting diseases, such as in sarcopenia, ii) reducing food and caloric intake to prevent metabolic syndrome, iii) blood pressure homeostasis via ACE-inhibitory components from connective tissue, and iv) maintaining functional gut environment through meat-derived nucleotides and nucleosides. In addition, meat could be an important source of phytanic acid, conjugated linoleic acids and antioxidants. Further, it becomes increasingly apparent that design of in vitro meat will be possible, and that this development may lead to improved health benefits from commercially viable and sustainable meat products.

Content and distribution of phytanic acid diastereomers in organic milk as affected by feed composition

Phytanic acid (PA) is a bioactive compound found in milk that is derived from the phytol chain of chlorophyll, and the content of PA in milk fat depends on the availability of phytol from feed. In this study, the content of PA diastereomers was analyzed in milk sampled from five organic herds twice during the grazing season (May and September). The total content of PA was higher in September compared to May, but was not affected by breed (Danish Holstein or Danish Jersey). Total PA could not be directly related to intake of green feed items. The distribution between diastereomers was closely related to the amount of grazed clovers, where a higher intake resulted in a higher share of the RRR isomer.

The chemical biology of branched-chain lipid metabolism

Mammalian metabolism of some lipids including 3-methyl and 2-methyl branched-chain fatty acids occurs within peroxisomes. Such lipids, including phytanic and pristanic acids, are commonly found within the human diet and may be derived from chlorophyll in plant extracts. Due to the presence of a methyl group at its beta-carbon, the well-characterised beta-oxidation pathway cannot degrade phytanic acid. Instead its alpha-methylene group is oxidatively excised to give pristanic acid, which can be metabolised by the beta-oxidation pathway. Many defects in the alpha-oxidation pathway result in an accumulation of phytanic acid, leading to neurological distress, deterioration of vision, deafness, loss of coordination and eventual death. Details of the alpha-oxidation pathway have only recently been elucidated, and considerable progress has been made in understanding the detailed enzymology of one of the oxidative steps within this pathway. This review summarises these recent advances and considers the roles and likely mechanisms of the enzymes within the alpha-oxidation pathway.

Alpha-oxidation of 3-methyl-substituted fatty acids and its thiamine dependence

3-Methyl-branched fatty acids, as phytanic acid, undergo peroxisomal alpha-oxidation in which they are shortened by 1 carbon atom. This process includes four steps: activation, 2-hydroxylation, thiamine pyrophosphate dependent cleavage and aldehyde dehydrogenation. The thiamine pyrophosphate dependence of the third step is unique in peroxisomal mammalian enzymology. Human pathology due to a deficient alpha-oxidation is mostly linked to mutations in the gene coding for the second enzyme of the sequence, phytanoyl-CoA hydroxylase.

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

Phytanic acid (3,7,10,14-tetramethylhexadecanoic acid) is a branched-chain fatty acid which is known to accumulate in a number of different genetic diseases including Refsum disease. Due to the presence of a methyl-group at the 3-position, phytanic acid and other 3-methyl fatty acids can not undergo beta-oxidation but are first subjected to fatty acid alpha-oxidation in which the terminal carboxyl-group is released as CO(2). The mechanism of alpha-oxidation has long remained obscure but has been resolved in recent years. Furthermore, peroxisomes have been found to play an indispensable role in fatty acid alpha-oxidation, and the complete alpha-oxidation machinery is probably localized in peroxisomes. This Review describes the current state of knowledge about fatty acid alpha-oxidation in mammals with particular emphasis on the mechanism involved and the enzymology of the pathway.

Utilization of sterol carrier protein-2 by phytanoyl-CoA 2-hydroxylase in the peroxisomal alpha oxidation of phytanic acid

Since it possesses a 3-methyl group, phytanic acid is degraded by a peroxisomal alpha-oxidation pathway, the first step of which is catalyzed by phytanoyl-CoA 2-hydroxylase (PAHX). Mutations in human PAHX cause phytanic acid accumulations leading to Adult Refsum's Disease (ARD), which is also observed in a sterol carrier protein 2 (SCP-2)-deficient mouse model. Phytanoyl-CoA is efficiently 2-hydroxylated by PAHX in vitro in the presence of mature SCP-2. Other straight-chain fatty acyl-CoA esters were also 2-hydroxylated and the products isolated and characterized. Use of SCP-2 increases discrimination between straight-chain (e.g., hexadecanoyl-CoA) and branched-chain (e.g., phytanoyl-CoA) substrates by PAHX. The results explain the phytanic acid accumulation in the SCP-2-deficient mouse model and suggest that some of the common symptoms of ARD and other peroxisomal diseases may arise in part due to defects in SCP-2 function caused by increased phytanic acid levels.

Refsum disease, peroxisomes and phytanic acid oxidation: a review

Refsum disease was first recognized as a distinct disease entity by Sigvald Refsum in the 1940s. The discovery of markedly elevated levels of the branched-chain fatty acid phytanic acid in certain patients marked Refsum disease as a disorder of lipid metabolism. Although it was immediately recognized that the accumulation of phytanic acid is due to its deficient breakdown in Refsum disease patients, the true enzymatic defect remained mysterious until recently. A major breakthrough in this respect was the resolution of the mechanism of phytanic acid alpha-oxidation in humans. In this review we describe the many aspects of Refsum disease from the clinical signs and symptoms to the enzyme and molecular defect plus the recent identification of genetic heterogeneity in Refsum disease.

Hepatic alpha-oxidation of phytanic acid. A revised pathway

Synthetic 3-methyl-branched chain fatty acids were used to decipher the breakdown of phytanic acid. Based on results obtained in intact or permeabilized rat hepatocytes, rat liver homogenates or subcellular fractions, a revised alpha-oxidation pathway is proposed which appears to be functioning in man as well. In a first step, the 3-methyl-branched chain fatty acid is activated by an acyl-CoA synthetase. This reaction requires CoA, ATP and Mg2+. Subsequently, the acyl-CoA ester is hydroxylated at position 2 by a peroxisomal dioxygenase. This step is dependent on alpha-oxoglutarate, ascorbate (or glutathione), Fe2+ and O2. The 2-hydroxy-3-methylacyl-CoA intermediate is cleaved by a peroxisomal lyase to formyl-CoA and a 2-methyl-branched fatty aldehyde. Formyl-CoA is (partly enzymically) hydrolyzed to formate, which is then converted, most likely in the cytosol, to CO2. In the presence of NAD+, the aldehyde is dehydrogenated to a 2-methyl-branched fatty acid, presumably by a peroxisomal aldehyde dehydrogenase. This acid can--after activation--be degraded via a D-specific peroxisomal beta-oxidation system.

Alpha-oxidation of 3-methyl-substituted fatty acids in rat liver

3-Methyl-substituted fatty acids are first oxidatively decarboxylated (alpha-oxidation) before they are degraded further via beta-oxidation. We synthesized [1-14C]phytanic and 3-[1-14C]methylmargaric acids in order to study their alpha-oxidation in isolated rat hepatocytes, rat liver homogenates and subcellular fractions. alpha-Oxidation was measured as the production of radioactive CO2. In isolated hepatocytes, maximal rates of alpha-oxidation amounted to 7 and 10 nmol/min x 10(8) cells with phytanic acid and 3-methylmargaric acid, respectively. At equimolar substrate concentrations, alpha-oxidation of branched fatty acids was approximately 10- to 15-fold slower than the beta-oxidation of the straight chain palmitate. In whole liver homogenates, rates of alpha-oxidation that equaled 60 to 70% of those observed in the hepatocytes were obtained. Optimum rates required O2, NADPH, Fe3+, and ATP. Fe3+ could be replaced by Fe2+ and ATP could be replaced by a number of other phosphorylated nucleosides and even inorganic phosphate without loss of activity. NADH could substitute for NADPH but not always with full restoration of activity. A variety of other cofactors and metal ions was either inhibitory or without effect. Scavengers of reactive oxygen species, known to be formed during the NADPH-dependent microsomal reduction of ferric-phosphate complexes, were without effect on alpha-oxidation. No evidence was found for the accumulation of NADPH-dependent or Fe(3+)-dependent reaction intermediates. Subcellular fractionation of liver homogenates demonstrated that alpha-oxidation was located predominantly, if not exclusively, in the endoplasmic reticulum. alpha-Oxidation, measured in microsomal fractions, was not inhibited by CO, cytochrome c, or ferricyanide, indicating that NADPH cytochrome P450 reductase and cytochrome P450 are not involved in alpha-oxidation. Our results indicate that, contrary to current belief, alpha-oxidation is catalyzed by the endoplasmic reticulum. The cofactor requirements suggest that alpha-oxidation involves the reduction of Fe3+ by electrons from NADPH and that it is stimulated by phosphate ions and nucleotides.

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.