Phytanic acid

Although /3-oxidation is universally important, there are some instances in which it cannot operate effectively. For example, branched-chain fatty acids with alkyl branches at odd-numbered carbons are not effective substrates for /3-oxidation. For such species, a-oxidation is a useful alternative. Consider phy-tol, a breakdown product of chlorophyll that occurs in the fat of ruminant animals such as sheep and cows and also in dairy products. Ruminants oxidize phytol to phytanic acid, and digestion of phytanic acid in dairy products is thus an important dietary consideration for humans. The methyl group at C-3 will block /3-oxidation, but, as shown in Figure 24.26, phytanic acid a-hydroxylase places an —OFI group at the a-carbon, and phytanic acid a-oxidase decar-boxylates it to yield pristanie add. The CoA ester of this metabolite can undergo /3-oxidation in the normal manner. The terminal product, isobutyryl-CoA, can be sent into the TCA cycle by conversion to succinyl-CoA. 

The a-oxidation pathway is defective in Refsum s disease, an inherited metabolic disorder that results in defective night vision, tremors, and other neurologic abnormalities. These symptoms are caused by accumulation of phytanic acid in the body. Treatment of Refsum s disease requires a diet free of chloro-... 

FIGURE 24.26 Branched-chain fatty acids are oxidized by o -oxidation, as shown for phytanic acid. The product of the phytanic acid oxidase, pristanic acid, is a suitable substrate for normal /3-oxidation. Isobutyryl-CoA and propionyl-CoA can both be converted to suc-cinyl-CoA, which can enter the TCA cycle. 

Phytanic acid, the product of chlorophyll that causes problems for individuals with Refsum s disease, is 3,7,11,15-tetramethyl hexa-decanoic acid. Suggest a route for its oxidation that is consistent with what you have learned in this chapter. Hint The methyl group at C-3 effectively blocks hydroxylation and normal /3-oxidation. You may wish to initiate breakdown in some other way.)... 

Schluter, A. et al.. The chlorophyll-derived metabolite phytanic acid induces white adipocyte differentiation, Int. J. Obes., 26, 1277, 2002. 

Arnhold, T, Ehnazar, M.M.A., and Nau, H., Prevention of vitamin A teratogenesis by phytol or phytanic acid results from reduced metabolism of retinol to the teratogenic metabolite, all frara-retinoic acid, Toxicol. Sci., 66, 274, 2002. 

Lipids from marine products have been studied less frequently. The detection of co-(o-alkylphenyl)alkanoic acids with 16,18 and 20 carbon atoms together with isoprenoid fatty acids (4,8,12-trimethyltetradecanoic acid and phytanic acid) and substantial quantities of bones from fish and molluscs has provided evidence for the processing of marine animal products in vessels [58 60]. C16, C18, and C20 co-(o-alkylphenyl)alkanoic acids are presumed to be formed during the heating of tri-unsaturated fatty acids (C16 3, C18 3 and C20 3), fatty acyl components of marine lipids, involving alkali isomerization, pericyclic (intermolecular Diels-Alder reaction) and aromatization reactions. 

Isoprenoid fatty acids (4,8,12 trimethyl tetradecanoic acid and phytanic acid), acids with 16, 18 and 20 carbon atoms Heated marine lipids [42,43]... 

Refsum s disease is an autosomal recessive peroxisomal disorder characterized by accumulation of phytanic acid and other 3-alkyl-branched fatty acids [55]. Patients develop hypertrophic demyelinative polyneuropathy, retinitis pigmentosa, ichthyosis and deafness. The disorder can be successfully treated by institution of a diet poor in phytanic acid, in conjunction with plasmapheresis to remove circulating phytanic acid. 

Refsum s disease AR Oxidation of branched chain fatty acids phytan-olyl CoA 2-hydroxylase in some cases Increase of branched-chain phytanic acid, especially in PNS myelin 1, 38, Ch. 40... 

Refsum s disease. This disorder, first described nearly 60 years ago, was recently been shown due to a defect in the enzyme phytanoyl-CoA hydroxylase. Phytanic acid is a 3-methyl fatty acid that because of this methyl group cannot be oxidized directly. It is degraded by a peroxisomal a-oxidation to pristanic acid, a 2-methyl fatty acid which can be degraded by P-oxidation. The principal clinical features of Refsum s disease are progressive polyneuropathy, retinal degeneration, hearing loss, cardiomyopathy and ichthyosis, beginning in late childhood or later. 

Most, and possibly all of these changes result from the tissue accumulation of phytanic acid. Phytanic is of dietary origin exclusively, and dietary restriction of phytanic and plasmapheresis are of clinical benefit. The defective gene and pathogenic mutations have been identified. The clinical manifestations can also be mimicked by defects of PEX7. 

Tsai, Su-Chen, Steinberg, D., Avigan, J., Fales, H. M. Studies on the stereospecificity of mitochondrial oxidation of phytanic acid and of a-hydroxyphytanic acid. J. Biol. Chem. 248, 1091-1097 (1973). 

Enzyme defects are also known to exist in the minor pathways of fatty acid degradation. In Refsum disease, the methyl-branched phytanic acid (obtained from vegetable foods) cannot be degraded by a-oxidation. In Zellweger syndrome, a peroxisomal defect means that long-chain fatty acids cannot be degraded. 

Very-Long-Chain Fatty Acids and Phytanic Acid... 

D-bifunctional protein deficiency [5], 2-methyl acyl-CoA racemase (AMACR) deficiency [3] and sterol carrier protein (SCP-x) deficiency [6], the disorders of etherphospholipid biosynthesis (dihydroxyacetone phosphate acyltransferase and alkyl- dihydroxyacetone phosphate synthase deficiency) [2], the disorders of phytanic acid alpha-oxidation (Refsum disease) [15], and the disorders of glyoxylate detoxification with hyperoxaluria type 1 as caused by alanine glyoxylate aminotransferase deficiency as a sole representative. 

The VLCFA as well as the branched-chain fatty acids phytanic acid and pristanic acid are extremely hydrophobic and practically insoluble in water. Their intracellular presence is in the form of coenzyme A esters. These acids are usually stored in lipid-containing tissues such as adipose tissue, but they may also be constituents of various physiologically important lipids such as myelin. In this respect, the VLCFA and the branched-chain fatty acids are abundant in many tissues/organs. 

Table 3.4.1 Levels of very-long-chain fatty acids (VLCFA), pristanic acid and phytanic acid in the different peroxisomal disorders. AMACR 2-methyl acyl-CoA racemase, N normal, RCDP rhizomelic chondrodysplasia punctata, SCP-x sterol carrier protein, ZSDs Zellweger spectrum disorders,...

Group 1 VLCFA Pristanic acid Phytanic acid... 

Several techniques have been described in the past for the analysis of VLCFA, pristanic acid and phytanic acid [1, 10]. In our hands gas chromatography-mass spectrometry (GC-MS) analysis after derivatisation with N-methyl-N-(tert-butyldi-methylsilyl) trifluoroacetamide (MTBSTFA), is a robust and reliable method for the quantitative analysis of VLCFA, pristanic acid and phytanic acid, especially when combined with stable isotopes for C26 0, C24 0, C22 0, phytanic acid and pristanic acid [13]. In order to allow measurement of the total pool of VLCFA, pristanic acid and phytanic acid, samples need to be subjected to both acidic and alkaline hydrolysis, followed by extraction into hexane. After the hexane phase is washed once more, the sample is dried under nitrogen followed by addition of pyridine and MTBSTFA and heating of the samples at 80°C. The sample is subsequently dried again under nitrogen and taken up in hexane, followed by GC-MS analysis. 

Phytanic acid-d3 (3-methyl-2H3-7,11,15-trimethyl) hexadecanoic acid. 

The intra- and inter-assay variation is determined in tenfold analyses of a pool plasma sample. Table 3.4.2 shows the results of these measurements. The linearity of this method should be assessed for all analytes. Pristanic acid and the C26 0 fatty acid were linear up to 16 pmol/1, phytanic acid to 100 pmol/l and the C22 0 and C24 0 fatty acid to 200 pmol/1. The lower detection limit for all analytes was at a level of less than 0.01 pmol/l, for the lower reporting levels (LOQ), an analysis of a blank solution was taken into account. The blank levels of the analytes phytanic acid, pristanic acid, and fatty acids C22 0, C24 0 and C26 0 were 0.04, 0.01, 0.41, 0.68 and... 

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