Metabolism of phytol to phytanic acid in the mouse, and the role of PPARa in its regulation

Phytol, a branched-chain fatty alcohol, is the naturally occurring precursor of phytanic and pristanic acid, branched-chain fatty acids that are both ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor a (PPARa). To investigate the metabolism of phytol and the role of PPARa in its regulation, wild-type and PPARa knockout (PPARa) mice were fed a phytolenriched diet or, for comparison, a diet enriched with Wy-14,643, a synthetic PPARa agonist. After the phytolenriched diet, phytol could only be detected in small intestine, the site of uptake, and liver. Upon longer duration of the diet, the level of the (E)-isomer of phytol increased significantly in the liver of PPARamice comparedwithwildtype mice. Activity measurements of the enzymes involved in phytol metabolism showed that treatment with a PPARa agonist resulted in a PPARa-dependent induction of at least two steps of the phytol degradation pathway in liver. Furthermore, the enzymes involved showed a higher activity toward the (E)-isomer than the (Z)-isomer of their respective substrates, indicating a stereospecificity toward the metabolismof (E)-phytol. In conclusion, the results describedhere show that the conversion of phytol to phytanic acid is regulated via PPARa and is specific for the breakdown of (E)phytol.—Gloerich, J., D. M. van den Brink, J. P. N. Ruiter, N. van Vlies, F. M. Vaz, R. J. A. Wanders, and S. Ferdinandusse. Metabolism of phytol to phytanic acid in the mouse, and the role of PPARa in its regulation. J. Lipid Res. 2007. 48: 77–85. Supplementary key words peroxisome proliferator-activated receptor a & fatty aldehyde dehydrogenase & branched-chain fatty acids Phytol is a branched-chain fatty alcohol (3,7,11,15tetramethylhexadec-2-en-1-ol) that is abundantly present in nature as part of the chlorophyll molecule. The release of phytol from chlorophyll occurs effectively in the digestive system of ruminant animals only, presumably by bacteria present in the gut (1). As a result, a relatively high amount of free phytol is present in dairy products (2). In mammals, free phytol is readily absorbed in the small intestine and is metabolized to phytanic acid, a fatty acid that accumulates in a number of metabolic disorders. Increased levels of phytanic acid in the body are toxic, so this fatty acid needs to be broken down (3–9). Because the methyl-group at the 3 position prevents b-oxidation, phytanic acid first has to undergo a round of a-oxidation. This results in the formation of pristanic acid, which is one carbon atom shorter than phytanic acid and can be normally b-oxidized (10). A deficiency in a-oxidation, such as in Refsum disease, leads to increased levels of phytanic acid in plasma and tissues of patients, and this is thought to cause the main clinical symptoms of this disorder: retinitis pigmentosa, peripheral neuropathy, and cerebellar ataxia (3, 4). Because the breakdown of phytol will contribute to the phytanic and pristanic acid levels in these patients, it is important to study its metabolism and regulation. In many animal studies, phytol is used as a precursor of phytanic acid. Addition of phytol to the diet results in an increase of phytol metabolites in tissues and plasma (6, 11– 13). This has been used as amodel to study the effects of the accumulation of phytol metabolites on fatty acid metabolism, inparticular via the activationof thenuclear hormone receptor peroxisome proliferator-activated receptor a (PPARa), which is an important transcription factor in the regulation of fatty acid metabolism. Both phytanic and pristanic acid have been shown to activate PPARa in vitro (14, 15), and more recently, PPARa was also shown to be activated in vivo in mice fed a phytol-enriched diet (13). Figure 1 shows a schematic representation of the metabolism of phytol to phytanic acid and the enzymes involved. First, phytol is converted into phytenal via a yet unknown alcohol dehydrogenase, and subsequently, phytenal is converted into phytenic acid by the microsomal Manuscript received 31 January 2006 and in revised form 16 May 2006 and in re-revised form 7 June 2006 and in re-re-revised form 27 September 2006. Published, JLR Papers in Press, October 2, 2006. DOI 10.1194/jlr.M600050-JLR200 Abbreviations: FALDH, fatty aldehyde dehydrogenase; MBP, maltose binding protein; MTBSTFA, N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide; PPARa, peroxisome proliferator-activated receptor a. To whom correspondence should be addressed. e-mail: [email protected] Copyright D 2007 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr.org Journal of Lipid Research Volume 48, 2007 77 by gest, on A uust 4, 2017 w w w .j.org D ow nladed fom