Phytanic acid alpha-oxidation in man: identification of 2-hydroxyphytanoyl-CoA lyase, a peroxisomal enzyme with normal activity in Zellweger syndrome

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

In humans, peroxisomes harbor a complex set of enzymes acting on various lipophilic carboxylic acids, organized in two basic pathways, alpha-oxidation and beta-oxidation; the latter pathway can also handle omega-oxidized compounds. Some oxidation products are crucial to human health (primary bile acids and polyunsaturated FAs), whereas other substrates have to be degraded in order to avoid neuropathology at a later age (very long-chain FAs and xenobiotic phytanic acid and pristanic acid). Whereas total absence of peroxisomes is lethal, single peroxisomal protein deficiencies can present with a mild or severe phenotype and are more informative to understand the pathogenic factors. The currently known single protein deficiencies equal about one-fourth of the number of proteins involved in peroxisomal FA metabolism. The biochemical properties of these proteins are highlighted, followed by an overview of the known diseases.

The role of 2-hydroxyacyl-CoA lyase, a thiamin pyrophosphate-dependent enzyme, in the peroxisomal metabolism of 3-methyl-branched fatty acids and 2-hydroxy straight-chain fatty acids

2-Hydroxyphytanoyl-CoA lyase (abbreviated as 2-HPCL), renamed to 2-hydroxyacyl-CoA lyase (abbreviated as HACL1), is the first peroxisomal enzyme in mammals that has been found to be dependent on TPP (thiamin pyrophosphate). It was discovered in 1999, when studying alpha-oxidation of phytanic acid. HACL1 has an important role in at least two pathways: (i) the degradation of 3-methyl-branched fatty acids like phytanic acid and (ii) the shortening of 2-hydroxy long-chain fatty acids. In both cases, HACL1 catalyses the cleavage step, which involves the splitting of a carbon-carbon bond between the first and second carbon atom in a 2-hydroxyacyl-CoA intermediate leading to the production of an (n-1) aldehyde and formyl-CoA. The latter is rapidly converted into formate and subsequently to CO(2). HACL1 is a homotetramer and has a PTS (peroxisomal targeting signal) at the C-terminal side (PTS1). No deficiency of HACL1 has been described yet in human, but thiamin deficiency might affect its activity.

Overexpression of Angiopoietin-Like Protein 4 Alters Mitochondria Activities and Modulates Methionine Metabolic Cycle in the Liver Tissues of db/db Diabetic Mice

Angiopoietin-like protein 4 (ANGPTL4) is a circulating protein predominantly produced from fat tissue and liver. Recent data from others and our laboratory have demonstrated this protein to be an important player in energy metabolism and insulin sensitivity. However, the molecular mechanisms underlying its metabolic actions remain elusive. In this study, we have employed a two-dimensional fluorescence difference gel electrophoresis technique to study the protein profiles in the livers of db/db mice treated with or without ANGPTL4. When compared with those of lean mice, 118 proteins were found to be up- or down-regulated in db/db mice. Adenovirus-mediated overexpression of ANGPTL4 could reverse a large portion of the up- or down-regulated proteins to control levels. Especially, a number of mitochondria proteins were down-regulated by ANGPTL4 to a great extent. Chronic treatment with ANGPTL4 resulted in an elevated activity of mitochondria respiratory chain complexes II-III and IV in db/db mice. Additionally, several key enzymes in the methionine/homocysteine metabolic cycle were found to be increased in db/db diabetic mice but decreased by ANGPTL4 treatment. HPLC analysis consistently revealed that ANGPTL4 could significantly restore the augmented S-adenosylmethionine levels and S-adenosylmethionine/S-adenosylhomocysteine ratios in livers of db/db mice. In summary, our results suggest that ANGPTL4 might elicit its metabolic effects through modulating the mitochondria functions and methionine metabolic cycles in the liver tissue.

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.

Human metabolism of phytanic acid and pristanic acid

Phytanic acid is a methyl-branched fatty acid present in the human diet. Due to its structure, degradation by beta-oxidation is impossible. Instead, phytanic acid is oxidized by alpha-oxidation, yielding pristanic acid. Despite many efforts to elucidate the alpha-oxidation pathway, it remained unknown for more than 30 years. In recent years, the mechanism of alpha-oxidation as well as the enzymes involved in the process have been elucidated. The process was found to involve activation, followed by hydroxylase, lyase and dehydrogenase reactions. Part, if not all of the reactions were found to take place in peroxisomes. The final product of phytanic acid alpha-oxidation is pristanic acid. This fatty acid is degraded by peroxisomal beta-oxidation. After 3 steps of beta-oxidation in the peroxisome, the product is esterified to carnitine and shuttled to the mitochondrion for further oxidation. Several inborn errors with one or more deficiencies in the phytanic acid and pristanic degradation have been described. The clinical expressions of these disorders are heterogeneous, and vary between severe neonatal and often fatal symptoms and milder syndromes with late onset. Biochemically, these disorders are characterized by accumulation of phytanic and/or pristanic acid in tissues and body fluids. Several of the inborn errors involving phytanic acid and/or pristanic acid metabolism have been characterized on the molecular level.