Phytol-induced pathology in 2-hydroxyacyl-CoA lyase (HACL1) deficient mice. Evidence for a second non-HACL1-related lyase

2-Hydroxyacyl-CoA lyase (HACL1) is a key enzyme of the peroxisomal α-oxidation of phytanic acid. To better understand its role in health and disease, a mouse model lacking HACL1 was investigated. Under normal conditions, these mice did not display a particular phenotype. However, upon dietary administration of phytol, phytanic acid accumulated in tissues, mainly in liver and serum of KO mice. As a consequence of phytanic acid (or a metabolite) toxicity, KO mice displayed a significant weight loss, absence of abdominal white adipose tissue, enlarged and mottled liver and reduced hepatic glycogen and triglycerides. In addition, hepatic PPARα was activated. The central nervous system of the phytol-treated mice was apparently not affected. In addition, 2OH-FA did not accumulate in the central nervous system of HACL1 deficient mice, likely due to the presence in the endoplasmic reticulum of an alternate HACL1-unrelated lyase. The latter may serve as a backup system in certain tissues and account for the formation of pristanic acid in the phytol-fed KO mice. As the degradation of pristanic acid is also impaired, both phytanoyl- and pristanoyl-CoA levels are increased in liver, and the ω-oxidized metabolites are excreted in urine. In conclusion, HACL1 deficiency is not associated with a severe phenotype, but in combination with phytanic acid intake, the normal situation in man, it might present with phytanic acid elevation and resemble a Refsum like disorder.

β-Oxidation in hepatocyte cultures from mice with peroxisomal gene knockouts

Beta-oxidation of carboxylates takes place both in mitochondria and peroxisomes and in each pathway parallel enzymes exist for each conversion step. In order to better define the substrate specificities of these enzymes and in particular the elusive role of peroxisomal MFP-1, hepatocyte cultures from mice with peroxisomal gene knockouts were used to assess the consequences on substrate degradation. Hepatocytes from mice with liver selective elimination of peroxisomes displayed severely impaired oxidation of 2-methylhexadecanoic acid, the bile acid intermediate trihydroxycholestanoic acid (THCA), and tetradecanedioic acid. In contrast, mitochondrial beta-oxidation rates of palmitate were doubled, despite the severely affected inner mitochondrial membrane. As expected, beta-oxidation of the branched chain compounds 2-methylhexadecanoic acid and THCA was reduced in hepatocytes from mice with inactivation of MFP-2. More surprisingly, dicarboxylic fatty acid oxidation was impaired in MFP-1 but not in MFP-2 knockout hepatocytes, indicating that MFP-1 might play more than an obsolete role in peroxisomal beta-oxidation.

Characterisation of human peroxisomal 2,4-dienoyl-CoA reductase

Based on the primary structure of the rat peroxisomal 2,4-dienoyl-CoA reductase (M. Fransen, P.P. Van Veldhoven, S. Subramani, Biochem. J. 340 (1999) 561-568), the cDNA of the human counterpart was cloned. It contained an open reading frame of 878 bases encoding a protein of 291 amino acids (calculated molecular mass 30778 Da), being 83% identical to the rat reductase. The gene, encompassing nine exons, is located at chromosome 16p13. Bacterially expressed poly(His)-tagged reductase was active not only towards short and medium chain 2,4-dienoyl-CoAs, but also towards 2,4,7,10,13,16,19-docosaheptaenoyl-CoA. Hence, the reductase does not seem to constitute a rate limiting step in the peroxisomal degradation of docosahexaenoic acid. The reduction of docosaheptaenoyl-CoA, however, was severely decreased in the presence of albumin.

Fibroblast studies documenting a case of peroxisomal 2-methylacyl-CoA racemase deficiency: possible link between racemase deficiency and malabsorption and vitamin K deficiency

BACKGROUND

2-Methylacyl-CoA racemase interconverts the 2-methyl group of pristanoyl-CoA or the 25-methyl group of hydroxylated cholestanoyl-CoAs, allowing further peroxisomal desaturation of these compounds in man by the branched chain acyl-CoA oxidase, which recognise only the S-isomers. Hence, oxidation studies in fibroblasts, currently based on the use of racemic substrates such as [1-14C] pristanic acid, do not allow us to distinguish between a deficient racemase or an impaired oxidase.

DESIGN

To evaluate the racemase activity directly, the 2R-isomer of[1-14C] pristanic acid, as well as the 2R-isomer of 2-methyl-[1-14C] hexadecanoic, a synthetic pristanic acid substitute, were prepared and their degradation by cultured human skin fibroblasts was compared to that of the racemic substrates.

RESULTS

In fibroblasts in a young girl, presenting with elevated urinary levels of trihydroxycholestanoic acid metabolites but normal plasma levels of very long chain fatty acids, a partial deficient degradation of racemic [1-14C] pristanic acid was observed. Incorporation of 2R-[1-14C] pristanic acid in glycerolipids of the patient's fibroblasts proceeded normally, but breakdown was impaired. Similar findings were seen with the 2R-isomer of 2-methyl-[1-14C] hexadecanoic. These data, combined with the fact that the branched chain acyl-CoA oxidase, catalyzing the first oxidation step of pristanic acid and bile acid intermediates in man, appeared normal, suggested a peroxisomal beta-oxidation defect in the patient at the level of 2-methylacyl-CoA racemase.

CONCLUSION

Carboxy-labelled 2R-methyl branched chain fatty acids might be useful tools to document cases of racemase deficiencies. Because a brother of the patient died with a diagnosis of vitamin K deficiency, an impaired racemase might be responsible for other cases of unexplicable malabsorption.