2,4 Dienoyl-CoA reductase (DCR) is an auxiliary beta-oxidation enzyme required for the degradation of unsaturated fatty acids with even-numbered double bounds. The enzyme activity is present in both mitochondria and peroxisomes, but catalysed by different proteins. The peroxisomal rat liver DCR, which was cloned before by means of phage-display technology (Fransen et al, 1999, BJ 340,561), was used as a query to recover sequences coding for the human DCR from the human EST1 and genomic DNA databases. A fragment of 900 bases from a human liver cDNA library was amplified and subcloned into the pBadHis vector. The bacterially expressed His-tagged 2,4-dienoyl-CoA reductase showed a clear activity not only towards 2-trans, 4-trans-hexadienoyl-CoA (sorboyl-CoA) and 2-trans, 4-trans-decadienoyl-CoA, but also towards 2-trans, +cis, 7-cis, 10-cis, 13-cis, 16-cis, 19-cis-docosaheptaenoyl-CoA. The latter CoA-ester is produced during the beta-oxidation of docosahexaenoic acid. DCR activity towards docosaheptaenoyl-CoA was inhibited in the presence of albumin whereas sorboyl-CoA reduction was not influenced by albumin. Hence, DCR is likely to be important for the peroxisomal degradation of polyunsaturated fatty acids.7 Is a microsomal aldehyde dehydrogenase involved in the degradation of 3-methyl branched fatty acids?The primary products of the degradation of 3-methyl branched fatty acids such as phytanic acid are formyl-CoA and a 2-methyl branched fatty aldehyde. Since this alpha-oxidation process and the subsequent beta-oxidation of the 2-methyl branched fatty acid were clearly shown to be peroxisomal, the role of a microsomal fatty aldehyde dehydrogenase (FALDH) in the conversion of 2-methyl fatty aldehydes appears somewhat intriguing. In order to investigate a putative peroxisome/ER transfer of alpha-oxidation intermediates, degradation of 3-methyl-[2-I4C]heptadecanoic acid was studied in intact cells. The release of labeled ASM produced in consecutive alpha-and beta-oxidation by cultured skin fibroblasts of patients with Sjogren-LarssonSyndrome (SLS), known to be deficient in fatty aldehyde dehydrogenation, was 30 to 60 % of the amount released by control fibroblasts. However, conversion of 2-methylpentadecanal into 2-methylpentadecanoic acid in homogenates of SLS fibroblasts, as measured by GC, was only marginally impaired.
Measurement of degradation of 3-methyl-[2-'4C]heptadecanoicacid in homogenates is difficult due to the need for different incubation conditions for alpha-and beta-oxidation. Hence, measuring consecutive alpha-and beta-oxidation could not prove the exclusive involvement of a microsomal FALDH in the degradation of 3-methyl branched fatty acids.
Beta-oxidation of docosahexaenoic acid and tetracosahexaenoic acid in isolated rat hepatocytes and human fibroblastsThe pathways involved in the synthesis and degradation of very long chain polyunsaturated fatty acids are still unclear. Peroxisomes have been implicated in the formation of docosahexaenoic acid, which would be formed by subjecting tetracosahexaen...