Localization of enzymes involved in glycero-ether bond formation in rat liver

Alkyl-dihydroxyacetonephosphate synthase. Fate in peroxisome biogenesis disorders and identification of the point mutation underlying a single enzyme deficiency

Peroxisomes play an indispensible role in ether lipid biosynthesis as evidenced by the deficiency of ether phospholipids in fibroblasts and tissues from patients suffering from a number of peroxisomal disorders. Alkyl-dihydroxyacetonephosphate synthase, a peroxisomal enzyme playing a key role in the biosynthesis of ether phospholipids, contains the peroxisomal targeting signal type 2 in a N-terminal cleavable presequence. Using a polyclonal antiserum raised against alkyl-dihydroxyacetonephosphate synthase, levels of this enzyme were examined in fibroblast cell lines from patients affected by peroxisomal disorders. Strongly reduced levels were found in fibroblasts of Zellweger syndrome and rhizomelic chondrodysplasia punctata patients, indicating that the enzyme is not stable in the cytoplasm as a result of defective import into peroxisomes. In a neonatal adrenoleukodystrophy patient with an isolated import deficiency of proteins carrying the peroxisomal targeting signal type 1, the precursor form of alkyl-dihydroxyacetonephosphate synthase was detected at a level comparable to that of the mature form in control fibroblasts, in line with an intraperoxisomal localization. A patient with an isolated deficiency in alkyl-dihydroxyacetonephosphate (DHAP) synthase activity had normal levels of this protein. Analysis at the cDNA level revealed a missense mutation leading to a R419H substitution in the enzyme of this patient. Expression of a recombinant protein carrying this mutation in Escherichia coli yielded an inactive enzyme, whereas a comparable control recombinant enzyme was active, providing further proof that this substitution is responsible for the inactivity of the enzyme and the phenotype. In line with this result is the observation that wild-type alkyl-DHAP synthase activity can be inactivated by the arginine-modifying agent phenylglyoxal. The enzyme is efficiently protected against this inactivation when the substrate palmitoyl-DHAP is present at a saturating concentration. The gene encoding human alkyl-dihydroxyacetonephosphate synthase was mapped on chromosome 2q31.

Biochemistry of peroxisomes in health and disease

The ubiquitous distribution of peroxisomes and the identification of a number of inherited diseases associated with peroxisomal dysfunction indicate that peroxisomes play an essential part in cellular metabolism. Some of the most important metabolic functions of peroxisomes include the synthesis of plasmalogens, bile acids, cholesterol and dolichol, and the oxidation of fatty acids (very long chain fatty acids > C22, branched chain fatty acids (e.g. phytanic acid), dicarboxylic acids, unsaturated fatty acids, prostaglandins, pipecolic acid and glutaric acid). Peroxisomes are also responsible for the metabolism of purines, polyamines, amino acids, glyoxylate and reactive oxygen species (e.g. O-2 and H2O2). Peroxisomal diseases result from the dysfunction of one or more peroxisomal metabolic functions, the majority of which manifest as neurological abnormalities. The quantitation of peroxisomal metabolic functions (e.g. levels of specific metabolites and/or enzyme activity) has become the basis of clinical diagnosis of diseases associated with the organelle. The study of peroxisomal diseases has also contributed towards the further elucidation of a number of metabolic functions of peroxisomes.

Purification of peroxisomal acyl-CoA: dihydroxyacetonephosphate acyltransferase from human placenta

The peroxisomal enzyme acyl-CoA:dihydroxyacetonephosphate acyltransferase (DHAPAT) was extracted from human placental membranes using CHAPS as a detergent in the presence of 1 M KCl. Prior to assay dipalmitoylphosphatidylcholine was added to the sample as eluted from the various columns in order to stabilize the protein for subsequent enzyme activity measurements at 37 degrees C. The enzyme was purified from the placental membrane using ocytl-Sepharose CL-4B chromatography, Hydroxyapatite HTP chromatography, CM-Sepharose CL-6B, PBE 94 chromatofocusing and TSK G3000 SW size exclusion chromatography. A final purification of more than 8000-fold with respect to the placental membranes was achieved with a final yield of about 5%. Upon chromatofocusing the peak of activity eluted at a pH of 5.1-5.3 indicating a low isoelectric point. A native M(r) of 60-80 kDa was calculated from HPLC size exclusion chromatography. SDS-PAGE of the final purified fraction showed one major band with a M(r) of 65 kDa. These results suggest that DHAPAT is a monomeric protein. A polyclonal antiserum raised against the purified fraction was prepared in rabbits. Immunoprecipitation experiments showed complete precipitation of DHAPAT activity in fractions prepared from human placenta, liver and skin fibroblasts. Immunoprecipitation was also used to determine the residual amount of DHAPAT protein in liver from a patient with the Zellweger syndrome. A value of about 10% was found, which closely corresponds to the residual amount of enzyme activity.

Fatty acid synthesis from [2-14C]acetate in normal and peroxisome-deficient (Zellweger) fibroblasts

The incorporation of [2-14C]acetate into the lipids of normal and peroxisome-deficient (Zellweger's syndrome) skin fibroblasts was examined. Most of the label was incorporated into triacylglycerol fatty acids in normal as well as Zellweger's syndrome cells. Triacylglycerols and cholesteryl esters in Zellweger's syndrome cells contained increased levels of labelled saturated and monounsaturated very long-chain fatty acids (VLCFA, that is fatty acids with more than 22 carbon atoms), in particular hexacosanoic (26:0) and hexacosaenoic (26:1) acids. As traces of labelled VLCFA with up to 32 carbon atoms were detected in triacylglycerols even in control cells it is probable that these fatty acids are formed naturally during the elongation process. Our data suggest that peroxisomes are involved in the chain shortening of the saturated and monounsaturated VLCFA.