Purification of Dihydroxyacetone Phosphate Acyltransferase from Guinea Pig Liver Peroxisomes

Abstract

Dihydroxyacetone phosphate acyltransferase (EC 2.3.1.42), a peroxisomal enzyme which initiates the biosynthesis of glycerolipids (especially the ether-linked glycerolipids) in higher eukaryotes, has been purified by over 3250-fold from guinea pig liver. Initial stages of purification entailed isolation of liver peroxisomes by a combination of differential and density-gradient centrifugation. Dihydroxyacetone phosphate acyltransferase was solubilized from peroxisomal membranes with 3-[3-cholamidopropyl)dimethylammonio]-1-propane sulfonate at moderate ionic strength (0.15 M NaCl). The solubilized enzyme was further purified by a regimen of size-exclusion chromatography, cation-exchange chromatography, and hydroxylapatite chromatography. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of different fractions during the purification of the enzyme, a 69-kDa protein band copurified with the enzyme activity, indicating that the monomeric enzyme may have a Mr of 69,000. This was verified by further purifying the enzyme by chromatofocusing, when a single 69-kDa band was observed on SDS-PAGE. The Mr of dihydroxyacetone phosphate acyltransferase determined by gel filtration is 90 kDa. The Vmax of the purified enzyme was 4 pmol acyldihydroxyacetone phosphate (acylDHAP) formed per minute per milligram protein and the Km(DHAP) is 70 [mu]M when assayed at saturating concentrations of palmitoylCoA. Free coenzyme A inhibits the acyltransferase reaction with an inhibition constant (Ki) of approximately 0.76 mM. To date, this is the most highly purified DHAP acyltransferase(>3200-fold) of mammalian origin.