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

Abstract

Studies on the localization of alkyldihydroxyacetone-phosphate synthase in rat liver are described. Less than 5% of the total activity was found in the cytosolic fraction, suggesting that the enzyme is membrane-bound. The ratio of the enzymatic (specific) activity over that of dihydroxyacetone-phosphate acyltransferase, a peroxisomal enzyme, is 10-fold higher in the microsomal fraction, when compared to the peroxisomal fraction. Studying the distribution of the enzyme in a linear density gradient, two activity peaks were found in the peroxisomal and the microsomal fraction indicating a bimodal localization of alkyldihydroxyacetone-phosphate synthase. Rabert et al. (Rabert, U., Völkl, A. and Debuch, H. (1986) Biol. Chem. Hoppe-Seyler 367, 215-222) have presented evidence that the activity in the microsomal fraction was mainly caused by a different enzyme that preferentially converted acyldihydroxyacetone to alkyldihydroxyacetone. We also found a radioactive product, different from alkyldihydroxyacetone-phosphate, upon incubation of microsomal protein in the presence of [14C]hexadecanol. However, it was shown that this product was formed independently of the presence of acyldihydroxyacetone. The product yielded [14C]hexadecanol upon alkaline hydrolysis, clearly showing that it did not contain an ether-bond. Upon incubation of microsomal protein with [14C]palmitic acid and hexadecanol the product was also observed and its chromatographic behaviour resembled that of a synthetically prepared palmitoyl ester of hexadecanol. From these data it was concluded that the product formed is most likely a wax and that the enzyme responsible for this conversion is clearly different from the alkyldihydroxyacetone-phosphate synthase. The implication is that acyldihydroxyacetone-phosphate cannot be replaced by acyldihydroxyacetone in the process of glycero-ether bond formation.