glycerophosphate acyltransferase and effect of divalent cations on neutral lipid formation

A sensitive radioactive assay of acyl CoA:sn-glycerol3-phosphate-0-acyltransferase (EC 2.3.1.15) was developed to study the properties and subcellular distribution of this enzyme in rat epididymal adipose tissue. The esterification of sn-glycerol-3-phosphate was measured in the presence of palmitoyl CoA or palmitate, ATP, CoA, and Mg2+ at pH 7.5. The presence of glycerophosphate acyltransferase was detected in both mitochondria and microsomes. The product of this reaction was identified as phosphatidate by thin-layer chromatography and dual isotope incorporation studies. Several divalent cations reduced the activity of this enzyme. Although Mg2+ was not required for the activity of glycerophosphate acyltransferase, its addition to the incubation mixture resulted in an increased formation of neutral lipids at the expense of phosphatidate. This result is explained by an activation of microsomal phosphatidate phosphatase (EC 3.1.3.4). The effect of Mg2+ was completely abolished by Ni2+, Co2+, Mn2+, and Zn2+. These studies suggest that the balance between Mg2 + and several other divalent ions may be important in the regulation of neutral lipid synthesis in adipose tissue. Supplementary key words phosphatidate formation . Mg2+ . phosphatidate phosphatase The esterification of sn-glycerol-3-phosphate by CoA thioesters of long-chain fatty acids is the initial reaction in the major pathway in adipose tissue glycerolipid biosynthesis. This reaction is mediated by the enzyme acyl CoA:sn-glycerol-3-P acyltransferase (EC 2.3.1.15) (1). Previous studies have reported that this enzyme is found in both the mitochondria and microsomes of epididymal adipose tissue (2-5). However, the quantitative distribution pattern of this enzyme is controversial. Steinberg (2) and Tzur and Shapiro (3) reported the enzyme to be primarily in microsomes, whereas Daniel and Rubinstein (5) and Roncori and coworkers (4, 6) found the activity large-