Solubilization and purification of rat liver microsomal 1,2-diacylglycerol: CDP-choline cholinephosphotransferase and 1,2-diacylglycerol: CDP-ethanolamine ethanolaminephosphotransferase.

1. The procedure, which involved 2-step sonication of microsomes at pH 7.4 and then at pH 8.5 in the presence of sodium deoxycholate and subsequent dialysis, resulted in 4-5-fold purification of choline-phosphotransferase and ethanolaminephosphotransferase with the yield of 40-50%. 2. Ethanolaminephosphotransferase was further purified 8.5-fold over microsomes by sucrose density gradient centrifugation of the partially purified preparation, while cholinephosphotransferase activity was considerably lost during this procedure. No separation of the two transferases from each other was achieved at this step. 3. Cholinephosphotransferase required Mg2+ as cofactor, and microsomal phospholipids for its maximal activity. On the other hand, Mn2+ was more effective than Mg2+ as cofactor for ethanol aminephosphotransferase, and this enzyme was inhibited by microsomal phospholipids. 4. Both transferases were stimulated several-fold by sodium deoxycholate and also showed similar optimal pH ranging from pH 8.0 to 8.5. 5. Km values for 1,2-diacylglycerol emulsion were 81.0 muM for cholinephosphotransferase and 63.0 muM for ethanolaminephosphotransferase, respectively. CDP-choline and CDP-ethanolamine competitively inhibited, with the same Ki value (both 350 muM), ethanolaminephosphotransferase and cholinephosphotransferase, respectively. The Ki values obtained were much greater than the corresponding Km values for the cytidine substrates (36.4 muM for CDP-choline and 22.0 muM for CDP-ethanolamine). 6. The partially purified enzymes were further treated with Triton X-100. When enzyme activities were assayed with Mg2+, cholinephosphotransferase, although considerably inactivated, was partially separated from ethanolaminephosphotransferase by sucrose density gradient centrifugation of Triton-treated preparations. Furthermore, cholinephosphotransferase (but not ethanol-aminephosphotransferase) itself was partially separated into Mg2+ -requiring and Mn2+ -requiring components. In contrast, ethanolaminephosphotransferase assayed with either Mg2+ or Mn2+ formed a single peak together with Mn2+ -requiring cholinephosphotransferase.