The synthesis of platelet-activating factor (PAF) was studied in microsomal fractions of cerebral cortices of 15-day-old rabbits. These included: a total microsomal fraction P3, rough and smooth microsomes, R and S, and microsomal fraction P derived from isolated nerve cell bodies. Cholinephosphotransferase (CPT) generating PAF from alkylacetylglycerol had the highest specific activities in fractions R and P (24 and 6 times the homogenate values, based on membrane phospholipid content). This CPT activity differed from that which synthesized phosphatidylcholine as the latter was sensitive to dithiothreitol inhibition and was more readily inhibited by Triton X-100. As the CPT activity for PAF synthesis relies on the production of alkylacetylglycerol we studied the acetyltransferase which forms 1-alkyl-2-acetyl-sn-glycero-3-phosphate (AAGP). This enzyme had the highest specific activity in fraction R, followed by fractions P3 and P. There was evidence that the acetyltransferase was more active in a phosphorylated form. NaF maximized the recovery of AAGP products in the assays. The pH optimium for acetylation was in a range of 8.0-9.0. Lyso PAF did not inhibit the formation of AAGP and the rates of formation of PAF by acetylation were less than 5% of values for AAGP synthesis. During AAGP formation there was no evidence for subsequent alkylacetylglycerol formation in the absence of NaF, but a small formation of radioactive PAF could be demonstrated from AAGP under the CPT assay conditions. The results of the CPT and acetyltransferase assays show a concentration of these activities particularly in rough microsomes and endoplasmic reticulum of nerve cell bodies and underline the potential for PAF synthesis de novo at these two sites.
