THE POTENTIAL FOR PLATELET-ACTIVATING-FACTOR SYNTHESIS IN BRAIN - PROPERTIES OF CHOLINEPHOSPHOTRANSFERASE AND 1-ALKYL-SN-GLYCERO-3-PHOSPHATE ACETYLTRANSFERASE IN MICROSOMAL FRACTIONS OF IMMATURE RABBIT CEREBRAL-CORTEX

The synthesis of platelet-activating factor (PAF) was studied in micro somal fractions of cerebral cortices of 15-day-old rabbits. These incl uded: a total microsomal fraction P3, rough and smooth microsomes, R a nd S, and microsomal fraction P derived from isolated nerve cell bodie s. Cholinephosphotransferase (CPT) generating PAF from alkylacetylglyc erol 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 phosphatidylc holine as the latter was sensitive to dithiothreitol inhibition and wa s more readily inhibited by Triton X-100. As the CPT activity for PAF synthesis relies on the production of alkylacetylglycerol we studied t he acetyltransferase which forms 1-alkyl-2-acetyl-sn-glycero-3-phospha te (AAGP). This enzyme had the highest specific activity in fraction R , followed by fractions P3 and P. There was evidence that the acetyltr ansferase was more active in a phosphorylated form. NaF maximized the recovery of AAGP products in the assays. The pH optimium for acetylati on was in a range of 8.0-9.0. Lyso PAF did not inhibit the formation o f 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 e vidence for subsequent alkylacetylglycerol formation in the absence of NaF, but a small formation of radioactive PAF could be demonstrated f rom AAGP under the CPT assay conditions. The results of the CPT and ac etyltransferase assays show a concentration of these activities partic ularly in rough microsomes and endoplasmic reticulum of nerve cell bod ies and underline the potential for PAF synthesis de novo at these two sites.