Binding and Metabolism of Platelet-Activating Factor (PAF) by Isolated Rat Type II Pneumonocytes

Abstract The specific binding of platelet-activating factor (PAF) to isolated type II pneumonocytes from rat lung has been investigated employing an intact cell preparation. The dissociation constant ( K d ) for the autacoid has been determined to be 0.46 × 10 −9 M and approximately 3000 receptor sites per cell are present. In studies conducted on the metabolism of PAF in these cells, it was demonstrated that PAF is rapidly converted into 1-alkyl-2-acylglycerophosphocholine (alkyl-acyl GPC). After longer time intervals there was a substantial conversion of alkyl-acyl GPC into alkyl-acylglycerophosphoethanolamine (allkyl-acyl GPE). Both the alkyl-acyl GPC and alkyl-acyl GPE fractions were devoid of plasmalogens. The alkyl-acyl GPC fraction was further characterized and a distinct double peak could be visualized following thin-layer chromatography and the same lyso-compound was produced from both peaks following mild alkaline hydrolysis. By using appropriate standards it was concluded that the dual alkyl-acyl GPC peaks represent differences in the fatty acid present in the sn -2 position. One peak corresponds to the presence of saturated fatty acid in the sn -2 position and is probably due to the characteristic high capacity of the type II cells to produce disaturated glycerophospholipids.