The final step in the de novo biosynthesis of platelet-activating factor. Properties of a unique CDP-choline:1-alkyl-2-acetyl-sn-glycerol choline-phosphotransferase in microsomes from the renal inner medulla of rats.

Final steps in the synthesis of platelet activating factor (PAF) occur via two enzymatic reactions: the acetylation of 1-alkyl-2-lyso-sn-glycero-3-phosphocholine by a specific acetyltransferase or the transfer of the phosphocholine base group from CDP-choline to 1-alkyl-2-acetyl-sn-glycerol by a dithiothreitol (DTT)-insensitive cholinephosphotransferase. Our studies demonstrate that rat kidney inner medulla microsomes synthesize PAF primarily via the DTT-insensitive cholinephosphotransferase since the specific activity of this enzyme is greater than 100-fold higher than the acetyltransferase. The two cholinephosphotransferases that catalyze the biosynthesis of phosphatidylcholine and PAF have similar Mg2+ or Mn2+ requirements and are inhibited by Ca2+. Also topographic experiments indicated that both activities are located on the cytoplasmic face of microsomal vesicles. PAF synthesis was slightly stimulated by 10 mM DTT, whereas the enzymatic synthesis of phosphatidylcholine was inhibited greater than 95% under the same conditions. The concept of two separate enzymes for PAF and phosphatidylcholine synthesis is further substantiated by the differences in the two microsomal cholinephosphotransferase activities with respect to pH optima, substrate specificities, and their sensitivities to temperature, deoxycholate, or ethanol. Study of the substrate specificities of the DTT-insensitive cholinephosphotransferase showed that the enzyme prefers a lipid substrate with 16:0 or 18:1 sn-1-alkyl chains. Short chain esters at the sn-2 position (acetate or propionate) are utilized by the DTT-insensitive cholinephosphotransferase, but analogs with acetamide or methoxy substituents at the sn-2 position are not substrates. Also, CDP-choline is the preferred water-soluble substrate when compared to CDP-ethanolamine. Utilization of endogenous neutral lipids as a substrate by the DTT-insensitive cholinephosphotransferase demonstrated that sufficient levels of alkylacetylglycerols are normally present in rat kidney microsomes to permit the synthesis of physiological quantities of PAF. These data suggest the renal DTT-insensitive cholinephosphotransferase could be a potentially important enzyme in the regulation of systemic blood pressure.