Kinetics of PAF transfer, distribution and metabolism in human blood in vitro.

The transfer kinetics of PAF (0.1-100 nM), deposited as a thin film on a plastic surface, to human blood were studied under conditions of physiological significance. Almost all (83-87%) available PAF was solubilized in two waves. Initially, 40-50% of the available PAF was transferred to blood very fast in less than 15 seconds while the rest, 30-45%, obeyed first order kinetics, 0.0226 < k(app) < 0.0319 sec(-1). Blood cells following a parallel to whole blood binding time course bound 20-25% of the solubilized PAF. Dilution of blood up to 1:9 with 0.15M NaCl did not affect PAF transfer parameters favouring an aqueous phase diffusion mechanism. Blood-bound PAF was allocated mainly to plasma, 67 +/- 4%, erythrocytes, 18 +/- 1% and platelets, 5 +/- 1%. These data indicated the role of the high affinity binding sites in human platelets versus the low affinity binding by erythrocytes, although the latter, due to their number, dominated cell bound PAF. Even at 100 nM there were no saturation signs for the transfer of PAF to blood or blood cells. PAF hydrolysis did not affect its binding to the blood elements. Given infinite time only 62 +/- 1% of the blood bound PAF would be metabolised by the rather slow acting, 11.5 > t(1/2) > 6.5 min, PAF-acetylhydrolase.