Release Of Lyso-PAF-Acether From Platelets
The release of PAF-acether from platelets, stimulated by thrombin or the ionophore A 23187, is now well-documented. PAF- acether, 1-O-alky 1-2-ace tyl-sn-glyceryl-phosphorylcholine, may represent the molecular intermediate for the third pathway (i.e. non-ADP, non-thromboxane A2~dependent) of platelet aggregation. We now report the release from stimulated platelets of the non-ace ty la ted compound, l-0-alkyl-2-lyso-sn-glyceryl-3-phos- phorylcholine (lyso-PAF-acether). We have already published the structural identification of this molecule released from blood leukocytes.Washed rabbit platelets (5 x 108/ml) were incubated in Tyrode’s for up to 7 min in the presence of 2.5 uM ionophore or 2.5 U/ml thrombin. PAF-acether was measured by aggregation of washed aspirin-pretreated rabbit platelets in the presence of ADP scavengers. Lyso-PAF-acether was measured by the same method after its chemical acetylation into PAF-acether, followed by treatment with lipase from Rhizoppus arrhizus. Results are given in g/ml, by comparison to known amounts of totally synthetic PAF-acether. PAF-acether and lyso-PAF-acether were released with similar kinetics, reaching a maximum at 5 min. Maximal releases of PAF- acether were 79.0 ± 16.9 and 12.4 ± 8.5 ng/ml (mean ± 1 SD) and that of lyso-PAF-acether were 2.6 ± 0.6 and 1.9 ± 0.8 ug/ml, i.e. about 4 uM, for the ionophore (3 exp.) and thrombin (7 exp.) respectively. PAF-acether directly released from platelets and that obtained through acetylation of lyso-PAF-acether maintained their aggregating activity after treatment with the lipase, thus behaving like the leukocyte-derived compound.The release of lyso-PAF-acether implies the activation of a phospholipase A2 acting on a membrane alkyl-ether phospholipid. Biosynthetic incorporation of acetate into platelet PAF-acether has been reported. Therefore, lyso-PAF-acether may be an intermediate for PAF-acether synthesis ; also potent biological activities of lyso-PAF-acether can be anticipated.