Inhibition of [3H]platelet activating factor (PAF) binding by Zn2+: a possible explanation for its specific PAF antiaggregating effects in human platelets.
Zinc ions in the micromolar range exhibited a strong inhibitory activity toward platelet activating factor (PAF)-induced human washed platelet activation, if added prior to this lipid chemical mediator. The concentration of Zn2+ required for 50% inhibition of aggregation (IC50) was inversely proportional to the concentration of PAF present. The IC50 values (in microM) for Zn2+ were 8.8 +/- 3.9, 27 +/- 5.8, and 34 +/- 1.7 against 2, 5, and 10 nM PAF, respectively (n = 3-6). Zn2+ exhibited comparable inhibitory effects on [3H]serotonin secretion and the IC50 values (in microM) were 10 +/- 1.2, 18 +/- 3.5, and 35 +/- 0.0 against 2, 5, and 10 nM PAF, respectively (n = 3). Under the same experimental conditions, aggregation and serotonin secretion induced by ADP (5 microM), arachidonic acid (3.3 microM), or thrombin (0.05 U/ml) were not inhibited. Introduction of Zn2+ within 0-2 min after PAF addition not only blocked further platelet aggregation and [3H]serotonin secretion but also caused reversal of aggregation. Analysis of [3H]PAF binding to platelets showed that Zn2+ as well as unlabeled PAF prevented the specific binding of [3H]PAF. The inhibition of [3H]PAF specific binding was proportional to the concentration of Zn2+ and the IC50 value was 18 +/- 2 microM against 1 nM [3H]PAF (n = 3). Other cations, such as Cd2+, Cu2+, and La3+, were ineffective as inhibitors of PAF at concentrations where Zn2+ showed its maximal effects. However, Cd2+ and Cu2+ at high concentrations exhibited a significant inhibition of the aggregation induced by 10 nM PAF with IC50 values being five- and sevenfold higher, respectively, than the IC50 for Zn2+, and with the IC50 values for inhibition of binding of 1 nM [3H]PAF being 5 and 19 times higher, respectively, than the IC50 for Zn2+. The specific inhibition of PAF-induced platelet activation and PAF binding to platelets suggested strongly that Zn2+ interacted with the functional receptor site of PAF or at a contiguous site.
Nunez
D
D
Department of Biochemistry, University of Texas Health Science Center, San Antonio 78284-7760.
Kumar
R
R
Hanahan
D J
DJ
D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000378:metabolism; D001792:Blood Platelets / Q000502:physiology; D002412:Cations / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D010712:Phosphatidic Acids / Q000378:metabolism; D010972:Platelet Activating Factor / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010974:Platelet Aggregation / Q000187:drug effects*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D012636:Secretory Rate / Q000187:drug effects; D012701:Serotonin / Q000378:metabolism*; D012710:Serum Albumin, Bovine / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology; D015032:Zinc / Q000494:pharmacology*
D001095:Arachidonic Acids; D002412:Cations; D010712:Phosphatidic Acids; D010972:Platelet Activating Factor; D010975:Platelet Aggregation Inhibitors; D012710:Serum Albumin, Bovine; D016718:Arachidonic Acid; D012701:Serotonin; D013917:Thrombin; D015032:Zinc