Arachidonic acid-induced malondialdehyde formation in rat platelets. Kinetic aspects and inhibition by acetylsalicylic acid and indomethacin.
Malondialdehyde (MDA) is a stable product of arachidonic acid metabolism, catalyzed by the enzyme cyclo-oxygenase. The experimental conditions for measuring the kinetic of MDA formation in rat platelet-rich plasma were defined. In platelets stimulated with arachidonic acid MDA formation was almost linear for a limited period of time (between 0 and 2 min) and was concentration-dependent with saturation kinetics. The hyperbolic curves obtained were recast in a linear plot (according to transformation S/V versus S) and straight lines fitting all experimental points were obtained. The apparent Km value of arachidonic acid was 0.49 +/- 0.09 mM and Vmax 1.44 +/- 0.06 nmoles MDA/1.4 x 10(9) platelets/min. The apparent type of inhibition and the relative potency of acetylsalicylic acid and indomethacin on MDA formation were also investigated. Inhibition by both drugs was concentration-dependent, and was much stronger when either drug was preincubated with platelets for 10 min than for 1 min. Analysis of the data by Dixon plots (1/V versus inhibitor concentrations) revealed an apparently competitive type of inhibition. It is suggested that both acetylsalicylic acid and indomethacin inhibit cyclo-oxygenase in platelet-rich plasma by a similar mechanism, not involving covalent binding of either drug to the enzyme.
D000818:Animals; D001095:Arachidonic Acids / Q000494:pharmacology*; D001241:Aspirin / Q000494:pharmacology*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D007213:Indomethacin / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D008314:Malonates / Q000097:blood*; D008315:Malondialdehyde / Q000097:blood*; D051381:Rats; D013997:Time Factors
D001095:Arachidonic Acids; D008314:Malonates; D008315:Malondialdehyde; D001241:Aspirin; D007213:Indomethacin