Calcium induces membrane translocation of 12-lipoxygenase in rat platelets

Translocation of soluble 12-lipoxygenase to membranes was examined in rat platelets. Preincubation of platelet homogenates with 0.1-10 microM Ca2+ resulted in an increase in 12-lipoxygenase activity of the particulate fraction with a concomitant decrease in that of the soluble fraction. Kinetic parameters of 12-lipoxygenase of the soluble and membrane fractions were not changes in the presence of 10 microM Ca2+. Ca2+-induced association of 12-lipoxygenase to the particulate fraction was dependent on the amounts of platelet-soluble and membrane fractions but not on the incubation temperature. 12-Lipoxygenase activity associated with the particulate fraction was completely dissociated by reducing the concentration of Ca2+ to 10 nM. Ca2+-induced association of the enzyme also occurred in the boiled- and trypsin-treated membranes but was significantly reduced in the phospholipase A2-treated membranes. Soluble 12-lipoxygenase also associated to liposomes in a Ca2+-dependent manner. Pretreatment of platelets with thrombin (0.5-5 units/ml) significantly caused a translocation of soluble 12-lipoxygenase to particulate fraction; in the time course study, the translocation was observed at the thrombin pretreatment of 1, 5, and 10 min. These results suggest that stimulation of platelets is followed by the translocation of soluble 12-lipoxygenase to membranes, which is mediated by physiological concentration of Ca2+.