Inhibition of the leukotriene synthetase of rat basophil leukemia cells by diethylcarbamazine, and synergism between diethylcarbamazine and piriprost, a 5-lipoxygenase inhibitor.

Diethylcarbamazine inhibited the formation of sulfidopeptide leukotrienes in rat basophil leukemia (RBL) cells (50% inhibitory concentration, EC50, 3 mM). Similar concentrations also inhibited the formation of leukotriene C4 (LTC4) by LTC synthetase, a detergent-solubilized cell free particulate enzyme from RBL cells which is capable of coupling LTA4 to glutathione. By contrast, the conversion of LTA4 to LTC4 using enzymes from rat liver was at least ten times less sensitive to this inhibitor. The EC50 for inhibition of the leukotriene C synthetase of RBL cells was directly proportional to the LTA4 concentration in the incubations, ranging from 1.5 mM at 10 microM LTA4 to over 40 mM at 500 microM LTA4. Kinetic analysis revealed that the inhibition of the leukotriene C synthetase reaction by diethylcarbamazine was competitive with respect to LTA4. In contrast to diethylcarbamazine, piriprost (U-60,257; 6,9-deepoxy-6,9-(phenylimino)-delta 6,8-prostaglandin I1), which inhibits the formation of sulfidopeptide leuktrienes in RBL cells at the 5-lipoxygenase step (EC50 5 microM), did not inhibit the leukotriene synthetase of these cells. On the other hand, low concentrations of piriprost, which had no demonstrable inhibitory activity on leukotriene formation by themselves, markedly synergized the inhibitory activity of diethylcarbamazine. These results are consistent with the interpretation that both piriprost and diethylcarbamazine inhibit leukotriene formation but that they act on sequential steps in the biosynthetic pathway in such a manner as to synergistically interfere with the availability or utilization of LTA4 in the leukotriene C synthetase reaction.