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

Abstract

Diethylcarbamazine inhibited the formation of sulfidopeptide leukotrienes in rat basophil leukemia (RBL) cells (50% inhibitory concentration, ec 50, 3 mM). Similar concentrations also inhibited the formation of leukotriene C 4 (LTC 4) by LTC synthetase, a detergent-solubilized cell free particulate enzyme from RBL cells which is capable of coupling LTA 4 to glutathione. By contrast, the conversion of LTA 4 to LTC 4 using enzymes from rat liver was at least ten times less sensitive to this inhibitor. The ec 50 for inhibition of the leukotriene C synthetase of RBL cells was directly proportional to the LTA 4 concentration in the incubations, ranging from 1.5 mM at 10 μM LTA 4 to over 40 mM at 500 μM LTA 4. Kinetic analysis revealed that the inhibition of the leukotriene C synthetase reaction by diethylcarbamazine was competitive with respect to LTA 4. In contrast to diethylcarbamazine, piriprost (U-60, 257; 6,9-deepoxy-6,9-(phenylimino)-Δ 6,8-prostaglandin I 1), which inhibits the formation of sulfidopeptide leuktrienes in RBL cells at the 5-lipoxygenase step ( ec 50 5 μM), 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 LTA 4 in the leukotriene C synthetase reaction.