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.