Inhibition of 5-lipoxygenase and Leukotriene C₄Synthase in Human Blood Cells by Thymoquinone

Abstract

Black cumin seed, Nigella sativa L., and its oils have traditionally been used for the treatment of asthma and other inflammatory diseases. Thymoquinone (TQ) has been proposed to be one of the major active components of the drug. Since leukotrienes (LTs) are important mediators in asthma and inflammatory processes, the effects of TQ on leukotriene formation were studied in human blood cells. TQ provoked a significant concentration-dependent inhibition of both LTC4 and LTB4 formation from endogenous substrate in human granulocyte suspensions with IC50 values of 1.8 and 2.3 microM, respectively, at 15 min. Major inhibitory effect was on the 5-lipoxygenase activity (IC50 3 microM) as evidenced by suppressed conversion of exogenous arachidonic acid into 5-hydroxy eicosatetraenoic acid (5HETE) in sonicated polymorphonuclear cell suspensions. In addition, TQ induced a significant inhibition of LTC4 synthase activity, with an IC50 of 10 microM, as judged by suppressed transformation of exogenous LTA4 into LTC4. In contrast, the drug was without any inhibitory effect on LTA4 hydrolase activity. When exogenous LTA4 was added to intact or sonicated platelet suspensions preincubated with TQ, a similar inhibition of LTC4 synthase activity was observed as in human granulocyte suspensions. The unselective protein kinase inhibitor, staurosporine failed to prevent inhibition of LTC4 synthase activity induced by TQ. The findings demonstrate that TQ potently inhibits the formation of leukotrienes in human blood cells. The inhibitory effect was dose- and time-dependent and was exerted on both 5-lipoxygenase and LTC4 synthase activity.

Inhibition of 5-Lipoxygenase Product Synthesis by Natural Compounds of Plant Origin

“…Direct inhibition of 5-LO in cellfree assays was demonstrated for the ortho-quinone aethiopinone 79 from Salvia aethiopsis (L.) [150] and thymoquinone 77 [63] with IC 50 values of 0.11 and 3 mM, respectively, and both compounds suppressed 5-LO in activated leukocytes with comparable efficacies ( Table 5). More detailed analysis of thymoquinone's 77 action revealed that, in addition to 5-LO (IC 50 = 3 mM), it also blocks LTC 4 synthase activity, albeit less potently (IC 50 = 10 mM) [63], and aethiopinone 79 inhibited synovial PLA 2 (IC 50 = 10 mM), but had no effects on COX enzymes [150]. Some of the investigated quinones {e. g., thymoquinone 77, atracylochromene 80 and 2-[(2E)-3,7-dimethyl-2,6-octadienyl]-6-methyl-2,5-cyclohexadiene-1,4-dione 81} also inhibited COX-1 at about 10-or more-fold higher concentrations [53], [118], [148].…”

“…[53], [118] might be superior, at least in intact cells (IC 50 = 0.2 ± 7.5 mM, see Table 5). Thymoquinone 77 and its polymer nigellone 78 from Nigella sativa (L.) that lack pronounced lipophilic residues are quite efficient as well [63], [148], [149]. Direct inhibition of 5-LO in cellfree assays was demonstrated for the ortho-quinone aethiopinone 79 from Salvia aethiopsis (L.) [150] and thymoquinone 77 [63] with IC 50 values of 0.11 and 3 mM, respectively, and both compounds suppressed 5-LO in activated leukocytes with comparable efficacies ( Table 5).…”

“…Thymoquinone 77 and its polymer nigellone 78 from Nigella sativa (L.) that lack pronounced lipophilic residues are quite efficient as well [63], [148], [149]. Direct inhibition of 5-LO in cellfree assays was demonstrated for the ortho-quinone aethiopinone 79 from Salvia aethiopsis (L.) [150] and thymoquinone 77 [63] with IC 50 values of 0.11 and 3 mM, respectively, and both compounds suppressed 5-LO in activated leukocytes with comparable efficacies ( Table 5). More detailed analysis of thymoquinone's 77 action revealed that, in addition to 5-LO (IC 50 = 3 mM), it also blocks LTC 4 synthase activity, albeit less potently (IC 50 = 10 mM) [63], and aethiopinone 79 inhibited synovial PLA 2 (IC 50 = 10 mM), but had no effects on COX enzymes [150].…”

“…Well-recognized inhibitors of LTA 4 hydrolase are bestatin, captopril, and kelatorphan that reduce formation of LTB 4 without affecting 5-LO [61]. The sesquiterpene lactone helenalin 93, present in species of the Asteraceae family, and thymoquinone 77 from Nigella sativa directly inhibit LTC 4 synthase besides inhibition of 5-LO [62], [63].…”

Broncho-relaxant activity of nigella sativa versus anthemis nobilis in chronic bronchial asthma; a comparative study of efficacy

“…Prednisolone which is used in group I, produces bronchial muscle relaxation indirectly through inhibiting the release of LTs and reversing mucosal edema. [11] In group II with chamomile, significant is poor when given by inhalation. No real improvement occurs in asthmatic symptoms in spite of its relaxant effect on rabbit tracheal muscle contraction when given by inhalation.…”