L-ascorbic acid modulates 5-lipoxygenase activity in human polymorphonuclear leukocytes.

Abstract

Leukotriene B4 (LTB4), a metabolite of the arachidonic acid pathway mediated by 5-lipoxygenase, is released by stimulated polymorphonuclear neutrophils (PMNL) and has been postulated to be an important mediator of the inflammatory response. Extracellular L-ascorbate at concentrations in the range of 0.5-5.0 mM effectively inhibits the biosynthesis of LTB4 and 5-hydroxyeicosatetraenoic acid (5-HETE) stimulated by the calcium ionophore A 23187. The ionophore-activated LTB4 production is reduced after incubation of PMNL with opsonized zymosan as a phagocytic stimulus. Extracellular L-ascorbate at concentrations above 0.1 mM reverses the zymosan-induced deactivation of 5-lipoxygenase, resulting in significantly higher LTB4 and 5-HETE yields. The inhibitory effect of zymosan preincubation on LTB4 production is independent of calcium or free arachidonic acid in the incubation medium. Interaction of L-ascorbate with the catabolism of LTB4 was excluded by measuring the trihydroxy metabolites which were unchanged. Furthermore, the formation of glutathione derivatives of leukotriene A4 can be excluded due to the lack of glutathione transferase activity in PMNL. In order to link the intracellular function of L-ascorbate with the serum level, the ascorbate uptake has been studied in more detail. The L-ascorbate transport into PMNL is stereospecific and can best be described by kinetics consisting of a saturation part, Km and Vmax being 39 microM and 0.28 nmol/10(8) cells.min, respectively, plus passive diffusion, the diffusion coefficient P being 1.75 microliter/10(8) cells.min. Furthermore, the uptake is inhibited by the isomers D-ascorbic acid and D-erythorbic acid.(ABSTRACT TRUNCATED AT 250 WORDS)