A comparison of effects of sulfasalazine and its metabolites on the metabolism of endogenous vs. exogenous arachidonic acid

Sulfasalazine and, to a lesser extent, 5-aminosalicylic acid and N-acetyl-aminosalicylic acid, were found to block production of 5-hydroxy-6,8,11,14-eicosatetraenoic acid, leukotriene B₄ (LTB₄), and LTB₄ stereoisomers from both exogenous and endogenous [¹⁴C]arachidonic acid (¹⁴C-AA) in ionophore A23187 (1 μg/ml)-stimulated human neutrophils. Lipids were assessed by thin-layer chromatography and reverse-phase high-pressure lipid chromatography. Sulfasalazine blocked the synthesis of these metabolites from both exogenous and endogenous AA, but was more effective in blocking the metabolism of exogenous than endogenous AA. The IC₅₀ for sulfasalazine in blocking the synthesis of LTB₄ was 0.8 mM when exogenous AA was the substrate and 2.8 mM when endogenous AA was the substrate. N-Acetyl-aminosalicylic acid showed a similar pattern, but was less effective than sulfasalazine (IC₅₀ for exogenous AA was 5.4 mM, and for endogenous AA was 8.0 mM). 5-Aminosalicylic acid had similar effects with an IC₅₀ of 6.0 and 6.4 mM respectively. Sulfasalazine but not 5-aminosalicylic acid inhibited the incorporation of arachidonic acid into phospholipids and triglycerides. Sulfasalazine, but not its metabolites, inhibited the release of ¹⁴C-AA from membrane phospholipids in a dose-dependent manner (46.0% inhibition with 4mM sulfasalazine). Sulfasalazine also blocked the metabolism of exogenously added LTB₄ to 20-OH LTB₄ and 20-COOH LTB₄ with an IC₅₀ of 2 mM. Our findings suggest that under physiologic conditions, with endogenous AA as a substrate, sulfasalazine acts an in inhibitor of lipoxygenase, of phospholipase A₂ and of LTB₄ metabolism, whereas 5-aminosalicylic acid and N-acetyl-aminosalicylic acid inhibit only lipoxygenase.