On the interference of boswellic acids with 5-lipoxygenase: Mechanistic studies in vitro and pharmacological relevance

Abstract

Boswellic acids are pharmacologically active ingredients of frankincense with anti-inflammatory properties. It was shown that in vitro 11-keto-boswellic acids inhibit 5-lipoxygenase (5-LO, EC 1.13.11.34), the key enzyme in leukotriene biosynthesis , which may account for their anti-inflammatory effectiveness. However, whether 11-keto-boswellic acids interfere with 5-LO under physiologically relevant conditions (i.e., in whole blood assays) and whether they inhibit 5-LO in vivo is unknown. Inhibition of human 5-LO by the major naturally occurring boswellic acids was analyzed in cell-free and cell-based activity assays. Moreover, interference of boswellic acids with 5-LO in neutrophil incubations in the presence of albumin and in human whole blood was assessed, and plasma leukotriene B 4 of frankincense-treated healthy volunteers was determined. Factors influencing 5-LO activity (i.e., Ca 2+, phospholipids, substrate concentration) significantly modulate the potency of 11-keto-boswellic acids to inhibit 5-LO. Moreover, 11-keto-boswellic acids efficiently suppressed 5-LO product formation in isolated neutrophils (IC 50 = 2.8 to 8.8 μM) but failed to inhibit 5-LO product formation in human whole blood. In the presence of albumin (10 mg/ml), 5-LO inhibition by 11-keto-boswellic acids (up to 30 μM) in neutrophils was abolished, apparently due to strong albumin-binding (> 95%) of 11-keto-boswellic acids. Finally, single dose (800 mg) oral administration of frankincense extracts to human healthy volunteers failed to suppress leukotriene B 4 plasma levels. Our data show that boswellic acids are direct 5-LO inhibitors that efficiently suppress 5-LO product synthesis in common in vitro test models, however, the pharmacological relevance of such interference in vivo seems questionable.