Examination of the Inhibitory Effect of Norathyriol in Formylmethionyl-Leucyl-Phenylalanine-Induced Respiratory Burst in Rat Neutrophils

Norathyriol, aglycone of a xanthone C-glycoside mangiferin isolated from Tripterospermum lanceolatum, concentration dependently inhibited the formylmethionyl-leucyl-phenylalanine (fMLP)-induced superoxide anion (O2˙−) generation and O2 consumption in rat neutrophils. In cell-free oxygen radical generating system, norathyriol inhibited the O2˙− generation during dihydroxyfumaric acid (DHF) autoxidation and in hypoxanthine-xanthine oxidase system. fMLP-induced transient elevation of [Ca2+]i and the formation of inositol trisphosphate (IP3) were significantly inhibited by norathyriol (30 μM) (about 30 and 46% inhibition, respectively). Norathyriol concentration dependently suppressed the neutrophil cytosolic phospholipase C (PLC). In contrast with the marked attenuation of fMLP-induced protein tyrosine phosphorylation (about 70% inhibition at 10 μM norathyriol), norathyriol only slightly modulated the phospholipase D (PLD) activity as determined by the formation of phosphatidic acid (PA) and, in the presence of ethanol, phosphatidylethanol (PEt). Norathyriol did not modulate the intracellular cyclic AMP level. In the presence of NADPH, the phorbol 12-myristate 13-acetate (PMA)-activated particulate NADPH oxidase activity was suppressed by norathyriol in a concentration-dependent manner and the inhibition was noncompetitive with respect to NADPH. Norathyriol inhibited the iodonitrotetrazolium violet (INT) reduction in arachidonic acid (AA)-activated cell-free NADPH oxidase system at the same concentration range as those used in the suppression of PMA-activated particulate NADPH oxidase activity. Taken together, these results suggest that the scavenging ability of norathyriol contributes to the reduction of generated O2˙−, however, the inhibition of O2˙− generation from neutrophils by norathyriol is attributed to the blockade of PLC pathway, the attenuation of protein tyrosine phosphorylation, and to the suppression of NADPH oxidase through the interruption of electrons transport.