Inhibition of neutrophil superoxide generation by hypericin, an antiretroviral agent

We previously reported that phorbol 12-myristate 13-acetate (PMA)-induced superoxide[formula]generation of neutrophils was inhibited by hypericin, a photosensitizing pigment found in St. Johnswort (herbHypericin triquetrifoliumTurra), via a mechanism involving protein kinase C (PKC). To obtain further insights into the mechanism of inhibition, the effects of hypericin on stimulation-dependent[formula]generation and related enzymes of neutrophils were investigated. Hypericin inhibited[formula]generation of neutrophils induced by PKC-dependent and -independent stimuli in a light- and concentration-dependent manner. Oxygen was required for the light-dependent inhibition by hypericin. NADPH oxidase activity in a cell-free system and TNF-α-induced tyrosyl phosphorylation of neutrophil proteins were also inhibited by hypericin in a concentration- and light-dependent manner. However, tyrosine kinase of p60^src, an enzyme not bound to a membrane, was not inhibited either in the light or in the dark. Oxygen uptake of neutrophils by photosensitization with hypericin resulted in the formation of singlet oxygen (¹O₂), [formula], and hydroxyl radical (·OH) and enhanced lipid peroxidation. The formation of¹O₂was inhibited by azide, a quencher of¹O₂, but not by desferrioxamine (DSF), a ferric ion chelator. By contrast, both generation of ·OH and lipid peroxidation were inhibited by DSF but not by azide. Furthermore, PMA-induced[formula]generation inhibited by hypericin partially recovered in the presence of azide but not DSF. These results suggested that the light-dependent inhibition of[formula]generation by hypericin might be due to inhibition of tyrosine kinase, PKC, and NADPH oxidase via an oxygen-dependent mechanism, possibly through both Type I and II photosensitization mechanisms.