The effect of the oxidized form of neopterin (NP) on the NADPH-dependent superoxide-generating oxidase (NADPH-oxidase) was investigated in both whole-cell and cell-free activation systems by using peritoneal macrophages of rats which had received an intraperitoneal injection of mineral oil. In the whole-cell system, NP remarkably inhibited the generation of Superoxides in macrophages stimulated with phorbol myristate acetate (PMA). NP also showed an significant suppression of the activation of superoxide-generating NADPH-oxidase in the cell-free system using solubilized membranes and sodium dodecyl sulfate (SDS) as a stimulant. The 50%-inhibitory concentration (IC50) of NP was about 1 μM in both assay systems. In a kinetic study, competitive inhibition of the NADPH-oxidase by NP was observed in the cell-free system with a calculated inhibition constant (K i) of 6.50 μM. These findings suggest that NP may play an important role in the suppression of Superoxide generation via the inhibition of the NADPH-oxidase in phagocytes.
