Nitrite generation in interleukin-4-treated human macrophage cultures does not involve the nitric oxide synthase pathway

The search continues for high-output nitric oxide biosynthesis in human macrophages analogous to murine phagocytes. Recently, generation of nitrite in culture supernatants of human macrophages exposed to interferon-gamma and interleukin-4 (IFN-gamma/IL-4) was reported. The present study reproduces these findings and shows that L-arginine is not consumed and L-citrulline is not produced during this process. Furthermore, the biosynthesis of the obligatory cofactor tetrahydrobiopterin is not coinduced. These biochemical data provide support against a nitric oxide synthase contribution to nitrite accumulation. Nitrite was generated from nitrate salts even in cell-free media. Nitric oxide synthase activity but not nitrate reduction depended on molecular oxygen. Nitrite accumulation in experiments with IFN-gamma/IL-4 in human monocytes appears to be an in vitro artifact produced by nitrate-reducing activities contained in cytokine preparations.