Abstract
Clinical measurement of neopterin has been extensively used as a marker of inflammation but the in vivo mechanism generating neopterin is poorly understood. Neopterin is described as the oxidation product of 7,8-dihydroneopterin, a potent antioxidant generated by monocyte/macrophages in response to interferon-γ. While peroxyl and hydroxyl scavenging generates dihydroxanthopterin, hypochlorite efficiently oxidises 7,8-dihydroneopterin into neopterin, but this reaction alone does not explain the high levels of neopterin seen in clinical data. Here, we examine whether superoxide scavenging by 7,8-dihydroneopterin generates neopterin.
U937 cells incubated with oxLDL showed a time dependent increase superoxide and 7,8-dihydroneopterin oxidation to neopterin. Neopterin generation in oxLDL or phorbol ester treated U937 cells or human monocytes was inhibited by apocynin and PEG-SOD. Addition of the myeloperoxidase inhibitor 4-aminobenzoic acid hydrazide (ABAH) had no effect of the superoxide generation or neopterin formation. 7,8-Dihydroneopterin reacted with superoxide/hydroxy radical mixtures generated by X-ray radiolysis to give neopterin. Formation of neopterin by superoxide derived from the xanthine/xanthine oxidase system was inhibited by superoxide dismutase. Neopterin formation was inhibited by apocynin in phorbol ester treated human carotid plaque rings in tissue culture.
These results indicate that 7,8-dihydroneopterin scavenges superoxide and is subsequently oxidised into neopterin in cellular and cell-free experimental systems.
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•7,8-dihydroneopterin is oxidised to neopterin by superoxide generated by xanthine oxidase.•Superoxide generation in response to oxLDL or PMA was associated with an increase in neopterin.•Neopterin formation was reduced by apocynin in PMA treated macrophages and excised human atheroma.•7,8-Dihydroneopterin scavenges superoxide and is subsequently oxidised into neopterin.•The research suggests a significant amount of the clinically measured neopterin is a product of superoxide scavenging.