EFFECTS OF NEOPTERIN-DERIVATIVES ON H2O2-INDUCED LUMINOL CHEMILUMINESCENCE - MECHANISTIC ASPECTS

Neopterin, 6-D-erythro-1',2',3'-trihydroxypropyl-pterin, and its dihyd roform, 7,8-dihydro-neopterin, are synthesized by human monocytes/macr ophages upon stimulation by interferon-gamma. In the presence of iron chelator complexes neopterin enhances hydrogen peroxide-induced lumino l chemiluminescence at neutral or slightly alkaline pH (7.5). In contr ast, 7,8-dihydroneopterin scavenges chemiluminescence independently fr om the pH value and iron. In this study, we explored in more detail th e mechanism possibly involved: analysis of the reaction products shows that 7,8-dihydroneopterin is oxidized and degraded to 7,8-dihydroxant hopterin and xanthopterin, whereas the neopterin molecule is not chemi cally altered during the chemiluminescence reaction. Investigations of the neopterin-induced effect show that mannitol. a scavenger of hydro xyl radicals, does nor alter the enhancing effect of neopterin. L-hist idine, which scavenges singlet oxygen almost as effective as hydroxyl radicals, reduces the enhancing effect of neopterin. However, singlet oxygen was not detectable during the reaction by measuring monomol lig ht emission (1270 nm). When replacing hydrogen peroxide by 3-morpholin osydnonimine, a generator of hydroxyl radicals, or naphthalene-endoper oxide, a generator of singlet oxygen, in the luminol chemiluminescence assay, neopterin shows no enhancing effect irrespective of the presen ce of iron-(III)-EDTA. The data suggest that neopterin enhances hydrog en peroxide-induced luminol chemiluminescence in the presence aa of ir on-(III)-EDTA by formation of a catalytic complex that seems to favor the formation of oxygen intermediates which derive from hydrogen perox ide and react with luminol.