Role of nitric oxide and superoxide in Giardia lamblia killing

ABSTRACT

Giardia lamblia trophozoites were incubated for 2 h with activated murine macrophages, nitric oxide (NO) donors or a superoxide anion generator (20 mU/ml xanthine oxidase plus 1 mM xanthine). Activated macrophages were cytotoxic to Giardia trophozoites (~60 per cent dead trophozoites). This effect was inhibited (>90 per cent) by an NO synthase inhibitor (200 muM) and unaffected by superoxide dismutase (SOD, 300 U/ml). Giardia trophozoites were killed by the NO donors S-nitroso-acetyl-penicillamine(SNAP)and sodium nitroprusside (SNP) in a dose-dependent manner (LD50 300 and 50 muM, respectively). A dual NO-superoxide anion donor, 3-morpholino-sydnonimine hydrochloride (SIN-1), did not have a killing effect in concentration up to 1 mM. However, when SOD (300 U/ml) was added simultaneously with SIN-1 to Giardia, a significant trophozoite-killing effect was observed (~35 per cent dead trophozoites at 1 mM). The mixture of SNAP or SNP with superoxide anion, which yields peroxynitrite, abolished the trophozoite killing induced by NO donors. Authentic peroxynitrite only killed trophozoites at very high concentrations (3 mM). These results indicate that NO accounts for Giardia trophozoite killing and this effect is not mediated by peroxynitrite.