Abstract: Objective To test the in vitro effect of daphnetin on cytochrome C oxidase （COX） and ribonucleotide reductase （RNR） activity of Plasmodium falciparum. Methods P. falciparum （FCC1/HN） was cultured in vitro using the method of Trager and Jensen. The effect of daphnetin and daphnetin-Fe complex on COX and RNR activity of P.falciparum was tested by ultraviolet spectrophotometer and electron spin resonance （ESR） respectively. Result The parasites synchronized with sorbitol in vitro was treated by daphnetin and daphnetin-Fe complex. The inhibition level of the COX activity by daphnetin after being treated for 2， 4， 8 and 12 h were 0.6%， 73% and 80% respectively and the inhibition level by daphnetin at different concentrations （0.1， 1， 100 and 1mmol/L） for 6h was 3%， 31%， 53% and 84%， respectively. No considerable effect was observed on the COX activity of P.falciparum treated with daphnetin-Fe complex. The tyrosyl free radical was tested to reflect the RNR activity of P.falciparum at various times by ESR. The inhibition level by daphnetin for 1， 2， 3 and 4 h were 7%， 51%， 69% and 75% respectively， while the values treated by daphnetin-Fe complex were 8%， 6%， 11% and 9% respectively. The inhibition level by daphnetin at different concentrations （0.1， 1， 100 and 1 mmol/L） for 6 h was 3%， 31%， 58% and 93% respectively and while the values treated by daphnetin-Fe complex were 8%， 6%， 11% and 9%. Conclusion Daphnetin significantly reduces the COX and RNR activities of Plasmodium falciparum in vitro.

Key words: Daphnetin, Plasmodium falciparum, Cytochrome C oxidase（COX）, Ribonucleotide reductase（RNR）