QINGHAOSU (ARTEMISININ)—A FANTASTIC ANTIMALARIAL DRUG FROM A TRADITIONAL CHINESE MEDICINE - Medicinal Chemistry of Bioactive Natural Products - page 233

Figure 5-6. Agarose gel illustrates the cleavage reactions of calf thymus DNA and salmon
DNA by qinghaosu or artemether and ferrous ion at 37 C for 12 hours in a phosphate buffer
solution.
parasite does. If qinghaosu could permeate the membrane, reach the nucleus of the
parasite, and react with Fe(II), it would be possible that the interaction with DNA
takes place. Therefore, this probability may explain why qinghaosu is only toxic to
the parasite, but not to the normal red blood cell.
Considering that the pH in blood serum is about 7.35-7.45 and that the ferrous
ion will precipitate above pH 7, the DNA damage experiments with qinghaosu and
stoichiometric ferrous ion were performed in aqueous acetonitrile (1:1), at 37 C
and at pH 6.5 adjusted with a phosphate buffer. It was interesting to find that the
cleavage of calf thymus, salmon, and a supercoiled DNA pUC 18 was observed
yielding a DNA fragment with about 100 base-pair (the marks not shown in the
figures) (Figures 5-6 and 5-7). 278,289 The phosphate buffer is important; otherwise,
the calf thymus DNA was totally cleaved, when the concentration of FeSO 4 was
200-2000 mM even in the absence of qinghaosu. Changing forms and no change
was only observed as the concentration of FeSO 4 was 10-25 mM and below
10 mM for the supercoiled DNA in the absence of a phosphate buffer and qinghaosu.
It was known that qinghaosu and its derivatives could react with a catalytic
amount of ferrous or ferric ions in the presence of excess reduced agents including
cysteine or glutathione (GSH) to afford the free radical reaction products
(vide ante). When the concentration of pUC18 and qinghaosu in aqueous acetonitril
was 0.040 mg/mL and 2 mM, the combination of 10 mM of FeSO 4 and 400 mMof
L-cysteine/NaHCO 3 would cleave the supercoiled DNA pUC18 totally. The
absence of either qinghaosu, FeSO 4 , or- L-cysteine/NaHCO 3 would cause the
DNA to not be cleaved. The results were similar, if the calf thymus DNA was
used instead of pUC18 as the substrate, but a fragment of 100 BP could be detected