Artemisinin, a secondary metabolite produced in plant species, besides having antimalarial

Artemisinin, a secondary metabolite produced in plant species, besides having antimalarial properties is also phytotoxic. 104615-18-1 manufacture The compound, on the other hand, when applied to plants (inhibitory effect appeared as a consequence of the formation of an unidentified artemisinin-metabolite rather than by the interaction of the compound and experimental conditions. The results from our and studies suggest the presence of two different modes of action of the compound on photosynthetic electron transport. Artemisinin is primarily an energy transfer inhibitor of isolated thylakoid membranes. However, the studies indicate that the compound impairs the thylakoid electron flow as an inhibitor of secondary quinone (QB) of PSII (Photosystem II). The inhibition was determined to be caused by a yet uncharacterized artemisinin- metabolite rather than its direct interaction with thylakoid membrane. Thus, artemisinin may act as a natural prophytotoxin. Results Inhibitory effect of artemisinin on electron transport activity: an study To find out the potency of artemisinin as an inhibitor of thylakoid photofunction, the effect of artemisinin on electron transport rate supported by the transfer of electrons from water to FeCN (potassium ferricyanide) was examined. The extent of inhibition was assessed relative to DMSO (dimethyl sulfoxide) supplemented reaction rates as a control (Fig. 1; trace 0), which was identical to the rate obtained without addition of DMSO. The degree of inhibition was concentration dependent and showed the onset of a saturating tendency around 744 M; the highest concentration tested in this report. About 65C70% inhibition of control activity was observed at this concentration of artemisinin (inset fig. 1). Figure 1 Effect of increasing concentration of artemisinin on FeCN supported O2 evolution activity of spinach thylakoids. The inhibitory effect of the compound was further evaluated under phosphorylating and uncoupled conditions of electron flow. Two different uncouplers known to dissipate the formation of high-energy state through different means were used in this study. The NH4Cl (ammonium chloride), an amine type of uncoupler is known to arrest the acidification (chemical potential, pH) of thylakoid lumen [14] and GS (gramicidin-S) is a pore forming ionophore [15] known to collapse the electrical potential () of high energy state of the membrane. In presence of NH4Cl or GS, the extent of artemisinin-mediated (744 M) inhibition of basal electron transport was reduced by nearly 50% (Fig. 2, A). Addition of ADP (adenosine diphosphate) and iP (inorganic phosphate), as expected, stimulated the basal electron transport rate. However, a decline in the artemisinin-mediated inhibition to 45C50% as against 65C70% in basal conditions was observed under phosphorylating condition (Fig. 2, B). Although the phosphorylating electron transport rate was further stimulated in presence of uncouplers, the toxicity of artemisinin, 104615-18-1 manufacture however drastically reduced it to the tune of 15C20% in presence of NH4Cl and about 20C25% with GS (Fig. 2, B). Figure 2 Effect of artemisinin on FeCN supported electron transport in thylakoids under basal, phosphorylating and uncoupled conditions. Fast chlorophyll (Chl.) (study To gain insight about the effect of artemisinin fluorescence emission rapidly increases from an initial Rabbit Polyclonal to CCBP2 level to a maximum through transient steps, the and uphill rise in fluorescence (Fand Fand a higher Ftransient (Fig. 3, A). It is known that Fis very sensitive to changes in the redox 104615-18-1 manufacture state of the PQ-pool. The phase is correlated to the transfer of electrons through PSI as shown by DBMIB (dibromothymoquinone) treatment [19], primarily an antagonist to cytochrome b563 but having limited effect on QB site as well [20], [21]. These alternations in fluorescence characteristics may have arisen due to reduced level of electron flow through PSI, accompanied with changes in the redox state of PQ. Figure 3 The room temperature Chl. fluorescence transient of control (DMSO) and artemisinin-treated (Artemisinin) leaves (A), and the effect of DCMU (B). A small but discernible rise in Ffluorescence intensity suggests the existence of a small effect on QA reduction in dark adapted leaves. However, a faster rise in Chl. fluorescence from to Flevel in artemisinin treated leaves suggest a slow reduction of QB resulting in formation of lower pool of PQH2 (plastoquinol), which in turn alters the overall redox chemistry of QA to QB to PQH2 in PSII complex. DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) treatment that inhibits the electron flow from QA to QB by binding to QB, results in the transformation of three step fluorescence rise into an rise only due to the accumulation of QA?. Comparison of the fluorescence induction curves of control and artemisinin treated leaves showed similar sensitivity to DCMU (Fig. 3, B). These results indicate that the electron flow from water to QA is insensitive to artemisinin toxicity and the compound renders its toxic affect in the span of electron flow beyond QA. The room temperature (25C) and 77 K fluorescence emission spectral analysis (See Fig. S1, Supporting Information) also suggest that.