Background & objectives: The study was undertaken to explore the locus of interaction of clofazimine and niclosamide which showed substantial growth inhibition property in Leishmania donovani promastigotes.
Methods: The uptake of final electron acceptor oxygen and 2,6-dichlorophenolindophenol (DCPIP) reduction in the electron transport chain were measured by constant volume Warburg respirometer and monitoring absorbance at 600 nm, respectively. Irreversibility of O2 uptake inhibition by clofazimine and niclosamide was determined by dilution of cell suspension followed by centrifugation.
Results: Clofazimine and niclosamide showed their minimum inhibitory concentration (MIC) at 33 and 150 micrograms/ml, respectively. Oxygen uptake inhibition by clofazimine and niclosamide was not reversed by removal of the drug by centrifugation. Rotenone, a potent inhibitor of mammalian electron transport chain showed no inhibition on the electron transport chain of L. donovani promastigotes. Cyanide at 1 mM concentration showed partial inhibition in L. donovani promastigotes. Oxygen uptake and DCPIP reduction by L. donovani promastigotes were highly sensitive to sulphhydryl group inhibitors. Strong inhibition of oxygen uptake (80-100%) by L. donovani promastigotes was achieved by clofazimine, niclosamide and amphotericin B. Amphotericin B failed to inhibit DCPIP reduction by L. donovani promastigotes, whereas DCPIP reduction was inhibited by clofazimine and niclosamide, respectively.
Interpretation & conclusion: DCPIP reduction was mediated by transplasma membrane electron transport as evidenced by its inhibition with membrane impermeable quinone 1,2-naphthoquinone-4-sulphonic acid (NQSA). Transplasma membrane electron transport requires b-cytochromes and sulphhydryl groups for its function and was inhibited by clofazimine and niclosamide.