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The respiratory quinone composition of the parasitic protozoa Leishmania donovani promastigote was investigated. 1'-oxomenaquinone-7, a chlorobiumquinone was found to be the major isoprenoid quinone. Substantial level of ubiquinone-9 was also present. Isolation and identification of the quinone from the purified plasma membrane yielded mainly 1'-oxomenaquinone-7 and ubiquinone-9; menaquinone was not detected. Membrane bound 1'-oxomenaquinone-7 could be destroyed by near-ultraviolet irradiation, with a concomitant loss or stimulation of plasma membrane electron transport activities. The abilities of different quinones to restore alpha-lipoic acid and ferricyanide reductase activity in near UV-irradiated cell preparations were compared. The order was; conjugate of chlorobiumquinone and sphingosine base approximately conjugate of 2-methyl-3-(1'-oxooctadecyl)-1,4-napthoquinone and octadecylamine >> chlorobiumquinone approximately 2-methyl-3-(1'-oxooctadecyl)-1,4-napthoquinone > menaquinone-4 approximately ubiquinone-10. After irradiation with near-UV light, transmembrane alpha-lipoic acid reduction was inhibited, while transmembrane ferricyanide reduction was stimulated. The result obtained indicates that chlorobiumquinone mediates the plasma membrane electron transport between cytosolic reductant and oxygen as well as alpha-lipoic acid. UV-inactivation of chlorobiumquinone shuts down the plasma membrane oxygen uptake and diverts the electron flux towards ferricyanide reduction via ubiquinone-9. Chlorobiumquinone is the only example of a polyisoprenoid quinone containing a side chain carbonyl group from photosynthetic green-sulphur bacteria. Recent work has revealed numerous genes of trypanosomatid sharing common ancestry with plants and/or bacteria. These observations pose some fascinating questions about the evolutionary biology of this important group of parasitic protozoa.

An investigation has been made of the points of coupling of four nonpermeable electron acceptors e.g., alpha-lipoic acid (ALA), 5,5'-dithiobis (2-nitroaniline-N-sulphonic acid) (DTNS), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide which are mainly reduced via an interaction with the redox sites present in the plasma membrane of Leishmania donovani promastigotes. ALA, DTNS, NQSA and ferricyanide reduction and part of O2 reduction is shown to take place on the exoplasmic face of the cell, for it is affected by external pH and agents that react with the external surface. Redox enzymes of the transplasma membrane electron transport system orderly transfer electron from one redox carrier to the next with the molecular oxygen as the final electron acceptor. The redox carriers mediate the transfer of electrons from metabolically generated reductant to nonpermeable electron acceptors and oxygen. At a pH of 6.4, respiration of Leishmania cells on glucose substrate shut down almost completely upon addition of an uncoupler FCCP and K+-ionophore valinomycin. The most pronounced effects on O2 uptake were obtained by treatment with antimycin A, 2-heptadecyl-4-hydroxyquinone-N-oxide, paracholoromercuribenzene sulphonic acid and trifluoperazine. Relatively smaller effects were obtained by treatment with potassium cyanide. Inhibition observed with respect to the reduction of the electron acceptors ALA, DTNS, NQSA and ferricyanide was not similar in most cases. The redox chain appears to be branched at several points and it is suggested that this redox chain incorporate iron-sulphur center, b-cytochromes, cyanide insensitive oxygen redox site, Na+ and K+ channel, capsaicin inhibited energy coupling site and trifluoperazine inhibited energy linked P-type ATPase. We analyzed the influence of ionic composition of the medium on reduction of electron acceptors in Leishmania donovani promastigotes. Our data suggest that K+ have some role for ALA reduction and Na+ for ferricyanide reduction. No significant effects were found with DTNS and NQSA reduction when Na+ or K+ was omitted from the medium. Stimulation of ALA, DTNS, NQSA and ferricyanide reduction was obtained by omitting Cl- from the medium. We propose that this redox system may be an energy source for control of membrane function in Leishmania cells.

Leishmania donovani cells, capable of reducing certain electron acceptors with redox potentials at pH 7.0 down to -290 mV, outside the plasma membrane, can reduce the oxidised form of alpha-lipoic acid. alpha-Lipoic acid has been used as natural electron acceptor probe for studying the mechanism of transplasma membrane electron transport. Transmembrane alpha-lipoic acid reduction by Leishmania was not inhibited by mitochondrial inhibitors as azide, cyanide, rotenone or antimycin A, but responded to hemin, modifiers of sulphhydryl groups and inhibitor of glycolysis. The protonophores carbonyl cyanide chlorophenylhydrazone and 2,4-dinitrophenol showed inhibition of alpha-lipoic acid reduction. This transmembrane redox system differs from that of mammalian cells in respect to its sensitivity of UV irradiation and stimulation by diphenylamine. Thus a naphthoquinone coenzyme appears to be involved in alpha-lipoic acid reduction by Leishmania cells.