Intraerythrocytic malaria parasites produce vast amounts of lactic acid through glycolysis. While the egress of lactate is very rapid, the mode of extrusion of H+ is not known. The possible involvement of a Na+/H+ antiport in the extrusion of protons across the plasma membrane of Plasmodium falciparum has been investigated by using the fluorescent pH probe 6-carboxyfluorescein. The resting cytosolic pH was 7.27 +/- 0.1 in ring stage parasites and 7.31 +/- 0.12 in trophozoites. Spontaneous acidification of parasite cytosol was observed in Na(+)-free medium and realkalinization occurred upon addition of Na+ to the medium in a concentration-dependent manner, with no apparent saturation. The rate of H(+)-efflux at the ring stage was higher than that at the trophozoite stage due to the larger surface/volume ratio of the young parasite stage. Na(+)-dependent H(+)-efflux was: 1) inhibited by the Na+/H+ inhibitors amiloride and 5-(N-ethyl-N-isopropyl) amiloride (EIPA), though at relatively high concentrations; 2) augmented with rising pH6 (pHi = 6.2, [Na+]o = 30 mM); and 3) decreased with increasing pHi (pHo = 7.4; [Na+]o = 30 mM). The pHi and the pHo dependencies of H(+)-efflux were almost identical at all parasite stages. Only at pHi > 7.6 efflux was totally obliterated. The target of this inhibitory effect is probably other than the antiport. Results indicate that H(+)-egress is mediated by a Na+/H+ antiport which is regulated by host and parasite pH and by the host cytosol sodium concentration. The proton transport capacity of the antiport can easily cope with all the protons of lactic acid produced by parasite's glycolysis.
1.94 electrons in a 2 with the most important weakly occupied orbitals being a a and equally occupied and orbitals. At 9.0 bohrs this has changed. The Is on Li is still doubly occupied, but the 2 now has changed somewhat (it has lost the 2p"Li and gained higher s orbitals on H) and holds only 1.2 electrons, while the a orbital that was weakly occupied at 3.1 bohrs now has 0.8 electrons and has also lost its 2p"Li component. At 14 bohrs the correlated AGP state has had to open up the core somewhat in order to separate into neutral atoms. The occupation of the lsLi is 1.96 and the 2 is 1.98 with all the other natural orbitals being very weakly but not negligibly occupied (8 X 1O"1 23456 < N¡ < 8 x 10"3). The A state at 3,1 bohrs has a doubly occupied lsLi orbital, 1.3 electrons in a 2 , and 0.7 electrons in a 3 , with both the la and the 3 a quite different from the corresponding ground-state orbitals. At 9.0 bohrs the la has 1.76, and the 3 a 0.23 electrons, while at 14 bohrs they have 1.13 and 0.87 electrons, respectively.
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