ABSTRACT
Leishmania are parasitic protozoa with a digenetic life cycle, proliferating as extracellular flagellated promastigotes in the alimentary tract of the insect vector and as obligate intracellular amastigotes in the phagolysosomal vacuoles of mammalian macrophages (Chang and Dwyer, 1978; Berman et al., 1979; Dwyer, 1979). During their life cycle, these organisms are subjected to marked changes in environmental pH, from relatively alkaline (pH 7-7.5) of the sandfly vector’s gut, to the neutral pH of the bloodstream, to the acid pH of the lysosomes of macrophages (pH 4.5-5). Furthermore, the transformation of promastigotes to amastigotes occurs during phagocytosis by host macrophages with exposure of the parasites to an acidic environment (Chang and Dwyer, 1976; Chang et al., 1985). Both promastigotes and amastigotes appear to be highly adapted to their corresponding environmental pHs. For example, L. donovani promastigotes metabolize glucose, proline and nucleosides most rapidly at pH 7-7.5. Amastigotes, on the other hand, catabolize these substrates with an optimal pH of 4.5-5 (Mukkada et al., 1985). Similarly, transport of proline is optimal at pH 7-7.5 with promastigotes and at pH 5.5 with amastigotes (Zilberstein et al., 1989; Mukkada et al., 1989).
