Leishmania amastigotes do not appear to alter the phagolysosomal pH (27), but rather possess membrane transport systems that are designed to exploit the acidic environment provided by the lysosome. Adaptation of amastigotes to high extracellular proton concentrations is strikingly evidenced by their metabolic behavior. Amastigotes optimally respire, metabolize glucose, and incorporate thymidine, uridine, and proline when bathed in medium between pH 4.0 and 5.5 (22). pH levels comparable to those within phagolysosomes. The effect of extracellular protons on the metabolic activity of amastigotes is probably due to increased nutrient transport through proton-coupled symporters, such as the glucose transporter (37,38). Proton influx is countered in both amastigotes and promastigotes by a plasmatemma proton pump, which is capable of generating a proton gradient across the plasma membrane (37; T. Glaser and A. Mukkada, personal communication). While the possible existence of other cation pumps has not been rigorously excluded, available evidence suggests that L. donotiani may rely solely on a proton pump to drive the uptake of nutrients and to regulate ion flow (37
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