The effect of some amino acids on carbohydrate uptake by Trypanosoma lewisi

D-Glucose transport in Trypanosoma brucei. D-Glucose transport is the rate-limiting step of its metabolism

D-Glucose and 2-deoxyglucose enter the 'long-slender' bloodstream form of Trypanosoma brucei only by means of a carrier-mediated process; no free diffusion can be observed. Permeation is not energy-dependent. The uptake is driven by the downhill concentration gradient of free substrate. The latter is maintained by the continual removal of sugar, due to the extremely high activities of the glycolytic enzymes. The permeation process is the rate-limiting step of glucose consumption, because permeation proceeds at a rate slower than metabolism. The inhibition of sugar uptake by glycerol was tested. Interactions at the carrier site can be ruled out since glucose and its 2-deoxy analog exhibit different inhibition kinetics.

Carbohydrate effects of amino acid transport by Trypanosoma equiperdum

Uptake of 14C-labeled alanine, glutamate, lysine, methionine, proline, and phenylalanine by Trypanosoma equiperdum during 2-minute incubations occurred by diffusion and membrane-mediated processes. Amino acid metabolism was not detected by paper chromatography of trypanosome extracts. Most of 18 carbohydrates tested for ability to alter amino acid transport neither changed nor significantly inhibited transport. Glucose, however, stimulated glutamate, lysine and proline transport; fructose stimulated lysine uptake and 2-deoxy-D-glucose increased phenylalanine and methionine absorption. No evidence was found that the carbohydrates acted by binding to amino acid transport "sites." Glucose inhibition of alanine, phenylalanine, and methionine uptake was linked to glycolysis. The rapid formation of alanine form glucose stimulated alanine release and, when glycolysis was blocked, glucose no longer inhibited alanine transport. Methionine and phenylalanine release was also stimulated by glucose. Glucose changed the ability of lysine, glutamate, and proline to inhibit each others' uptake, indicating that certain amino acids are preferentially absorbed by respiring cells. Analysis of free pool amino acid levels suggested that some amino acid transport systems in T. equiperdum are linked in such a way to glycolysis as to control the cell concentrations of these amino acids.