Effects of various metabolic conditions and of the trivalent arsenical melarsen oxide on the intracellular levels of fructose 2,6‐bisphosphate and of glycolytic intermediates in Trypanosoma brucei

Abstract

Upon differential centrifugation of cell-free extracts of Trypanosoma brucei, 6-phosphofructo-2-kinase and fructose-2,6-bisphosphatase behaved as cytosolic enzymes. The two activities could be separated from each other by chromatography on both blue Sepharose and anion exchangers. 6-phosphofructo-2-kinase had a K_m for both its substrates in the millimolar range. Its activity was dependent on the presence of inorganic phosphate and was inhibited by phosphoenolpyruvate but not by citrate or glycèrol 3-phosphate. The K_m of fructose-2,6-bisphosphatase was 7 μM; this enzyme was inhibited by fructose 1,6-bisphosphate (K_i= 10 μM) and, less potently, by fructose 6-phosphate, phosphoenolpyruvate and glycerol 3-phosphate. Melarsen oxide inhibited 6-phosphofructo-2-kinase (K_i < 1 μM) and fructose-2,6-bisphosphatase (K_i= 2 μM) much more potently than pyruvate kinase (K_i > 100 μM). The intracellular concentrations of fructose 2,6-bisphosphate and hexose 6-phosphate were highest with glucose, intermediate with fructose and lowest with glycerol and dihydroxyacetone as glycolytic substrates. When added with glucose, salicylhydroxamic acid caused a decrease in the concentration of fructose 2,6-bisphosphate, ATP, hexose 6-phosphate and fructose 1,6-bisphosphate. These studies indicate that the concentration of fructose 2,6-bisphosphate is mainly controlled by the concentration of the substrates of 6-phosphofructo-2-kinase. The changes in the concentration of phosphoenolpyruvate were in agreement with the stimulatory effect of fructose 2,6-bisphosphate on pyruvate kinase. At micromolar concentrations, melarsen oxide blocked almost completely the the formation of fructose 2,6-bisphosphate induced by glucose, without changing the intracellular concentrations of ATP and of hexose 6-phosphates. At higher concentrations (3–10 μM), this drug caused cell lysis, a proportional decrease in the glycolytic flux, as well as an increase in the phosphoenolpyruvate concentrations which was restricted to the extracellular compartment. Similar changes were induced by digitonin. It is concluded that the lytic effect of melarsen oxide on the bloodstream form of T. brucei is not the result of an inhibition of pyruvate kinase.