O2-polarographic studies on soluble and mitochondrial enzymes of Crithidia fasciculata; glycerophosphage enzymes

Carbon-13 nuclear magnetic resonance analysis of [1-13C]glucose metabolism in Crithidia fasciculata. Evidence of CO2 fixation by phosphoenolpyruvate carboxykinase

The non-invasive technique of 13C nuclear magnetic resonance was applied to study glucose metabolism in vivo in the insect parasite Crithidia fasciculata. It was found that under anaerobic conditions [1-13C]glucose underwent a glycolytic pathway whose main metabolic products were identified as [2-13C]ethanol, [2-13C]succinate and [1,3-13C2]glycerol. These metabolites were excreted by C. fasciculata into the incubation medium, while in the cells [3-13C]phosphoenolpyruvate was also detected in addition to the aforementioned compounds. The C3 acid is apparently the acceptor of the primary CO2 fixation reaction, which leads in Trypanosomatids to the synthesis of succinate. By addition of sodium bicarbonate to the incubation mixture L-[3-13C]malate was detected among the excretion products, while the ethanol:succinate ratio of 2.0 in the absence of bicarbonate changed to a ratio of 0.6 in the presence of the latter. This was due to a shift of the balance between carboxylation of phosphoenolpyruvate, leading to succinate, and pyruvate decarboxylation leading to ethanol. The addition of 25% 2H2O to the incubation mixture led to the formation of [2-13C, 2-2H]ethanol derived from the prior incorporation of 2H+ into pyruvate in the reactions mediated by either pyruvate kinase or malic enzyme. However, no 2H+ incorporation into L-malate was detected, excluding the possibility that the latter was formed by carboxylation of pyruvate, and lending support to the idea that L-malate results from the carboxylation of phosphoenolpyruvate to oxaloacetate by phosphoenolpyruvate carboxykinase. The formation of [2-13C, 2-2H]-succinate under the same conditions reflected the uptake of 2H+ during the reduction of fumarate. When the incubations were carried out in the presence of 100% 2H2O, several [1-13C, 1-2H]ethanol species were detected, as well as [2-13C, 2-2H]malate and [1,3-13C2, 1,3-2H2]glycerol. The former deuterated compounds reflect the existence of NAD2H species when the incubations were carried out in 100% 2H2O, while the incorporation of 2H+ into [1,3-13C2]glycerol must be attributed to the phosphoglucose-isomerase-mediated reaction during glycolysis.

Uptake of the trypanocidal drug suramin by bloodstream forms of Trypanosoma brucei and its effect on respiration and growth rate in vivo

After a single intravenous injection of suramin the rate of removal of the drug from the plasma into other tissue compartments of the rat is independent of initial concentration. The data can be fitted to the sum of two exponential functions, consistent with a two-compartment, open model system. Trypanosomes take up only small amounts of suramin in vivo and do not actively concentrate the drug within the cell. Uptake is apparently by a non-saturable process that decreases with time and is dependent on the amount of suramin already taken up. Once within the cell, suramin progressively inhibits respiration and glycolysis, such that, for a given exposure in vivo, inhibition of oxygen consumption is proportional to the total amount of suramin absorbed. It can be calculated that only a fraction (4--9%) of this total is required to inhibit respiration to the extent found in broken cell preparations. The combined inhibition of two key enzymes in glycolysis--the sn-glycerol-3-phosphate oxidase (EC unassigned) and the glycerol-3-phosphate dehydrogenase (NAD+) (sn-glycerol-3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8)--are sufficient to account for the differential inhibition of glucose and oxygen consumption and of pyruvate production, together with the small, but significant, production of glycerol. Even at the highest dose of suramin tolerated by the rat, trypanosomes continue to increase exponentially in the bloodstream for at least 6 h. The mean doubling time is increased from 4.6 h to a maximum of about 12.5 h in rats treated with doses of suramin in the range 25--150 mg/kg. In the light of these and other findings, it is concluded that part of the trypanocidal action of suramin results from the inhibition of ATP production by glycolysis.

Effect of ethidium bromide on the oxidative metabolism and enzyme profiles of Crithidia fasciculata

Changes in the metabolism of Crithidia fasciculata ATCC 11745 when grown in the presence of ethidium bromide were studied. Ethidium bromide-grown cells had decreased respiratory activity as measured by oxygen consumption. More than 50% of the organisms cultivated in a defined medium containing 1.0 mg/liter of ethidium bromide became dyskinetoplastic and had decreased activities of particulate succinate and NADH-linked dehydrogenases as well as of soluble isocitrate dehydrogenase. These cells also had increased activities of particulate alpha-glycerophosphate dehydrogenase, soluble alpha-glycerophosphate dehydrogenase, malic enzyme, hexokinase, and malate dehydrogenase. Ethidium bromide-grown cells had a lower level of ATP and contained less DNA than cells grown in its absence.

Biochemical properties of trypanosomatid lactate dehydrogenases

Lactate dehydrogenase (LDH, E.C.1.1.1.27) was found in supernatant (cytoplasmic enzyme) fractions of the trypanosomatid flagellates Trypanosoma conorhini and Crithidia fasciculata if 10 mm cysteine was present in the homogenizing medium. The T. conorhini LDH activity with pyruvate as substrate was increased 35% if 5 mm cysteine was also included in reaction mixtures. K(m) values for the T. conorhini enzyme were 3.3 x 10(-4)m with pyruvate, and 1.6 x 10(-4)m with alpha-ketobutyrate. Cysteine inhibited alpha-ketobutyrate reduction. Comparison of trypanosomatid and human serum LDH enzymes with respect to K(m), substrate activity and inhibition, pH optima, and K(i) values for oxalate and oxamate indicated that the trypanosomatid isoenzymes differed significantly from serum LDH. C. fasciculata LDH was extremely labile, since 59% of the activity was lost 90 min after isolation. The role of LDH enzymes in trypanosomatid metabolism is discussed, and the results are related to other trypanosomatid LDH enzymes. The comparison of homologous enzymes in host and parasite is discussed with regard to metabolic function and a possible model system for chemotherapy.

Effects of some antitumor agents on growth and glycolytic enzymes of the flagellate Crithidia

Some antitumor agents known to specifically inhibit certain tumor cell enzymes were examined for activity against glycolytic enzymes and growth of the insect trypanosomatid, Crithidia fasciculata. The cytoplasmic enzymes hexokinase, alpha-glycerophosphate dehydrogenase, malic dehydrogenase, and glucose-6-phosphate dehydrogenase were tested. Agaricic acid (2-hydroxy-1,2,3-nonadecane tricarboxylic acid) was highly inhibitory (50 to 100%) to malic and alpha-glycerophosphate dehydrogenases at approximately 3 x 10(-5)m; 2-(p-hydroxyphenyl)-2-phenylpropane (2 x 10(-4)m), and 5,6-dichloro-2-benzoxazolinone (5 x 10(-4)m) were less effective (50% inhibition) against them. The antiprotozoal agents primaquine (4 x 10(-4)m) and Melarsoprol (8 x 10(-4)m) were 30 to 40% inhibitory. Agaricic acid, 2-(p-hydroxyphenyl)-2-phenylpropane, and 5,6-dichloro-2-benzoxazolinone inhibited growth of Crithidia at less than 10(-4)m. Eight other test compounds from the Cancer Chemotherapy National Service Center (CCNSC) were not toxic to cell growth, although two (4-biphenylcarboxylic acid and 1-[p-chlorobenzyl]-2-ethyl-5-methyl-indole-3-acetic acid) inhibited Crithidia alpha-glycerophosphate dehydrogenase below 1 mm. All of the compounds used specifically inhibited cancer cell alpha-glycerophosphate dehydrogenase. The corresponding enzyme in pathogenic African trypanosomes is important in their terminal respiration. C. fasciculata may be useful in preliminary evaluation of chemotherapeutic agents as potential trypanocides.