Kinetics of Methionine Transport and Metabolism by Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense

Abstract

Methionine is an essential amino acid for both prokaryotic and eukaryotic organisms; however, little is known concerning its utilization in African trypanosomes, protozoa of the Trypanosoma brucei group. This study explored the Michaelis–Menten kinetic constants for transport and pool formation as well as metabolic utilization of methionine by two divergent strains of African trypanosomes, Trypanosoma brucei brucei (a veterinary pathogen), highly sensitive to trypanocidal agents, and Trypanosoma brucei rhodesiense (a human pathogenic isolate), highly refractory to trypanocidal arsenicals. The Michaelis–Menten constants derived by Hanes–Woolf analysis for transport of methionine for T. b. brucei and T. b. rhodesiense, respectively, were as follows: KM values, 1.15 and 1.75 mM; Vmax values, 3.97 × 10−5 and 4.86 × 10−5 mol/L/min. Very similar values were obtained by Lineweaver–Burk analysis (KM, 0.25 and 1.0 mM; Vmax, 1 × 10−5 and 2.0 × 10−5 mol/L/min, T. b. brucei and T. b. rhodesiense, respectively). Cooperativity analyses by Hill (log–log) plot gave Hill coefficients (n) of 6 and 2 for T. b. brucei and T. b. rhodesiense, respectively. Cytosolic accumulation of methionine after 10-min incubation with 25 mM exogenous methionine was 1.8-fold greater in T. b. rhodesiense than T. b. brucei (2.1 vs 1.1 mM, respectively). In African trypanosomes as in their mammalian host, S-adenosylmethionine (AdoMet) is the major product of methionine metabolism. Accumulation of AdoMet was measured by HPLC analysis of cytosolic extracts incubated in the presence of increasing cytosolic methionine. In trypanosomes incubated for 10 min with saturating methionine, both organisms accumulated similar amounts of AdoMet (∼23 μM), but the level of trans-sulfuration products (cystathionine and cysteine) in T. b. rhodesiense was double that of T. b. brucei. Methionine incorporation during protein synthesis in T. b. brucei was 2.5 times that of T. b. rhodesiense. These results further confirm our belief that the major pathways of methionine utilization, for polyamine synthesis, protein transmethylation and the trans-sulfuration pathway, are excellent targets for chemotherapeutic intervention against African trypanosomes.