Kinetics ofS-Adenosylmethionine Cellular Transport and Protein Methylation inTrypanosoma brucei bruceiandTrypanosoma brucei rhodesiense

African trypanosomes of theTrypanosoma bruceigroup are agents of disease in man and animals. They present unique biochemical characteristics such as the need for preformed purines and have extensive salvage mechanisms for nucleoside recovery. In this regard we have shown that trypanosomes have a dedicated transporter forS-adenosylmethionine (AdoMet), a key metabolite in transmethylation reactions and polyamine synthesis. In this study we compared the apparent kinetics of AdoMet transport, cytosolic AdoMet pool formation, and utilization of AdoMet in protein methylation reactions using two isolates:Trypanosoma brucei brucei,a veterinary parasite, andTrypanosoma brucei rhodesiense,a human pathogen that is highly refractory and has greatly reduced susceptibility to standard trypanocidal agents active againstT. b. brucei.The apparentKmvalues for [methyl-3H]AdoMet transport, derived by Hanes–Woolf analysis, forT. b. bruceiwas 4.2 and 10 mM forT. b. rhodesiense,and theVmaxvalues were 124 and 400 μmol/liter/min, respectively. Both strains formed substantial cytosolic pools of AdoMet, 1600 nmol/109T. b. bruceiand 3500 nmol/109T. b. rhodesienseafter 10 min incubation with 25 mM exogenous AdoMet. Data obtained from washed trichloroacetic acid precipitates of cells incubated with [methyl-3H]AdoMet indicated that the rate of protein methylation inT. b. bruceiwas fourfold greater than inT. b. rhodesiense.These results demonstrate that the unique rapid uptake and utilization of AdoMet by African trypanosomes is an important consideration in the design and development of new agents of potential use in chemotherapy.