Metabolic effects of a methylthioadenosine phosphorylase substrate analog on African trypanosomes

The effects of 5'-deoxy-5'-(hydroxyethylthio)adenosine (HETA), a trypanocidal analog of 5'-deoxy-5'-(methylthio)adenosine (MTA), on polyamine synthesis and S-adenosylmethionine (AdoMet) metabolism were examined in bloodstream forms of Trypanosoma brucei brucei. HETA was cleaved by trypanosome MTA phosphorylase at the same rate as the natural substrate, MTA, in a phosphate-dependent reaction. Fluorine substitution at the 2-position of the purine ring increased activity by approximately 50%, whereas substitution with an amino group reduced activity to about one-third of the control. HETA was accumulated by trypanosomes with internal concentrations of 100-250 microM and >800 microM after a 15-min incubation with 1 and 10 microM, respectively. Trypanosomes preincubated with HETA metabolized it at a rate of 21.9 nmol/hr/mg protein. Preincubation of cells with HETA at 1 or 10 microM inhibited spermidine synthesis from [3H]ornithine by 22-37%, and increased the cytosolic levels of AdoMet by 2- to 5-fold and that of MTA by up to 8-fold. S-Adenosylhomocysteine (AdoHcy) levels also increased 1.5- to 7-fold in treated cells, whereas decarboxylated AdoMet decreased 65%. Preincubation of trypanosomes with HETA for 4 hr also reduced the incorporation of [35S]methionine in trichloroacetic acid-precipitable material by 50-60%, and reduced the methyl group incorporation into protein from [U-14C]methionine by 65-70%. Thus, HETA interferes with a series of biochemical events involving the participation of AdoMet and methionine in polyamine synthesis, protein synthesis, and transmethylation reactions.