Metabolic effects of a methylthioadenosine phosphorylase substrate analog on African trypanosomes

The effects of 5'-deoxy-5'-(hydroxyethylthio)adenosine (HETA), a trypanocid al analog of 5'-deoxy-5'-(methylthio)adenosine (MTA), on polyamine synthesi s and S-adenosylmethionine (AdoMet) metabolism were examined in bloodstream forms of Trypanosoma brucei brucei. HETA was cleaved by trypanosome MTA ph osphorylase at the same rate as the natural substrate, MTA, in a phosphate- dependent reaction. Fluorine substitution at the 2-position of the purine r ing increased activity by similar to 50%, whereas substitution with an amin o group reduced activity to about one-third of the control. HETA was accumu lated by trypanosomes with internal concentrations of 100-250 mu M and >800 mu M after a 15-min incubation with 1 and 10 mu M, respectively. Trypanoso mes preincubated with HETA metabolized it at a rate of 21.9 nmol/hr/mg prot ein. Preincubation of cells with HETA at 1 or 10 mu M inhibited spermidine synthesis from [H-3]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-Adenosylhomoc ysteine (AdoHcy) levels also increased 1.5- to 7-fold in treated cells, whe reas decarboxylated AdoMet decreased 65%. Preincubation of trypanosomes wit h HETA for 4 hr also reduced the incorporation of [S-35]methionine in trich loroacetic acid-precipitable material by 50-60%, and reduced the methyl gro up incorporation into protein from [U-C-14]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 tr ansmethylation reactions. (C) 1998 Elsevier Science Inc.