title: The trypanocidal effect of NO-releasing agents is not due to inhibition of the major cysteine proteinase in Trypanosoma brucei.
abstract: The lysosomal cysteine proteinase activity of bloodstream forms of Trypanosoma brucei is a validated drug target. Previously, it was reported that nitric oxide (NO)-releasing agents inhibit the catalytic activity of cysteine proteinases of the protozoan parasites Leishmania infantum, Trypanosoma cruzi and Plasmodium falciparum. In this study, we investigated the effect of the NO-donors S-nitrosoglutathione, (+/-)-(E)-4-ethyl-2[(E)-hydroxyimino]-5-nitro-3-hexenamide, 3-morpholinosydnonimine (SIN-1) and S-nitroso-N-acetyl-DL-penicillamine on the activity of the cysteine proteinase of T. brucei. At a concentration of 1 mM, the NO donors inhibited the catalytic activity of purified T. brucei cysteine proteinase by 50-90%. With the exception of SIN-1, all NO donors displayed trypanocidal activities against bloodstream forms of T. brucei in vitro with 50% growth inhibition values of around 30 microM. However, the NO donors were ineffective in significantly inhibiting the cysteine proteinase activity within the parasites. This finding was confirmed by the ineffectiveness of the NO donors to block proteinolysis in the lysosome of the parasites. The results show that the trypanocidal activity of NO donors cannot be attributed to the inhibition of the major lysosomal cysteine proteinase in bloodstream forms of T. brucei.
authors.affiliation: BioMedical Research Centre, School of Medicine, Health Policy and Practice, University of East Anglia, Norwich NR4 7TJ, UK. dsteverding@hotmail.com
authors.forename: Dietmar
authors.initials: D
authors.lastname: Steverding
authors.forename: Xia
authors.initials: X
authors.lastname: Wang
authors.forename: Darren W
authors.initials: DW
authors.lastname: Sexton
mesh_terms: D000818:Animals; D002470:Cell Survival / Q000187:drug effects; D003546:Cysteine Endopeptidases / Q000302:isolation & purification / Q000378:metabolism*; D008247:Lysosomes / Q000201:enzymology / Q000378:metabolism; D009569:Nitric Oxide / Q000494:pharmacology*; D014344:Trypanocidal Agents / Q000494:pharmacology*; D014346:Trypanosoma brucei brucei / Q000187:drug effects*
chemical_list: D014344:Trypanocidal Agents; D009569:Nitric Oxide; D003546:Cysteine Endopeptidases