Proteases from Trypanosoma brucei brucei: Purification, characterisation and interactions with host regulatory molecules

African trypanosomes contain proteases that may be released into the bloodstream of their infected hosts. This paper describes a novel, combined isolation of a cysteine proteinase (called trypanopain-Tb) and a serine oligopeptidase (which we call oligopeptidase-Tb) from Trypanosoma brucei brucei, as well as a comparison of the activities of these two enzymes against several host regulatory molecules. The enzymes differed in various respects. Firstly, purified trypanopain-Tb was shown to readily cleave proteins such as gelatin maximally at acidic pH. In contrast, oligopeptidase- Tb, which is optimally active at alkaline pH, did not hydrolyase proteins larger than 4 kDa. However, it readily hydrolysed various polypeptides, including neurotensin and atrial natriuretic factor. The interaction of the two enzymes with mammalian protease inhibitors also differed. Cystatins and α₂-macroglobulin effectively inhibited trypanopain-Tb, with the K(i) values for cystatin C and low-molecular-mass kininogen (≃10^-11 M) predicting that trypanopain-Tb is likely to be effectively controlled by these inhibitors if released into the host bloodstream. In contrast, oligopeptidase-Tb was not inhibited by serpins or α₂-macroglobulin, suggesting that it may remain active if released into the host bloodstream. In support of these in vitro results, the blood of trypanosome-infected rats displayed no trypanopain-Tb-like activity, but exhibited high oligopeptidase- Tb-like activity. Thus, while trypanopain-Tb seems likely to be confined to an intracellular role within the parasite, oligopeptidase-Tb has the potential to remain active in the host bloodstream and so contribute directly to pathogenesis.