Previously, it was reported that caffeic acid esters inhibit the growth of bloodstream forms of Trypanosoma brucei and the activity of its major lysosomal cathepsin L-like cysteine protease, TbCATL. However, whether this trypanocidal activity is due to inactivation of TbCATL has not so far been demonstrated. Caffeic acid isopentyl ester (isopentyl caffeate) displayed antitrypanosomal activity against T. brucei bloodstream forms with minimum inhibitory concentration (MIC) and 50 % growth inhibition (GI50) values of 1 and 0.31 μg/ml, respectively. The ester also inhibited the activity of purified TbCATL but with a 27-fold higher half maximal inhibitory concentration (IC50) value of 8.5 μg/ml compared to its GI50 value. In contrast to previous suggestion, isopentyl caffeate did not interact with the active site of TbCATL but inhibited the enzyme in a non-competitive way. In addition, the ester was ineffective in blocking the proteolysis in the lysosome of the parasite, which, however, is a hallmark for inhibitors whose trypanocidal action is through inactivation of TbCATL. These results suggest that the antitrypanosomal activity of isopentyl caffeate (and probably of other caffeic acid esters) cannot be attributed to inhibition of TbCATL. Nevertheless, caffeic acid esters are interesting compounds with promising antitrypanosomal activity. This is supported by a more than 100 times less sensitivity of human HL-60 cells to isopentyl caffeate indicating that the ester has a favourable selectivity profile.
Keywords: Cysteine protease; Isopentyl caffeate; Non-competitive inhibition; Trypanosoma brucei.