Cytotoxic interactions of methylene blue with trypanosomatid-specific disulfide reductases and their dithiol products

Methylene blue (MB) is known to have trypanocidal activity. We tested the interactions of MB with a number of trypanosomatid-specific molecules of the antioxidant metabolism. At pH 7, trypanothione and other (di)thiols were oxidized to disulfides by the phenothiazine drug. MB inhibited Trypanosoma cruzi trypanothione reductase (TR) (Ki = 1.9 μM), and served as a significant subversive substrate of this enzyme (KM = 30 μM, kcat = 4.9 s−1). With lipoamide dehydrogenase, the second thiol-generating flavoenzyme of T. cruzi, the catalytic efficiency for MB reduction was found to be almost 106 M−1 s−1. When the system MB-enzyme-molecular oxygen acts as a NAD(P)H-driven redox cycler, a reactive oxygen species, H2O2 or superoxide, is produced in each cycle. Since MB is an affordable, available, and accessible drug it might be tested – alone or in drug combinations – against trypanosomatid-caused diseases of animal and man.