Photoinactivation of trypanothione reductase and glutathione reductase by A1-phthalocyanine tetrasulfonate and hematoporphyrin

The irradiation of Trypanosoma congolense trypanothione reductase (TR), human erythrocyte (HGR) and yeast glutathione reductase (YGR) with visible light in the presence of A1-phthalocyanine tetrasulfonate (A1PcS₄) or hematoporphyrin (Hp) caused a time-dependent inactivation of these enzymes. TR was inactivated more rapidly than either HGR or YGR. Half-maximal rates of inactivation were determined in the presence of 100 μM Hp and 1.4-17 μM A1PcS₄. The photosensitized irradiation modified the disulfide substrate-binding sites of these enzymes, most likely the conserved catalytic histidine residue. In the dark, A1PcS₄ acted as a reversible inhibitor competitive with the disulfide substrate of TR and HGR. These findings suggest the possible use of photosensitized irradiation for preventing the transmission of trypanosomiasis by blood transfusion.