Inhibition of human glutathione S-transferases by basic triphenylmethane dyes

Human glutathione S-transferases (GSTs) of the Alpha-, Mu- and Pi- classes were expressed in E. coli and isolated by affinity chromatography. They were tested for their susceptibility to inhibition by basic triphenylmethane dyes. hGSTA 1-1 was inhibited by Malachite Green with a Ki value of the order of 10 microM. The inhibitory species appeared to be the dye-GSH adduct. This isoenzyme was not inhibited by either Crystal Violet or Ethyl Violet at concentrations up to 50 microM. hGSTM 2-2 was weakly inhibited by all three dyes tested with Ki values being in the range 40-80 microM. For all dyes the inhibition was best characterised as non-competitive. hGSTP 1-1 was not inhibited by Crystal Violet or by Ethyl Violet but was strongly inhibited by Malachite Green (Ki = 0.3 microM). The mode of inhibition appeared to be non-competitive but it seems probable that the mechanism is complex. There is at present no evidence to show clearly whether the dominant inhibitory species is the free dye or the adduct.