Detoxication of microcystins mediated by human GSTs: comparison among variants with different hydrophilicity

It has been hypothesized that kinetics can be a key determinant of (MC) congener-specific toxicity. The in vitro inhibition potency of PP1/PP2A by single MC is comparable: therefore, the toxicokinetic of MC seems to be the critical point to explain congener-dependent toxicity. Those variants, such as MC-LW and MC-YR, having hydrophobic amino acids (e.g. tyrosine, tryptophan) may be more cell permeable than MC-LR. Glutathione conjugation, occurring either spontaneously or catalyzed by GST, is the accepted main step in MC detoxification. Recently, we characterized the in vitro human conjugation of MC-LR and MC-RR showing some differences in the presence of GSH depletion. This study was carried out to understand possible dependence of detoxication reaction on lipophilicity. Using single human hepatic recombinant isoforms (GSTA1, A2, A4, M1, T1 T2, P1, and O1) and human liver cytosol (HLC, pool of 200 donors) the kinetic parameters Vmax, Km and Cli were calculated. The efficiencies of recombinant GSTs used are quite similar (0.022-0.066 pmolGSMCLW/( μgprot*min*μM and 0.048-0.09 pmolGSMC-YR/(μgprot*min*μM); the highest Cli were shown by GSTP1 and A1 for MC-LW and P1=O1>A1 for MC-YR. Since GSTA1 is the most abundant hepatic GST, it is expected to give the highest contribution to MCLW and MCYR detoxification. Using the HLC a typical saturation curve was evidenced for MC-LW whilst the reaction was still linear for MCYR. Beside the differences in the kinetic behavior, comparing in the Cli of MC-LR, MC-LW, MC-YR and MC-RR, the variants which is most efficiently detoxified is MC-RR, which is the least acutely toxic. Acknowledgements: This study was partially supported by : the European Food Safety Authority (EFSA) under the grant agreement no.GA/EFSA/SCER/2015/01.