An integrated in vitro approach to study Microcystyn detoxification by Glutathione-S-Transferases: from toxicokinetic parameters to the possible identification of differently susceptible population groups

Daily we are exposed to xenobiotics via the environment, water and food which can be toxic, in relation to the exposure dose. These substances are generally lipophilic, therefore organisms, as a defence, promote their excretion, through biotransformation or conjugation catalysed by specific enzymes as Glutathione-S-Transferases (GSTs) characterized by different polymorphic isoforms. Our work has been focused on the application of an integrated approach to study the in vitro detoxification (that is conjugation with glutathione mediated by GSTs) of two variants of Microcystins (MC), a group of natural hepatotoxic compounds with more than 100 congeners. MC are characterized by very similar structure but the difference in some amino-acidic groups leads to different in vivo toxicity. The in vitro inhibition potency of protein phosphatase by single MC congener, the first step of their mechanism of toxicity is comparable: therefore the toxicokinetic of MC seems to be the critical point to explain congener-dependent toxicity. Human hepatic recombinant isoforms (GST A1, A2, A4, M1, T1 T2, P1, and O1) were used followed by human hepatic cytosol, where all the isoforms are present. The different affinity of the single recombinant isoforms was evidenced and the kinetic parameters Vmax, Km and Cli, were derived. Differences in GSTs contribution at low and high MC and/or GSH concentrations have been evidenced. Considering that some GSTs, as T1 and M1 are highly polymorphic (these enzymes are lacking in the 50% of Caucasian population) the toxicokinetic information can suggest different susceptibility of population groups to MC toxic effects.