Binding of antibiotics to human liver glutathione S-transferases

Glutathione S-transferases (GSH S-transferases) are multifunctional enzymes and are known to play an important role as intracellular binding protein in human liver. Binding of several antibiotics, including cefotetan, benzylpenicillin, cefazolin, chloramphenicol and gentamicin, to cationic and anionic GSH S-transferases isolated from human liver, and to human serum albumin, has been investigated by using the centrifuge column technique, which is supposed to be an excellent one for its sensitivity and rapidity in ligand binding studies. The dissociation constants (Kd) of the antibiotics for human liver GSH S-transferases and for human serum albumin, and the number of binding sites for the antibiotics on a molecule of human liver GSH S-transferases and of human serum albumin have been evaluated by means of Scatchard plots. This study has shown that cefotetan is bound to cationic GSH S-transferases to the greatest extent, followed by benzylpenicillin, cefazolin and chloramphenicol, gentamicin being bound to the smallest extent. It has been confirmed that not only cationic GSH S-transferases but also anionic GSH S-transferases have the binding capacity to the antibiotics, and that the extent of binding of the antibiotics to anionic GSH S-transferases is similar to the one to cationic GSH S-transferases. Therefore, anionic GSH S-transferases are supposed to play nearly the same role as cationic GSH S-transferases in transport of the antibiotics in human liver. The fact that the extent of binding of the antibiotics to human liver GSH S-transferases is closely correlated with the extent of biliary excretion of the antibiotics suggests that human liver GSH S-transferases play an important role in the transport of certain antibiotics from plasma, through hepatocytes, into bile.