Relationship between cytocidal activity and glutathione-S-transferase inhibition using doxorubicin coupled to stereoisomers of glutathione with different substrate specificity

To determine the cytotoxic mode of action of a glutathione (GSH)-doxorubici n (DXR) conjugate, which exhibited point cytotoxicity against various multi drug-resistant as well as DXR-sensitive cell lines, the molecular interacti on between covalent GSH-DXR conjugates and glutathione-S-transferase (GST), a possible molecular target of the conjugates, was investigated. The follo wing four GSH molecules with stereoisomeric forms were prepared: L-Glu-L-Cy s-Gly (LL-GSH), D-Glu-L-Cys-Gly (DL-GSH), L-Glu-D-Cys-Gly (LD-GSH) anal D-G lu-D-Cys-Gly (DD-GSH). The enzymic activity of GST against each GSH stereoi somer was 88, 38, 8 and 4 nmol/ mg/min, respectively, suggesting that the L -form cysteine residue in the molecule was an important substrate of GST. A ddition of DXR conjugated with each isomer (10 muM) to a GSH-containing GST assay mixture inhibited the GST activity to 32% for LL-GSH-DXR, 16% for DL -GSH-DXR and 61% for LD-GSH-DXR as compared with the solvent control. Moreo ver, IC,, values for these conjugates were 30, 20 and 250 nM, respectively. The cytocidal activity of each conjugate corresponded to the substrate spe cificity of GST activity for the GSH isomer. These conjugates bound to the GST molecule, and the binding ability was 0.746, 0.627 and 0.462 mol/mol of GST for LL-GSH-DXR, DL-GSH-DXR and LD-GSH-DXR, respectively. These finding s suggested that GSH-DXR interacted with the substrate-binding site of the GST molecule and inhibition of GST activity exhibited potent cytotoxicity. [(C) 2001 Lippincott Williams & Wilkins.].