Inhibition of human glutathione transferase by catechin and gossypol: comparative structural analysis by kinetic properties, molecular docking and their efficacy on the viability of human MCF-7 cells.

Glutathione transferase Pi (GSTP1) expression is increased in many cancer types and is associated with multidrug resistance and apoptosis inhibition. Inhibitors of GSTP1-1 have the potential to overcome drug resistance and improve chemotherapy efficacy as adjuvant agents. This study investigated the effects of catechin and gossypol on human glutathione transferase Pi (GSTP1-1) activity and their cytotoxic effects on breast cancer cells (MCF-7) individually and in combination with tamoxifen (TAM). Gossypol effectively inhibited the enzyme with an IC50 value of 40 µM, compared to 200 µM for catechin. Gossypol showed stronger inhibition of GSTP1-1 activity (Ki = 63.3 ± 17.5 µM) compared to catechin (Ki = 220 ± 44 µM). Molecular docking analysis revealed their binding conformations to GSTP1-1, with gossypol binding at the subunit interface in an un-competitive manner and catechin showing mixed non-competitive inhibition. Gossypol had severe cytotoxic effects on both MCF-7 cells and normal BJ1 cells, while catechin had a weak cytotoxic effect on MCF-7 cells only. Combination therapy with TAM resulted in cytotoxicity of 27.3% and 35.2% when combined with catechin and gossypol, respectively. Gossypol showed higher toxicity to MCF-7 cells, but its strong effects on normal cells raised concerns about selectivity and potential side effects.