Studies of the inhibitory activities of tiopronin and mercaptosuccinic acid on glutathione peroxidase and their cytotoxic and antioxidant properties.

Glutathione peroxidase (GPx), an important antioxidative enzmye, can be inhibited by various thiols, including of tiopronin and mercaptosuccinic acid (MSA). Recently, there has been discussion regarding the combination of tiopronin in anticancer therapy to overcome acquired resistance to anticancer drugs. However, thiols are also known to act as antioxidants, which can be contraindicated in cancer chemotherapy. This article focuses on the inhibitory effects of tiopronin and MSA on bovine and human glutathione peroxidase activities, and their effects on the redox status of cancer cells. IC50 values for the inhibition for the bovine erythrocyte enzyme were 356 and 24.7 µM for tiopronin and MSA, respectively, with the corresponding Ki values of 343 µM and 14.6 µM, respectively at pH 7.4 and 25 °C. MSA inhibited human GPx activity in human cancer cell lysates at its IC50 while tiopronin did not. Both compounds were cytotoxic to human cancer cell lines GUMBUS and HL-60, with IC50 values between 42.7 and 149.4 µM. Neither had an effect on cell cycle. Only MSA induced apoptosis in HL-60 cells but not in GUMBUS cells, while tiopronin resulted in no apoptosis in either cell line. Combination studies of the MSA with hydrogen peroxide in living cells enhanced the production of reactive oxygen species in GUMBUS cells while tiopronin acted as antioxidant in HL-60 cells. MSA and tiopronin antagonized the cytotoxic effect of cisplatin, doxorubicin and methotrexate in combination studies. Our findings indicate that the antioxidant properties of both thiols prevail over their GPx inhibitory activity in human cancer cell lines.