Disassembling the glutathione molecule at the plasma membrane (before it can enter the cell as individual amino acids) is accomplished by the enzyme GGT (gamma-glutamyl transpeptidase). Ironically, in the process of splitting glutathione, GGT produces a significant amount of hydrogen peroxide. Since cancer cells are generally under oxidative stress and therefore need additional glutathione, many cell lines overexpress GGT. When adequate extracellular glutathione is present, enough hydrogen peroxide can be generated to either stimulate cell proliferation in vitro (at low hydrogen peroxide concentrations) or inhibit it (at high concentra-tions).34-39 In most conditions seen in vivo, however, extracellular concentrations of glutathione and overexpression of GGT are probably not high enough to produce so much hydrogen peroxide that cancer cells are killed. Rather, in in-vivo conditions, GGT overexpression tends to increase tumor growth and reduce the effectiveness of chemotherapy drugs like cisplatin (since the glutathione produced assists in drug detoxifica-tion).40
Although this mechanism may be one way cancer cells make low concentrations of hydrogen peroxide in vivo (and low concentrations of hydrogen peroxide can assist cancer cell progression), this is no reason to avoid using antioxidants. Antioxidants increase glutathione concentrations by sparing its destruction, and thus would not be expected to increase hydrogen peroxide production. Moreover, they can affect cancer in several different ways, and their overall effect is of greatest concern. Antioxidants are likely to be beneficial to the patient if they are used in combination with other anticancer compounds.
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