Mediation of the in vitro cytotoxicity of green and black tea polyphenols by cobalt chloride

Abstract

The effects of Co 2+ (as CoCl 2) on the cytotoxicity of green tea polyphenol (GTP) and black tea polyphenol (BTP) extracts towards proliferation of immortalized human gingival epithelial-like S-G cells were studied. The 24 h potencies of GTP and BTP extracts, as determined with the neutral red (NR) cell viability assay, were greatly reduced in the presence of 250, but not of 50, μM Co 2+ . The cytotoxicities of the GTP and BTP extracts were due, in part, to their generation of hydrogen peroxide (H 2O 2) in the cell culture medium (DMEM). Progressively increasing the concentration of Co 2+ in the tea polyphenol-amended cell culture medium resulted in a lowering of the level of H 2O 2. The cytotoxicity of freshly added H 2O 2 to S-G cells was abolished in the presence of 250 μM Co 2+ and the level of freshly added H 2O 2 to cell culture medium was progressively lowered as the concentration of Co 2+ was increased. Apparently, under the conditions of these studies, the decreases in the cytotoxicity of GTP and BTP extracts in the presence of CoCl 2 were due to the rapid catalytic decomposition by Co 2+ of the H 2O 2 generated in the tea polyphenol-amended cell culture medium.