Mechanisms of toxicity of 2- and 5-hydroxy-1,4-naphthoquinone; absence of a role for redox cycling in the toxicity of 2-hydroxy-1,4-naphthoquinone to isolated hepatocytes

The mechanisms of toxicity to isolated rat hepatocytes of two structurally related naphthoquinones have been studied. Both 5‐OH‐1,4‐naphthoquinone (5‐OH‐1,4‐NQ; juglone) and 2‐OH‐1,4‐naphthoquinone (2‐OH‐1,4‐NQ; lawsone) caused a concentration‐dependent cytotoxicity to hepatocytes which was preceded by a depletion of intracellular glutathione. 5‐OH‐1,4‐NQ caused a depletion of intracellular glutathione when incubated either at 4°C or 37°C whereas 2‐OH‐1,4‐NQ caused a depletion of intracellular glutathione when the hepatocytes were incubated at 37°C but not at 4°C. 5‐OH‐1,4‐NQ but not 2‐OH‐1,4‐NQ reacted with glutathione in buffered solution. These results suggested that the depletion of intracellular glutathione by 2‐OH‐1,4‐NQ is enzyme mediated whereas in the case of 5‐OH‐1,4‐NQ the direct chemical reaction with glutathione may be largely responsible for the depletion. A critical role for depletion of protein thiols in menadione‐induced cytotoxicity has been proposed. In agreement with earlier work, menadione caused a decrease in protein sulphydryls prior to cell death, however, at cytotoxic concentrations of both 2‐OH‐1,4‐NQ and 5‐OH‐1,4‐NQ this decrease only accompanied rather than preceeded cell death. The mechanism of toxicity of 5‐OH‐1,4‐NQ is similar to that of other naphthoquinones and involves formation of its corresponding naphthosemiquinone, active oxygen species and redox cycling as it stimulated a disproportionate increase in both microsomal NADPH oxidation and oxygen consumption. No evidence for redox cycling of 2‐OH‐1,4‐NQ was obtained either with microsomes or hepatocytes and thus its cytotoxicity must involve a different mechanism to that currently proposed for other naphthoquinones. This inability of 2‐OH‐1,4‐NQ to redox cycle may be due to its very low one‐electron reduction potential (E1/7‐415 mV). Thus, it is a very poor substrate for reduction to a semiquinone by cellular reductases.