Interconversion of NAD(H) TO NADP(H)

Quinones may be toxic by a number of mechanisms, including oxidative stress caused by redox cycling and arylation. This study has compared the cytotoxicity of four quinones, with differing abilities to arylate cellular nucleophiles and redox cycle, in relation to their effects on cellular pyridine nucleotides and ATP levels in rat hepatocytes. Non-toxic concentrations (50 μM) of menadione (redox cycles and arylates), 2-hydroxy-1,4-naphthoquinone (neither arylates nor redox cycles via a one electron reduction) and 2,3-dimethoxy-1,4-naphthoquinone (a pure redox cycler) all caused markedly similar changes in cellular pyridine nucleotides. An initial decrease in NAD + was accompanied by a small, transient increase in NADP + and followed by a larger, prolonged increased in NADPH and total NADP + + NADPH. At toxic concentrations (200 μM), the quinones caused an extensive depletion of NAD(H), an increase in levels of NADP + and an initial rise in total NADP + + NADPH, prior to a decrease in ATP levels and cell death. Nucleotide changes were not observed with non-toxic (20 μM) or toxic (100 μM) concentrations of p -benzoquinone (a pure arylator) and ATP loss accompanied or followed cell death. A novel mechanism for the activation of 2-hydroxy-1,4-naphthoquinone has been implicated. Our findings also suggest that a primary event in the response of the cell to redox cycling quinones is to bring about an interconversion of pyridine nucleotides, possibly mediated by an NAD + reduction, in an attempt to combat the effects of oxidative stress.