Quinoneimines as substrates for quinone reductase (NAD(P)H: (quinone-acceptor)oxidoreductase) and the effect of dicumarol on their cytotoxicity.

Several quinoneimines have been shown to be substrates for partly purified rat liver cytosolic quinone reductase with either NADH or NADPH as cofactor. Km and Vmax values with NADH as cofactor for N-acetyl-p-benzoquinoneimine were 54.9 microM and 278 mumol/min/mg; for 2-amino-1,4-naphthoquinoneimine, 2.8 microM and 38 mumol/min/mg; for N,N-dimethylindoaniline, 1.7 microM and 22 mumol/min/mg; and 2-acetamido-N,N-dimethylindoaniline, 0.4 microM and 9 mumol/min/mg. All the quinoneimines showed substrate inhibition at high concentrations. At 30 microM dicumarol, an inhibitor of quinone reductase, potentiated the acute toxicity of quinoneimines to cultured phenobarbital-induced rat hepatocytes by 0.7- to 2.9-fold. Dicumarol was toxic to cultured non-induced rat hepatocytes and produced little or no increase in quinoneimine toxicity. Dicumarol potentiated the toxicity of 2-methyl-1,4-naphthoquinone (menadione) to cultured non-induced, as well as phenobarbital-induced, hepatocytes. Levels of quinone reductase in both types of hepatocytes were similar. Quinoneimines exhibited strong growth inhibitory properties with Chinese hamster ovary (CHO) cells and A204 human rhabdomyosarcoma cells. Dicumarol, 0.1 mM, potentiated growth inhibition by N,N-dimethylindoaniline and 2-acetamido-N,N-dimethylindoaniline in A204 but not in CHO cells. Growth inhibition by 2-amino-1,4-naphthoquinoneimine was inhibited by dicumarol in both cell lines. Dicumarol potentiated growth inhibition by 2-methyl-1,4-naphthoquinone in A204 and CHO cells. Quinone reductase activity in A204 cells was 48% and in CHO cells 1% of the activity in cultured hepatocytes. The lack of a correlation between the effects of dicumarol on quinoneimine and quinone growth inhibition and levels of cellular quinone reductase suggests that dicumarol has effects in cells in addition to, or other than, inhibition of quinone reductase. It is concluded that quinone reductase may protect cells against quinoneimine toxicity under certain conditions, as with phenobarbital-induced hepatocytes, but does not appear to play a major role in modifying quinoneimine toxicity in non-induced hepatocytes, or growth inhibition in CHO cells or A204 cells.