The cytotoxic effects and biotransformation of
harmine and
harmaline, which are known β-
carboline alkaloids and potent
hallucinogens, were studied in freshly isolated rat hepatocytes. The exposure of hepatocytes to
harmine caused not only concentration (0-0.50mM)- and time (0-3h)-dependent cell death accompanied by the formation of cell
blebs and the loss of cellular
ATP,
reduced glutathione, and
protein thiols but also the accumulation of
glutathione disulfide. Of the other analogues examined, the cytotoxic effects of
harmaline and
harmol (a metabolite of
harmine) at a concentration of 0.5mM were less than those of
harmine. The loss of mitochondrial membrane potential and generation of
oxygen radical species in hepatocytes treated with
harmine were greater than those with
harmaline and
harmol. In the oxygen consumption of mitochondria isolated from rat liver, the ratios of state-3/state-4 respiration of these β-
carbolines were decreased in a concentration-dependent manner. In addition,
harmine resulted in the induction of the mitochondrial permeability transition (MPT), and the effects of
harmol and
harmaline were less than those of
harmine. At a weakly toxic level of
harmine (0.25mM), it was metabolized to
harmol and its monoglucuronide and monosulfate conjugates, and the amounts of
sulfate rather than
glucuronide predominantly increased with time. In the presence of 2,5-dichloro-4-nitrophenol (50μM; an inhibitor of
sulfotransferase),
harmine-induced cytotoxicity was enhanced, accompanied by decrease in the amount of
harmol-sulfate conjugate, due to an increase in the amount of unconjugated
harmol and the inhibition of
harmine loss. Taken collectively, these results indicate that (a) mitochondria are target organelles for
harmine, which elicits cytotoxicity through mitochondrial failure related to the induction of the MPT, mitochondrial depolarization, and inhibition of
ATP synthesis; and (b) the toxic effects of
harmine are greater than those of either its metabolite
harmol or its analogue
harmaline, suggesting that the onset of
harmine-induced cytotoxicity may depend on the initial and/or residual concentrations of
harmine rather than on those of its metabolites.