Modulating carbonyl cytotoxicity in intact rat hepatocytes by inhibiting carbonyl-metabolizing enzymes. I. Aliphatic alkenals.

The cytotoxicity of alkenals towards hepatocytes was related to their electrophilicity not their hydrophobicity as cytotoxicity decreased as the chain length increased from acrolein to hexenal and then cytotoxicity increased from hexenal to nonenal. The sequence of events found was rapid glutathione depletion, lipid peroxidation, and inhibition of respiration before cell lysis occurred. Cytotoxicity markedly increased if glutathione was depleted beforehand. Although acrolein-induced cytotoxicity was only delayed by antioxidants or glycolytic substrates (e.g. fructose), it was prevented by NADH generators (e.g. xylitol and sorbitol) due to increased metabolism by ADH1. Cytotoxicity induced by trans,trans-2,4-decadienal (decadienal), on the other hand, was prevented by antioxidants and/or glycolytic substrates but was not prevented by NADH generators. Decadienal-induced cytotoxicity was also more increased by mitochondrial ALDH2 inhibitors than acrolein and was more increased by decreasing mitochondrial NAD+ with rotenone or decreased by increasing mitochondrial NAD+ with oxaloacetate. This suggests that the high electrophilicity of acrolein makes acrolein a more promiscuous inhibitor than decadienal. This results in the inactivation of more enzymes required for cell viability including the cytosolic and mitochondrial ALDHs as well as other enzymes (e.g. mitochondrial) making the reductive detoxication of acrolein by ADH1 more important than the oxidative detoxification by ALDHs. Decadienal is detoxified by all cytosolic and mitochondrial ALDHs and is less dependent on ADH1 for detoxication. There was also marked cytotoxic synergism between acrolein and decadienal presumably because of ALDH inactivation by acrolein.

Modulating carbonyl cytotoxicity in intact rat hepatocytes by inhibiting carbonyl metabolizing enzymes. II. Aromatic aldehydes.

The molecular cytotoxic mechanisms of dietary benzaldehydes towards hepatocytes and its modulation by metabolizing enzymes were compared. Salicylaldehyde was found to be the most cytotoxic followed by cinnamaldehyde and both rapidly depleted some glutathione before an inhibition of respiration occurred, which preceded cell lysis. Reactive oxygen species were formed, but lipid peroxidation was induced with cinnamaldehyde, but not salicylaldehyde. Glutathione depleted hepatocytes were more susceptible to cytotoxicity. Mitochondrial toxicity and cytotoxicity were prevented by glycolytic substrates (e.g. fructose), citric acid cycle substrates (e.g. glutamine) or cyclosporin, the mitochondrial permeability transition inhibitor. Inhibition of mitochondrial ALDH with chloral hydrate, crotonaldehyde or citral or decreasing mitochondrial NAD+ with rotenone increased cinnamaldehyde induced cytotoxicity with a much smaller effect on salicylaldehyde induced cytotoxicity. Cyanamide was the most effective ALDH inhibitor for increasing cinnamaldehyde induced cytotoxicity, presumably because cyanamide also inhibits microsomal ALDH. Although cinnamaldehyde was a better substrate than salicylaldehyde for ADH1, cytosolic NADH generators (e.g. xylitol) prevented salicylaldehyde and cinnamaldehyde cytotoxicity similarly. This could be explained as salicylaldehyde was not a substrate for the ALDHs and would then be more dependent on ADH for detoxification.