Ethanol metabolism by a transplantable hepatocellular carcinoma. Role of microsomes and mitochondria

Involvement of cell oxidant status and redox state in the increased non-enzymatic ethanol oxidation by the regenerating rat liver

Ethanol administration is capable of inhibiting or delaying the partial hepatectomy (PH)-induced liver regeneration, probably altering liver metabolism by means of its oxidative metabolism. Since the regenerating liver has increased capacity for oxidizing ethanol, the present study was aimed to address the contribution of the ethanol-oxidizing metabolic pathways in the regenerating liver cells. Isolated hepatocytes were prepared from control livers and from animals subjected to two-thirds PH. In both preparations, ethanol oxidation was largely increased by incubation with glucose and was highly sensitive to inhibitors of ethanol-oxidizing enzymatic pathways (alcohol dehydrogenase, catalase and cytochrome P-4502E1 activities). The latter led to a total blockade of ethanol disposal by control hepatocytes, while liver cells from PH-rats only showed an early 70-75% inhibition of ethanol catabolism with the inhibitors used. In regenerating hepatocytes, the enhanced ethanol oxidation was blocked by scavengers of reactive oxygen species, an effect that correlated with enhanced cytoplasmic lipid peroxidation by-products. Both cell preparations showed similar sensitivity to inhibitors for the malate-aspartate shuttle and mitochondrial electron transport chain; the shift of the cytoplasmic redox state was also quite similar after ethanol oxidation. A more oxidized mitochondrial redox state was found in hepatocytes from PH-rats and more shifted to the reduced state during ethanol oxidation this effect was not abolished by inhibiting alcohol dehydrogenase activity. In conclusion, data clearly show that an important fraction of ethanol is metabolized through a non-enzymatic-mediated oxidative event, which could largely contribute to the deleterious effect of ethanol on the proliferating liver.

The impairment of respiration by glycolysis in the Lewis lung carcinoma

Isolated vegetative tumour cells from mice bearing the Lewis lung carcinoma showed low rates of basal respiration with both low oxygen uptake rates and cytochrome-c oxidase activity. The cells were affected by a marked Crabtree effect and a high rate of lactate production in the presence of 10 mM glucose. The glycolytic capacity of the tumour was also assessed through the measurement of the maximum activities for hexokinase, phosphofructokinase, pyruvate kinase and lactate dehydrogenase. These activities were similar to the ones found in other fast-growing, undifferentiated tumours. The concentration of fructose-2,6-bisphosphate in the tumour was 2,3 nmoles/g fresh tissue wt., a value which is of the same order of magnitude as that found in other types of highly glycolytic cells. It is concluded that the Lewis lung carcinoma follows the same pattern as other undifferentiated tumours with a high capacity for both glucose and amino acid utilization.

Chemical evidence for production of hydroxyl radicals during microsomal electron transfer

Rat liver microsomes generate methane from dimethyl sulfoxide and ethylene from either methional or 2-keto-4-thiomethylbutyric acid during electron transfer initiated by reduced nicotinamide-adenine dinucleotide phosphate (NADPH). Hydrocarbon gas production is suppressed by hydroxyl radical scavenging agents. Azide, an inhibitor of catalase, augments the production of hydrocarbon gases. These observations constitute chemical evidence for the generation of hydroxyl radicals by microsomes.

Effect of thiourea on microsomal oxidation of alcohols and associated microsomal functions

Thiourea and diethylthiourea, two compounds which react with hydroxyl radicals, inhibited NADPH-dependent microsomal oxidation of ethanol and 1-butanol. Inhibition by both compounds was more effective in the presence of the catalase inhibitor, azide. Inhibition by thiourea was noncompetitive with respect to ethanol in the absence of azide but was competitive in the presence of azide. Urea, a compound which does not react with hydroxyl radicals or H2O2, was without effect. Thiourea had no effect on NADH- and NADH-cytochrome c reductase, NADPH oxidase, and NADH- and NADPH-dependent oxygen uptake. Thiourea inhibited the activities of aniline hydroxylase and aminopyrine demethylase. Thiourea, but no other hydroxyl radical scavengers, e.g., dimethyl sulfoxide, mannitol, and benzoate, reacted directly with H202 and decreased H2O2 accumulation in the presence of azide. Therefore the actions of thiourea are complex because it can react with both hydroxyl radicals and H2O2. Differences between the actions of thiourea and those previously reported for dimethyl sulfoxide, mannitol, and benzoate, e.g., effects on drug metabolism, effectiveness of inhibition in the absence of azide, or kinetics of the inhibition, probably reflect the fact that thiourea reacts directly with H2O2 whereas the other agents do not. The current results remain consistent with the concept that microsomal oxidation of alcohols involves interactions of the alcohols with hydroxyl radicals generated from microsomal electron transfer.