Sensitivity to acetaldehyde of pyruvate oxidation by mitochondria from liver, kidney, brain and muscle

Cytotoxicity of short-chain alcohols

Ethanol and other short-chain alcohols elicit a number of cellular responses that are potentially cytotoxic and, to some extent, independent of cell type. Aberrations in phospholipid and fatty acid metabolism, changes in the cellular redox state, disruptions of the energy state, and increased production of reactive oxygen metabolites have been implicated in cellular damage resulting from acute or chronic exposure to short-chain alcohols. Resulting disruptions of intracellular signaling cascades through interference with the synthesis of phosphatidic acid, decreases in phosphorylation potential and lipid peroxidation are mechanisms by which solvent alcohols can affect the rate of cell proliferation and, consequently, cell number. Nonoxidative metabolism of short-chain alcohols, including phospholipase D-mediated synthesis of alcohol phospholipids, and the synthesis of fatty acid alcohol esters are additional mechanisms by which alcohols can affect membrane structure and compromise cell function.

Biochemical and morphological studies of skeletal muscle in experimental chronic alcoholic myopathy

Experimental alcoholic myopathy was induced in rats by a combination of prolonged alcohol intake (mean 15.3 g ethanol/kg/day for up to 10 weeks) and a short fast. In view of literature evidence for impairment of both glycolytic and oxidative metabolism in alcoholic myopathy, we combined histological and histochemical observations with biochemical studies comprising assay of all glycolytic enzymes and measurement of respiration rates and cytochrome content in isolated intact mitochondria. The predominant histological finding was Type IIb fibre atrophy, while levels of the glycolytic enzymes aldolase, pyruvate kinase and lactate dehydrogenase were significantly depressed. Evidence of rhabdomyolysis was seen in a minority of animals. Mean mitochondrial respiratory rates were significantly lower with the Site I substrate glutamate in alcohol-treated animals. It is postulated that chronic alcoholic myopathy is associated with glycolytic deficiency and that acute rhabdomyolysis may arise from a superimposed mitochondrial failure, resulting in a severe energy crisis in muscle.

Mitochondrial respiration after cardiac perfusion with ethanol or acetaldehyde

Hemodynamics, energetics, and mechanics of isolated working rat hearts were studied during perfusion with Krebs-Henseleit medium containing either ethanol (24 +/- 2 mM) or acetaldehyde (74 +/- 6 microM). After 60-min perfusion with agent, mitochondria were isolated from the hearts and assayed polarographically using glutamate, alpha-keto-glutarate, pyruvate-malate, and succinate-rotenone substrates. Hemodynamic and mechanical function of the ethanol-treated hearts declined significantly more than that of control hearts during the 60-min perfusion. The acetaldehyde-perfused hearts did not differ from the controls. A 90-sec washout period reversed the ethanol-induced depression of physiologic function. Respiratory function (respiratory control ratio and Qo2) of mitochondria isolated from the ethanol- and acetaldehyde-treated hearts was slightly, though not significantly, depressed compared with control hearts. These data indicate that ethanol and acetaldehyde, at physiologically significant concentrations, have dissimilar effects on cardiac hemodynamics and mechanics: ethanol has a direct depressive effect which is reversible; acetaldehyde has no significant effect. Mitochondrial respiration is not irreversibly affected by brief exposure to ethanol or acetaldehyde at the concentrations tested.

Inhibition of the oxidation of acetaldehyde and formaldehyde by hepatocytes and mitochondria by crotonaldehyde

Crotonaldehyde was oxidized by disrupted rat liver mitochondrial fractions or by intact mitochondria at rates that were only 10 to 15% that of acetaldehyde. Although a poor substrate for oxidation, crotonaldehyde is an effective inhibitor of the oxidation of acetaldehyde by mitochondrial aldehyde dehydrogenase, by intact mitochondria, and by isolated hepatocytes. Inhibition by crotonaldehyde was competitive with respect to acetaldehyde, and the Ki for crotonaldehyde was about 5 to 20 microM. Crotonaldehyde had no effect on the oxidation of glutamate or succinate. Very low levels of acetaldehyde were detected during the metabolism of ethanol. Crotonaldehyde increased the accumulation of acetaldehyde more than 10-fold, indicating that crotonaldehyde, besides inhibiting the oxidation of added acetaldehyde, also inhibited the oxidation of acetaldehyde generated by the metabolism of ethanol. Formaldehyde was a substrate for the low-Km mitochondrial aldehyde dehydrogenase, as well as for a cytosolic, glutathione-dependent formaldehyde dehydrogenase. Crotonaldehyde was a potent inhibitor of mitochondrial oxidation of formaldehyde, but had no effect on the activity of formaldehyde dehydrogenase. In hepatocytes, crotonaldehyde produced about 30 to 40% inhibition of formaldehyde oxidation, which was similar to the inhibition produced by cyanamide. This suggested that part of the formaldehyde oxidation occurred via the mitochondrial aldehyde dehydrogenase, and part via formaldehyde dehydrogenase. The fact that inhibition by crotonaldehyde is competitive may be of value since other commonly used inhibitors of aldehyde dehydrogenase are irreversible inhibitors of the enzyme.

Does consumption of alcohol in the presence of disulfiram cause brain damage?

After blocking aldehyde dehydrogenase activity with disulfiram in rats, a single injection of ethanol or a single day of inhaling ethanol vapor resulted in neuronal degeneration in the cerebral cortex and elsewhere that was detected by a specific silver stain and by electron microscopy. Treatment of rats with alcohol alone and with disulfiram alone did not cause neuronal degeneration. The minimum blood alcohol levels and disulfiram doses administered to the rats which caused neural degeneration are not distant from those values described in a clinical case of abuse of disulfiram.

Mitochondrial chemiluminescence elicited by acetaldehyde

Acetaldehyde-dependent chemiluminescence has been found to be a sensitive technique for the study of superoxide and hydrogen peroxide formation in beef heart mitochondria. The system responds to ATP and antimycin A with increased emission intensities and to ADP and rotenone with decreased intensities, indicating that the chemiluminescence reflects the energy status of the mitochondrion. These effects are based on the ability of acetaldehyde to react with superoxide and hydrogen peroxide to form metastable intermediates which decay spontaneously with the emission of light. Additionally, these intermediates can react with cyanide to give alternative products which can also decay with the emission of light, the cyanide-evokable chemiluminescence. The interaction of acetaldehyde with mitochondria is complex because acetaldehyde can serve as a hydrogen source for NADH and as an inhibitor (at high concentration) of electron transport, and appears to be a reducing agent for a heat-stable site that autoxidatively generates HOOH from O2-.. Inasmuch as acetaldehyde is a metabolite of ethanol, this broad spectrum of reactivity may play a role in the hepatic and cardiac toxicity that is associated with alcoholism. The heat-stable site that generates HOOH from O2-. has been studied further and appears to contain vicinal dithiol which is primarily responsible for the cyanide-evokable chemiluminescence.

The toxicity to rat cerebral cortex or topical applications of acetaldehyde, ammonia or bilirubin

The relative toxicity of acetaldehyde, ammonia and bilirubin to cerebral cortex of rats has been tested by superfusion of the intact dura for 1 h with each toxin separately. The cortex was examined histologically after 6 days survival using a silver strain which selectively impregnates degenerating axons and their synaptic terminals. A tissue concentration of 30 mM acetaldehyde was found to cause axonal and terminal degeneration, whereas 11 mM acetaldehyde was not toxic, and did not produce any structural changes detectable by electron microscopy. In terms of tissue concentrations, acetaldehyde was at least 27 times more toxic than ethanol on a molar basis. However, the acetaldehyde concentration necessary to produce neuronal degeneration in a 1 h exposure is many times greater than ever reported in CSF in human alcoholism. Both ammonia and bilirubin are capable of causing neuronal degeneration, but we have not measured the tissue concentrations that are toxic. Comparison of molar concentrations in the superfusing fluids showed that ammonia is at least 39 times more toxic than acetaldehyde, and at least 1000 times more toxic than ethanol. Superfusion with an ethanolic solution of bilirubin (2.5mM) was strongly toxic, but this solution diluted to 50% with saline was not toxic. The possibility exists that ammonia or bilirubin may reach concentrations toxic to neurons in alcoholism, especially in the presence of liver damage.

The effect of cyanamide on acetaldehyde oxidation by isolated rat liver mitochondria and on the inhibition of pyruvate oxidation by acetaldehyde

Compared to other substrates, the oxidation of pyruvate by isolated mitochondria is especially sensitive to inhibition by acetaldehyde. It is not known whether this inhibition represents a direct effect of acetaldehyde or requires the metabolism of acetaldehyde. Experiments were therefore carried out in the presence of cyanamide, an inhibitor of aldehyde dehydrogenase. After a brief incubation period, cyanamide inhibited the state 4 and state 3 rate of acetaldehyde (0.1-1.0 mM) oxidation by isolated rat liver mitochondria. Little inhibition was found in the absence of the incubation period. Maximum inhibition was found at cyanamide concentrations of 0.01 to 0.033 mM. Cyanamide also inhibited the activity of aldehyde dehydrogenase assayed in disrupted mitochondrial fractions. The inhibition by cyanamide was specific since cyanamide did not affect mitochondrial oxidation of succinate, glutamate, or pyruvate. Acetaldehyde inhibited the state 3 rate of pyruvate oxidation by liver mitochondria. Despite preventing acetaldehyde oxidation, cyanamide did not prevent the inhibition of pyruvate oxidation by acetaldehyde. These results indicate that (a) cyanamide can be used as an effective in vitro inhibitor of acetaldehyde oxidation and (b) the unique sensitivity of pyruvate oxidation to acetaldehyde represents a direct effect of acetaldehyde on pyruvate dehydrogenase.