Contribution of "substrate shuttles" in the transport of extramitochondrial reducing equivalents by hepatic mitochondria from chronic alcohol-fed mice

Metabolic alterations produced in the liver by chronic ethanol administration. Comparison between the effects produced by ethanol and by thyroid hormones

1. Liver slices from rats treated with thyroxine show an increased rate of O(2) consumption. The extra consumption, but not the basal respiration, can be abolished by ouabain. 2. Dinitrophenol is not effective in increasing the rate of O(2) consumption of liver slices from thyroxine-treated animals but its effectiveness can be recovered in the presence of ouabain. 3. (Na(+)+K(+))-stimulated adenosine triphosphatase activity of liver was increased by administration of thyroxine in vivo. No changes were found in total Mg(2+)-stimulated adenosine triphosphatase activity. 4. Mitochondrial alpha-glycerophosphate dehydrogenase and microsomal NADPH oxidase activity were increased by both thyroxine and chronic ethanol treatment. 5. Liver slices from animals chronically treated with ethanol synthesize urea at an increased rate. 6. Mitochondrial size (section area) is markedly increased in the liver of animals chronically treated with ethanol. 7. Acute administration of ethanol in doses of 4 and 6g/kg significantly increases the uptake of (131)I-labelled thyroxine by the liver. 8. Work reported here, along with results from other investigators, indicates marked similarities between the effects produced in the liver by chronic administration of ethanol and by thyroid hormones.

Effects of moderate chronic ethanol consumption on rat liver mitochondrial functions

The biochemical consequences of moderate chronic ethanol ingestion has been scarcely investigated in spite of the fact that most of the human population drinks ethanol on a moderate basis. This paper describes some metabolic effects produced by moderate ethanol consumption. The substitution of drinking water in rats for a 10% ethanol solution during 4 weeks resulted in: a) a decrease of blood urea and citrulline synthesis in liver mitochondria; b) a slight inhibition in state 3 and state 4 respiration either with glutamate-malate as substrates or succinate as substrate; c) no change in ADP/O ratio with succinate but slight increase with glutamate-malate; d) a reduction of the cytochrome oxidase activity and cytochromes a+a3 content; e) a 42% increase in the succinate dehydrogenase activity and a small but constant increase in the Vmax (no change in the Km) of the adenine nucleotide translocase activity in liver mitochondria. These results show that even moderate, but continuous ethanol ingestion, produces metabolic responses that must be carefully evaluated to define health risk in larger human groups.

Effects of acetaldehyde on human red cell metabolism: evidence for the formation of enzyme inhibitors

Since red cells transport and metabolize acetaldehyde in vivo, the effects of acetaldehyde on human red cell enzyme activities were studied. Incubation of intact red cells or undiluted red cell lysates at 37 degrees C for 4 h with 1-10 mmol/l acetaldehyde decreased only GOT, GPT and aldolase activities among the 26 enzymes tested. No inhibition occurred at 4 degrees C or when acetaldehyde was incubated with dilute hemolysates. Incubation of lysates with other reducing substrates or with acetate inhibited aldolase but not GOT or GPT. Preincubation of lysates with cyanate or fluoride markedly decreased acetaldehyde-mediated transaminase inhibition but not aldolase inhibition. Addition of pyridoxal phosphate, the vitamin B6 transaminase coenzyme, to GOT and GPT assay mixes did not reverse acetaldehyde-mediated transaminase inhibition. These findings suggest that acetaldehyde-mediated aldolase inhibition results from oxidation of acetaldehyde while transaminase inhibition results from nonoxidative acetaldehyde metabolism. When 100-200 mumol/l acetaldehyde is added to lysates at 2-h intervals and when lysates are incubated with ethanol, alcohol dehydrogenase and an NAD-regenerating system, enzyme inhibition occurs at acetaldehyde levels approaching those seen in vivo. Thus, the role of acetaldehyde-mediated enzyme inhibition in the toxicity of alcohol abuse warrants further study.

Studies on the mechanism of acetaldehyde-mediated inhibition of rat liver transaminases

Incubation of mitochondria-depleted rat liver homogenates with 5 mmol/l acetaldehyde at 37 degrees C for 1 h inhibited both aspartate and alanine aminotransferases by 30%. Inhibition was prevented by decreasing temperature to 4 degrees C or by preincubating homogenates with cyanate but was unaffected by cyanamide and methylpyrazole which block acetaldehyde oxidation and reduction respectively. Cyanate-sensitive acetaldehyde-mediated inhibition of purified porcine heart transaminases was also demonstrated in the presence of rat liver homogenate but not in Tris/sucrose medium. Moreover, porcine transaminases were inhibited by trichloroacetic acid extracts of rat liver homogenates previously incubated with acetaldehyde but not by extracts of homogenates incubated with both acetaldehyde and cyanate. These findings suggest that acetaldehyde-mediated transaminase inhibition requires further non-oxidative metabolism of acetaldehyde. Since transaminase activities were not restored by addition of pyridoxal 5'-phosphate to the assay systems, acetaldehyde-induced transaminase inhibition does not appear to be mediated by displacement or depletion of this B6 coenzyme.

Evidence for the generation of transaminase inhibitor(s) during ethanol metabolism by rat liver homogenates: a potential mechanism for alcohol toxicity

Since ethanol consumption decreases hepatic aminotransferase activities in vivo, mechanisms of ethanol-mediated transaminase inhibition were explored in vitro using mitochondria-depleted rat liver homogenates. When homogenates were incubated at 37 degrees with 50 mM ethanol for 1 hr, alanine aminotransferase decreased by 20%, while aspartate aminotransferase was unchanged. After 2 hr, aspartate aminotransferase decreased by 20% and by 3 hr, alanine and aspartate aminotransferases were decreased by 31 and 23%, respectively. Levels of acetaldehyde generated during ethanol oxidation were 525 +/- 47 microM at 1 hr, 855 +/- 14 microM at 2 hr, and 1293 +/- 140 microM at 3 hr. Although inhibition of alcohol oxidation with methylpyrazole or cyanide markedly decreased ethanol-mediated transaminase inhibition, neither incubation with acetate nor generation of reducing equivalents by oxidation of lactate, malate, xylitol, or sorbitol altered the activity of either enzyme. However, semicarbazide, an aldehyde scavenger, prevented inhibition of both aminotransferases by ethanol. Moreover, incubation with 5 mM acetaldehyde for 1 hr inhibited alanine and aspartate aminotransferases by 36 and 26%, respectively. Cyanamide, an aldehyde dehydrogenase inhibitor, had little effect on ethanol-mediated transaminase inhibition. Thus, metabolism of ethanol by rat liver homogenates produces transaminase inhibition similar to that described in vivo and this effect requires acetaldehyde generation but not acetaldehyde oxidation. Since addition of pyridoxal 5'-phosphate to assay mixes did not reverse ethanol effects, aminotransferase inhibition does not result from displacement of vitamin B6 coenzymes.

Sources of energy for the brain and physical dependence on ethanol

This paper describes a proposed biochemical mechanism to account for physical dependence on ethanol. Long-term exposure to a continuous and plentiful supply of ethanol-derived hydrogens in NADH and ethanol-derived acetyl-CoA (AcCoA) may lead to a decreased rate of production of endogenous hydrogens in NADH and endogenous AcCoA in compensation. It is proposed that physical dependence on ethanol occurs when ketolytic- and glycolytic-based NADH and AcCoA synthetic rates fall below the rate of utilization by the brain; the brain then "depends" on alcohol-derived NADH and AcCoA for normal function.

Alcohol metabolism in man: effect of intravenous fructose infusion on blood ethanol elimination rate following stimulation by phenobarbital treatment or chronic alcohol consumption

The effect of phenobarbital (PB) pretreatment and of chronic alcoholism on blood ethanol elimination rate (BEER) was investigated in man. In order to gain additional information concerning the mechanism of possible changes BEER was determined before and during intravenous infusion of fructose, a compound known to increase the NADH-oxidizing capacity of the liver and thereby stimulating alcohol oxidation rate. Following PB-treatment (300 mg/day for 5-6 days, n = 8) a marked increase in unstimulated (U-) BEER was obtained. But the fructose stimulated (FS-) BEER was not significantly changed by PB-treatment. In chronic alcoholics (n = 15) U-BEER values above the upper limit (chi + 2 S D) obtained in healthy controls, were observed only when the time of sobriety was less than one week (n = 6). Values of FS-BEER in chronic alcoholics with increased basal alcohol oxidation rates were in the same range as those of healthy controls. In 5 out of the 6 alcoholics in whom the values were elevated on admission, BEER decreased significantly after withdrawal of alcohol for 2-4 weeks. Since FS-BEER was nearly identical in all conditions tested, the distinct changes in U-BEER are probably independent of changes in the activity of enzymes involved in alcohol oxidation. It is assumed that alcohol metabolism in man is mainly controlled by the rate of NADH reoxidation in the liver.

Effect of maternal ethanol consumption on foetal and neonatal rat hepatic protein synthesis

Effects of maternal ethanol consumption were investigated on the rates of protein synthehsis by livers of foetal and neonatal rats both in vivo and in vitro, and on the activities of enzymes involved in protein synthesis and degradation. The rates of general protein synthesis by ribosomes in vitro studied by measuring the incorporation of [14C]leucine into ribosomal protein showed that maternal ethanol consumption resulted in an inhibition of the rates of protein synthesis by both foetal and neonatal livers from the ethanol-fed group. The rates of incorporation of intravenously injected [14C]leucine into hepatic proteins were also significantly lower in the foetal, neonatal and adult livers from the ethanol-fed group. Incubation of adult-rat liver slices with ethanol resulted in an inhibition of the incorporation of [14C]leucine into hepatic proteins; however, this effect was not observed in the foetal liver slices. This effect of externally added ethanol was at least partially prevented by the addition of pyrazole to the adult liver slices. Pyrazole addition to foetal liver slices was without significant effect on the rates of protein synthesis. Cross-mixing experiments showed that the capacity of both hepatic ribosomes and pH5 enzyme fractions to synthesize proteins was decreased in the foetal liver from the ethanol-fed group. Maternal ethanol consumption resulted in a decrease in hepatic total RNA content, RNA/DNA ratio and ribosomal protein content in the foetal liver. Foetal hepatic DNA content was not significantly affected. Ethanol consumption resulted in a significant decrease in proteolytic activity and the activity of tryptophan oxygenase in the foetal, neonatal and adult livers. It is possible that the mechanisms of inhibition of protein synthesis observed here in the foetal liver after maternal ethanol consumption may be responsible for at least some of the changes observed in 'foetal alcohol syndrome'.

Effects of ethanol on brain metabolism

The influence of acute or chronic ethanol administration on the biochemical processes in brain and cerebral metabolic pathways has been discussed. Ethanol seems to affect cerebral carbohydrate metabolism mainly through increased glycogenolysis, although the possibility of decreased cerebral glucose utilization remains eminent. Ethanol affects the consumption of oxygen by the brain tissue presumably through alterations in the brain cell membranes. Inhibition of Na+-K+-ATP-ase observed during ethanol intoxication is suspected to result in alterations in the membranes of the nerve cells. Isotope studies in addition to total respiratory carbon dioxide production strongly suggest the inhibition of citric acid cycle function during ethanol metabolism. Although synthetic pathways for lipids do not seem to be affected by ethanol, lipid oxidation in the cerebral tissue is significantly inhibited. In addition to above mentioned alterations in the cerebral metabolic processes, ethanol also affects ionic transport processes, adenine nucleotides, and amino acid and protein metabolism. The metabolic consequences of such effects of ethanol have been discussed.

Rate-limiting steps in ethanol metabolism and approaches to changing these rates biochemically

Ethanol is oxidized to acetate primarily by a system involving liver alcohol and aldehyde dehydrogenases coupled with reoxidation of NADH by the mitochondria. All of these steps are at least partially rate-limiting in ethanol metabolism, with alcohol dehydrogenase and oxidative phosphorylation probably slower than the others. More research is required to assess the quantitative roles of various steps. Many agents are ineffective in changing the rate of metabolism of ethanol, but fructose and dinitrophenol may increase the rate by up to 1.5-fold in vivo. The failure of single agents to increase the rate substantially may indicate that when one step is accelerated, another step becomes rate-limited. Therefore, combinations of agents that affect several steps simultaneously may be required for acceleration. Effective experimental methods for inhibiting alcohol dehydrogenase in vivo are available.