Effect of Fructose and Glyceraldehyde on Ethanol Metabolism in Human Liver and in Rat Liver

Abstract

The basic kinetic parameters, V and Km, have been determined for liver enzymes involved in the metabolism of fructose and ethanol in rats and man. Values, previously not reported, or which deviate significantly from those reported in the literature are as follows: The maximal activity of aldehyde dehydrogenase from human liver with acetaldehyde as substrate was determined as 43 pmol x min-l x g wet wt-l. The activity of NADP-dependent alcohol dehydrogenase with ethanol as substrate both in rat liver and in human liver was very low. The presence of glycerate kinase (12 pmol x min-l x g wet &-I) in human liver has been established. The K,-value of human-liver alcohol dehydrogenase (NAD) for D-glyceraldehyde was determined as 80-90mM compared to 8-10mM for the rat liver enzyme. The NADP-dependent alcohol dehydrogenase from human liver had a Km-value for D-glyceraldehyde of 2.5-3.3 mM. Glycerate kinase from human and rat liver had K,-values for D-glyceraldehyde of 2.5-3.0 mM and 0.03 mM, respectively.I n slices of human liver the increase in ethanol-oxidation rate caused by 11 mM fructose ('the fructose effect') or by 2 mM D-glyceraldehyde was 76O/, and 56O/,, respectively.I n rat-liver slices the effect of D-glyceraldehyde upon ethanol metabolism was inhibited 1000/o by rotenone. I n the same experiments rotenone caused a 5001, inhibition of the basic ethanol oxidation rate and of the oxygen consumption.10 mM pyruvate, in experiments with liver slices from fasted rats, caused a 9001, increase in the ethanol oxidation rate in the absence of CO,, but only a 200/, increase in the presence of CO,. CO, in itself caused a 700/, increase in the unstimulated oxidation of ethanol.Kinetic considerations and results reported in this paper held together with results from other laboratories lead to the conclusion that the theories previously proposed for the effect of fructose, D-glyceraldehyde or pyruvate upon ethanol metabolism are irreconcilable with the experimental results. A new hypothesis for the 'fructose effect' is proposed.Malate dehydrogenase, malic enzyme, pyruvate carboxylase and transfer of oxaloacetate from the mitochondria1 to the extramitochondrial compartment constitute a mechanism by which transfer of hydrogen from NADH to NADP may take place, thus increasing the rate of removal of NADH from the cytoplasm.I n the liver ethanol is oxidized to acetate via acetaldehyde. The oxidation to acetaldehyde, catalyzed by the cytosolic alcohol dehydrogenase, has been thought to be the rate-limiting step in ethanol metabolism [I]. There is, however, no correlation between the alcohol dehydrogenase activity measured in vitro and the rate of ethanol oxidation in vivo or in liver slices [2,3,4]. As the activity of the enzyme does not appear to be the rate-limiting factor, it is assumed that the oxidation of NADH is ratelimiting for the ethanol oxidation in vivo.During ethanol metabolism acetaldehyde does not accumulate in the liver and is probably oxidized immediately by aldehyde dehydrogenase to acetate [ 5 ] . Thus 2 mol N...

Effect of Long-Term 'Corn Pepffde' Ingestion on Alcohol Metabolism in Stroke-Prone Spontaneously Hypertensive Rats with Alcohol Loading.

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Effect of Amino Acids on Alcohol Intake in Stroke-Prone Spontaneously Hypertensive Rats.

“…Concerning the relationship between the alcohol metabolism and amino acids, Tieden et al suggested that alanine and glutamine abundantly supply pyruvic acid that performs the NAD generating system and thus alcohol metabolism itself can readily progress to the TCA cycle (9,10). Ethanol oxidation has been shown to be accelerated in the presence of certain L-amino acids, such as proline and lysine (11)(12)(13)(14).…”

Sugars are complementary resources to ethanol in foods consumed by Egyptian fruit bats

“…During ethanol catabolism NAD is reduced to NADH, and the supply of NAD might limit ethanol oxidation. It has been proposed that during fructose metabolism NADH is transformed into NAD, and thus fructose increases ethanol oxidation (Scholz and Nohl, 1976;Thieden et al, 1972;Tygstrup et al, 1965). However, this idea is not fully accepted yet (Mascord et al, 1991).…”