Hepatic microsomal ethanol oxidation. Hydrogen peroxide formation and the role of catalase

The oxygen sensing characteristics of microsomal enzymes

The microsomal electron transport complex, in particular the segment associated with cytochrome P-450 function, is both qualitatively and quantitatively an important contributor to the cellular respiration of tissues such as liver. Although our knowledge is still limited, it is apparent that oxygen plays a pivitol role in dictating the mode of substrate hydroxylation or the generation of hydrogen peroxide. As illustrated in Figure 7, current evidence suggests that hydrogen peroxide is formed by the dismutation of the superoxide anion resulting from the dissociation of oxycytochrome P-450. Of interest are recent studies demonstrating the ability of hydrogen peroxide to initiate a cytochrome P-450 dependent peroxidatic reaction competent for supporting substrate hydroxylation. The fact that the function of cytochrome P-450 is sensitive to changes in oxygen tension establishes its role as an "oxygen sensor" for cellular metabolism.

Evidence for superoxide-dependent reduction of Fe3+ and its role in enzyme-generated hydroxyl radical formation

This report describes studies yielding additional evidence that superoxide anion (O2) production by some biological oxidoreductase systems is a potential source of hydroxyl radical production. The phenomenon appears to be an intrinsic property of certain enzyme systems which produce superoxide and H2O2, and can result in extensive oxidative degradation of membrane lipids. Earlier studies had suggested that iron (chelated to maintain solubility) augmented production of the hydroxyl radical in such systems according to the following reaction sequence: O2 + Fe3+ leads to O2 + Fe2+ Fe2+ + H2O2 leads to Fe3+ + HO-+OH-. The data reported below provide additional support for the occurrence of these reactions, especially the reduction of Fe3+ by superoxide. Because the conditions for such reactions appear to exist in animal tissues, the results indicate a mechanism for the initiation and promotion of peroxidative attacks on membrane lipids and also suggest that the role of antioxidants in intracellular metabolism may be to inhibit initiation of degradative reactions by the highly reactive radicals formed extraneously during metabolic activity. This report presents the following new information: (1) Fe3+ is reduced to Fe2+ during xanthine oxidase activity and a significant part of the reduction was oxygen dependent. (2) Mn2+ appears to function as an efficient superoxide anion scavenger, and this function can be inhibited by EDTA. (3) The O2-dependent reduction of Fe3+ to Fe2+ by xanthine oxidase activity is inhibited by Mn2+, which, in view of statement 2 above, is a further indication that the reduction of the iron involves superoxide anion. (4) Free radical scavengers prevent or reverse the Fe3+ inhibiton of cytochrome c3+ reduction by xanthine oxidase. (5) The inhibition of xanthine oxidase-catalyzed reduction of cyt c3+ by Fe3+ does not affect uric acid production by the xanthine oxidase system. (6) The reoxidation of reduced cyt c in the xanthine oxidase system is markedly enhanced by Fe3+ and is apparently due to enhanced HO-RADICAL formation since the Fe3+-stimulated reoxidation is inhibited by free radical scavengers, including those with specificity for the hydroxyl radical.

Ethanol metabolism and lipid synthesis by isolated liver cells from fed rats

1. The fatty acid synthesis in isolated liver cells from fed rats was studied with tritiated water as the radioactive precursor. The cells incorporated 3H20 at a rate of 1.26 mumol per min per g packed cells. 2. Addition of ethanol caused a 20% decrease in the incorporation of tritium into fatty acids. The decrease was correlated to the increase in the NAD-redox level. Probably, the decreased tritium incorporation into fatty acids during ethanol metabolism is due to a decrease in the specific activity of the NADPH used for the synthesis of fatty acids, rather than to a real inhibition of the fatty acid synthesis. 3. Ethanol oxidation via NADPH-consuming pathways and ethanol per se at a concentration of 80 mM had no effect upon the incorporation of tritium into fatty acids. 4. Fructose in a concentration of 15 mM inhibited the fatty acid synthesis by 75%, and this inhibition was further augmented by ethanol. 5. The ioslated rat liver cells oxidized ethanol at a rate of 2.72, 2.93 and 3.48 mumol per min per g packed cells at 5, 20 and 80 mM ethanol, respectively. Fructose had no effect upon ethanol oxidation neither at low nor at high concentrations of ethanol. 6. Ethanol oxidation via the non alcohol dehydrogenase pathway(s) may involve a transfer of reducing equivalents from mitochondrial NADH to cyctosolic NADP+ as judged from measurements of metabolite levels. This conclusion is supported by determinations of 14C yield in glucose from [1-14C] ethanol, and the results are taken as evidence for the presence of hydrogen shuttle activity during metabolism of ethanol, catalyzed by the NAD-dependent alcohol dehydrogenase. A metabolic scheme is proposed to account for the observed changes at low and high concentrations of ethanol.

Enhancement of hepatic microsomal drug metabolism in vitro following ethanol administration

1. Administration of ethanol intraperitoneally at low dosages (10-25 mg/kg) to rats stimulates hepatic microsomal mixed-function oxidase activity in vitro. 2. Pretreatment with ethanol administered orally has no effect on in vivo drug metabolism as measured by pentobarbitone plasma half-life and has no effect on the excretion of ascorbic acid. Ethanol administration does not enhance its own binding to cytochrome P-450. 3. These observations suggest that the administration of ethanol, at moderate dosage, does not give rise to induction of hepatic cytochrome P-450. 4. Unwashed hepatic microsomes are contaminated with alcohol dehydrogenase, but pretreatment with ethanol does not increase microsomal generation of NADH. 5. Pretreatment with ethanol has no stimulatory effect on NADH-NADP+ transhydrogenation. 6. The stimulation of hepatic drug metabolism in vitro following administration of ethanol is not due to increased cytochrome P-450 nor to increased NADPH, per se, but appears to result from an increase in the activity of NADPH-cytochrome c reductase.

The mechanism of liver microsomal lipid peroxidation

In the presence of Fe-3+ and complexing anions, the peroxidation of unsaturated liver microsomal lipid in both intact microsomes and in a model system containing extracted microsomal lipid can be promoted by either NADPH and NADPH : cytochrome c reductase or by xanthine and xanthine oxidase. Erythrocuprein effectively inhibits the activity promoted by xanthine and xanthine oxidase but produces much less inhibition of NADPH-dependent peroxidation. The singlet-oxygen trapping agent, 1, 3-diphenylisobenzofuran, had no effect on NADPH-dependent peroxidation but strongly inhibited the peroxidation promoted by xanthine and xanthine oxidase. NADPH-dependent lipid peroxidation was also shown to be unaffected by hydroxyl radical scavengers.. The addition of catalase had no effect on NADPH-dependent lipid peroxidation, but it significantly increased the rate of malondialdehyde formation in the reaction promoted by xanthine and xanthine oxidase. The results demonstrate that NADPH-dependent lipid peroxidation is promoted by a reaction mechanism which does not involve either superoxide, singlet oxygen, HOOH, or the hydroxyl radical. It is concluded that NADPH-dependent lipid peroxidation is initiated by the reduction of Fe-3+ followed by the decomposition of hydroperoxides to generate alkoxyl radicals. The initiation reaction may involve some form of the perferryl ion or other metal ion species generated during oxidation of Fe-2+ by oxygen.

High blood acetaldehyde levels after ethanol administration. Difference between alcoholic and nonalcoholic subjects

Blood actaldehyde and ethanol levels were measured in 11 subjects, six with chronic alcholoism and five nonalcholic controls, after alcohol had been given intravenously. Despite a progressive fall in blood ethanol over a range of 54 to 33 mM/acetaldehyde did not decrease in any of the 11 subjects. The mean acetaldehyde plateau level was significantly (p less than 0.001) higher in alcoholic (42.7 plus or minus 1.2 mum) than in nonalcoholic (26.5 plus or minus 1.5 mum) subjects. When the mean blood ethanol concentration reached 24 mM,the acetaldehyde plateau ended abruptly in each subject. The ethanol concentration at which this fall of blood acetaldehyde occurred suggests desaturation of an ethanol oxidizing system other than alcohol dehydrogenase and indicates that at high ethanol blood levels, such a system contributes to ethanol oxidation. The highet acetaldehyde levels in alcholism may result from both greater activity of this system and mitochondrial damage, and could contribut to the neurologic, hepatic and cardiac complications of alcoholism.

Effect of chronic alcohol consumption on ethanol and acetaldehyde metabolism

Hepatic metabolism of ethanol to acetaldehyde by the alcohol dehydrogenase (ADH) pathway is associated with the generation of reducing equivalents as NADH. Conversely, reducing equivalents are consumed when ethanol oxidation is catalyzed by the NADPH dependent microsomal ethanol oxidizing system (MEOS). Since the major fraction of ethanol metabolism proceeds via ADH and since the oxidation of acetaldehyde also generates NADH, an excess of reducing equivalents is produced. This explains a variety of effects following acute ethanol administration, including hyperlactacidemia, hyperuricemia, enhanced lipogenesis and depressed lipid oxidation. To the extent that ethanol is oxidized by the alternate MEOS pathway, it slows the metabolism of other microsomal substrates. Following chronic ethanol consumption, adaptive microsomal changes prevail, which include enhanced ethanol and drug metabolism, and increased lipoprotein production. Eventually, injury develops with alterations of the rough endoplasmic reticulum and structural and functional abnormalities of the mitochondria.

Microsomal ethanol oxidation: activity in vitro and in vivo

Studies by several investigators have confirmed that the microsomal fraction of mammalian liver oxidizes ethanol to acetaldehyde in a reaction that requires NADPH and oxygen. Efforts to identify the enzymes involved have produced conflicting opinions of the reaction mechanism, however. Initially, the microsomal mixed function oxidase system was assumed to be capable of oxidizing ethanol in a mechanism that did not involve either alcohol dehydrogenase or catalase. Later evidence suggested that the oxidative enzyme was, in fact, catalase, a contaminant of microsomal preparations and that the mixed function oxidase system merely furnished hydrogen peroxide to the reaction. Much current research supports the latter interpretation. Other workers provide evidence that favors a system in which catalase does not participate. Attempts to define the reaction process have involved studies with catalase inhibitors, kinetic studies of the different reaction systems, and physical separation of catalase from the microsomal components. Questions of the mechanism of microsomal ethanol oxidation may prove to be purely academic, however. Efforts to prove that the system has significant in vivo activity generally have not been successful.

Pathways of ethanol metabolism in perfused rat liver

The primary pathway of hepatic ethanol metabolism involves alcohol dehydrogenase. Hydrogen generated from ethanol metabolism enters the mitochondrial space most likely as malate over a substrate shuttle mechanism, and is subsequently oxidized by the mitochondrial respiratory chain. The rate-limiting step in this overall multicompartmental process is the rate of reduced cofactor (NADH) reoxidation by the respiratory chain. Since the electron flux in the respiratory chain is controlled by the ADP supply, alcohol dehydrogenase-dependent ethanol metabolism can be activated by perturbations which circumvent the rate-limiting step, such as artificial electron acceptors, gluconeogenic precursors, and uncoupling agents. Moreover, an ATP utilizing process is responsible for the stimulation of ethanol metabilism observed following chronic pretreatment with ethanol. In perfused rat liver catalase also participates in ethanol metabolism to a lesser extent than alcohol dehydrogenase. Quantitative assessments indicate that the predominant ethanol oxidase at low ethanol concentrations (less than 20 mM) is a alcohol dehydrogenase; however, at higher ethanol concentrations, a significant portion of total ethanol metabolism (up to 50%) is mediated by catalase-hydrogen peroxide complex. This pathway is limited by the rate of generation of hydrogen peroxide in the hepatocyte, and can be stimulated with substrates for intraperoxisomal hydrogen peroxide generation such as glycolate, urate and D-amino acids. Considerable evidence implicates catalase-hydrogen peroxide complex in the mechanism of NADPH-dependent microsomal ethanol oxidation.

The characteristics of the "peroxidatic" reaction of catalase in ethanol oxidation

Ethanol oxidation by rat liver catalase (the ;peroxidatic' reaction) was studied quantitatively with respect to the rate of H(2)O(2) generation, catalase haem concentration, ethanol concentration and the steady-state concentration of the catalase-H(2)O(2) intermediate (Compound I). At a low ratio of H(2)O(2)-generation rate to catalase haem concentration, the rate of ethanol oxidation was independent of the catalase haem concentration. The magnitude of the inhibition of ethanol oxidation by cyanide was not paralleled by the formation of the catalase-cyanide complex and was altered greatly by varying either the ethanol concentration or the ratio of the rate of H(2)O(2) generation to catalase haem concentration. The ethanol concentration producing a half-maximal activity was also dependent on the ratio of the H(2)O(2)-generation rate to catalase haem concentration. These phenomena are explained by changes in the proportion of the ;catalatic' and ;peroxidatic' reactions in the overall H(2)O(2)-decomposition reaction. There was a correlation between the proportion of the ;peroxidatic' reaction in the overall catalase reaction and the steady-state concentration of the catalase-H(2)O(2) intermediate. Regardless of the concentration of ethanol and the rate of H(2)O(2) generation, a half-saturation of the steady state of the catalase-H(2)O(2) intermediate indicated that about 45% of the H(2)O(2) was being utilized by the ethanol-oxidation reaction. The results reported show that the experimental results in the study on the ;microsomal ethanol-oxidation system' may be reinterpreted and the catalase ;peroxidatic' reaction provides a quantitative explanation for the activity hitherto attributed to the ;microsomal ethanol-oxidation system'.