Formation of 20-oxoleukotriene B4 by an alcohol dehydrogenase isolated from human neutrophils

A new view of alcohol metabolism and alcoholism--role of the high-Km Class III alcohol dehydrogenase (ADH3)

The conventional view is that alcohol metabolism is carried out by ADH1 (Class I) in the liver. However, it has been suggested that another pathway plays an important role in alcohol metabolism, especially when the level of blood ethanol is high or when drinking is chronic. Over the past three decades, vigorous attempts to identify the enzyme responsible for the non-ADH1 pathway have focused on the microsomal ethanol oxidizing system (MEOS) and catalase, but have failed to clarify their roles in systemic alcohol metabolism. Recently, using ADH3-null mutant mice, we demonstrated that ADH3 (Class III), which has a high K(m) and is a ubiquitous enzyme of ancient origin, contributes to systemic alcohol metabolism in a dose-dependent manner, thereby diminishing acute alcohol intoxication. Although the activity of ADH3 toward ethanol is usually low in vitro due to its very high K(m), the catalytic efficiency (k(cat)/K(m)) is markedly enhanced when the solution hydrophobicity of the reaction medium increases. Activation of ADH3 by increasing hydrophobicity should also occur in liver cells; a cytoplasmic solution of mouse liver cells was shown to be much more hydrophobic than a buffer solution when using Nile red as a hydrophobicity probe. When various doses of ethanol are administered to mice, liver ADH3 activity is dynamically regulated through induction or kinetic activation, while ADH1 activity is markedly lower at high doses (3-5 g/kg). These data suggest that ADH3 plays a dynamic role in alcohol metabolism, either collaborating with ADH1 or compensating for the reduced role of ADH1. A complex two-ADH model that ascribes total liver ADH activity to both ADH1 and ADH3 explains the dose-dependent changes in the pharmacokinetic parameters (beta, CL(T), AUC) of blood ethanol very well, suggesting that alcohol metabolism in mice is primarily governed by these two ADHs. In patients with alcoholic liver disease, liver ADH3 activity increases, while ADH1 activity decreases, as alcohol intake increases. Furthermore, ADH3 is induced in damaged cells that have greater hydrophobicity, whereas ADH1 activity is lower when there is severe liver disease. These data suggest that chronic binge drinking and the resulting liver disease shifts the key enzyme in alcohol metabolism from low-K(m) ADH1 to high-K(m) ADH3, thereby reducing the rate of alcohol metabolism. The interdependent increase in the ADH3/ADH1 activity ratio and AUC may be a factor in the development of alcoholic liver disease. However, the adaptive increase in ADH3 sustains alcohol metabolism, even in patients with alcoholic liver cirrhosis, which makes it possible for them to drink themselves to death. Thus, the regulation of ADH3 activity may be important in preventing alcoholism development.

The Janus face of alcohol dehydrogenase 3

Many carbonyl metabolizing enzymes are equally involved in xenobiotic and endogenous metabolism, but few have been investigated in terms of substrate competition and interference between different cellular pathways. Mammalian alcohol dehydrogenase 3 (ADH3) represents the key enzyme in the formaldehyde detoxification pathway by oxidation of S-hydroxymethylglutathione [HMGSH; the glutathione (GSH) adduct of formaldehyde]. In addition, several studies have established ADH3 as S-nitrosoglutathione (GSNO) reductase in endogenous NO homeostasis during the last decade. GSNO depletion associates with various diseases including asthma, and evidence for a causal relationship between ADH3 and asthma pathology has been put forward. In a recent study, we showed that ADH3-mediated alcohol oxidation, including HMGSH oxidation, is accelerated in presence of GSNO which is concurrently reduced under immediate cofactor recycling [C.A. Staab, J. Alander, M. Brandt, J. Lengqvist, R. Morgenstern, R.C. Grafström, J.-O. Höög, Reduction of S-nitrosoglutathione by alcohol dehydrogenase 3 is facilitated by substrate alcohols via direct cofactor recycling and leads to GSH-controlled formation of glutathione transferase inhibitors, Biochem. J. 413 (2008) 493-504]. Thus, considering the usually low cytosolic free NADH/NAD(+) ratio, formaldehyde may trigger and promote GSNO reduction by enzyme-bound cofactor recycling. These findings provided evidence for formaldehyde-induced, ADH3-mediated GSNO depletion with potential direct implications for asthma. Furthermore, analysis of product formation as a function of GSH concentrations suggested that, under conditions of oxidative stress, GSNO reduction can lead to the formation of glutathione sulfinamide and its hydrolysis product glutathione sulfinic acid, both potent inhibitors of glutathione transferase activity.

Merging protein, gene and genomic data: the evolution of the MDR-ADH family

Multiple members of the MDR-ADH (MDR: Medium-chain dehydrogenases/reductases; ADH: alcohol dehydrogenase) family are found in vertebrates, although the enzymes that belong to this family have also been isolated from bacteria, yeast, plant and animal sources. Initial understanding of the physiological roles and evolution of the family relied on biochemical studies, protein alignments and protein structure comparisons. Subsequently, studies at the genetic level yielded new information: the expression pattern, exon-intron distribution, in silico-derived protein sequences and murine knockout phenotypes. More recently, genomic and EST databases have revealed new family members and the chromosomal location and position in the cluster of both the first and new forms. The data now available provide a comprehensive scenario, from which a reliable picture of the evolutionary history of this family can be made.

Induction of cytochrome CYP4F3A in all-trans-retinoic acid-treated HL60 cells

Cytochrome P-450 CYP4F3A catalyzes the inactivation of leukotriene B(4) by omega-hydroxylation, an activity of which is specifically expressed in human neutrophils. Here, we examined expression of the LTB(4) omega-hydroxylating activity during the differentiation of HL60 cells, an acute promyelocytic leukemia cell line, in the presence of various inducers. Among the inducers used, all-trans-retinoic acid (ATRA) most strongly induces the LTB(4) omega-hydroxylating activity in a dose-dependent manner. The time course of the induction of the omega-hydroxylating activity correlates well with that of the superoxide-generating activity, indicative of cell differentiation. ATRA-treated cell microsomes convert LTB(4) to its 20-hydroxyl derivative under aerobic conditions in the present of NADPH. The reaction is inhibited by carbon monoxide, an inhibitor of cytochrome P-450, and by antibodies raised against NADPH-P-450 reductase. CYP4F3A appears to be responsible for the LTB(4) omega-hydroxylase activity, based on the following observations: expression of the mRNA for CYP4F3A is observed together with the induction of LTB(4) omega-hydroxylating activity in ATRA-treated HL60 cells; and the apparent K(m) values for the omega-hydroxylation of LTB(4) and lipoxin B(4) by ATRA-treated cell microsomes are essentially the same as those of CYP4F3A in human neutrophil microsomes.

The metabolic role of human ADH3 functioning as ethanol dehydrogenase

Human class III alcohol dehydrogenase (ADH3), also known as glutathione-dependent formaldehyde dehydrogenase, exhibited non-hyperbolic kinetics with ethanol at a near physiological pH 7.5. The S(0.5) and k(cat) were determined to be 3.4+/-0.3 M and 33+/-3 min(-1), and the Hill coefficient (h) 2.21+/-0.09, indicating positive cooperativity. Strikingly, the S(0.5) for ethanol was found to be 5.4 x 10(6)-fold higher than the K(m) for S-(hydroxymethyl)glutathione, a classic substrate for the enzyme, whereas the k(cat) for the former was 41% lower than that for the latter. Isotope effects on enzyme activity suggest that hydride transfer may be rate-limiting in the oxidation of ethanol. Kinetic simulations using the experimentally determined Hill constant suggest that gastric ADH3 may highly effectively contribute to the first-pass metabolism at 0.5-3 M ethanol, an attainable range in the gastric lumen during alcohol consumption. The positive cooperativity mainly accounts for this metabolic role of ADH3.

Histological distribution of class III alcohol dehydrogenase in human brain

The distributions of class III alcohol dehydrogenase (ADH), a glutathione-dependent formaldehyde dehydrogenase, and class I ADH in the human brain were examined immunohistochemically. The most intense immunostaining of class III ADH was observed in the dendrites and cytoplasm of cerebellar Purkinje cells. Scattered cerebral cortical neurons in layers IV and V, and some hippocampal pyramidal neurons were also immunopositive. The neuronal distribution of class III ADH resembled that of the vulnerable neurons in patients with hypoxic encephalopathy, which in view of the intense staining in the Purkinje cells, raises the possibility that this enzyme contributes to the hypoxia and cerebellar degeneration suffered by chronic alcoholics. Perivascular and subependymal astrocytes, which contribute to the maintenance of the cerebral cellular milieu and isolate the brain from the systemic circulation and cerebrospinal fluid, were also class III ADH positive. As the substrates of this enzyme include intrinsic toxic formaldehyde, inflammatory intermediate of 20-hydroxy-leukoteiene B4, and possibly ethanol, the distribution of class III ADH immunostaining indicates this enzyme contributes to the defence of the brain against degenerative processes. The finding that, unlike ependymal cells, subependymal astrocytes were class III ADH positive, suggests this enzyme may be useful for differentiating astrocytes and ependymal cells.

Cellular disposition of sulphamethoxazole and its metabolites: implications for hypersensitivity

1. Bioactivation of sulphamethoxazole (SMX) to chemically-reactive metabolites and subsequent protein conjugation is thought to be involved in SMX hypersensitivity. We have therefore examined the cellular metabolism, disposition and conjugation of SMX and its metabolites in vitro. 2. Flow cytometry revealed binding of N-hydroxy (SMX-NHOH) and nitroso (SMX-NO) metabolites of SMX, but not of SMX itself, to the surface of viable white blood cells. Cellular haptenation by SMX-NO was reduced by exogenous glutathione (GSH). 3. SMX-NHOH and SMX-NO were rapidly reduced back to the parent compound by cysteine (CYS), GSH, human peripheral blood cells and plasma, suggesting that this is an important and ubiquitous bioinactivation mechanism. 4. Fluorescence HPLC showed that SMX-NHOH and SMX-NO depleted CYS and GSH in buffer, and to a lesser extent, in cells and plasma. 5. Neutrophil apoptosis and inhibition of neutrophil function were induced at lower concentrations of SMX-NHOH and SMX-NO than those inducing loss of membrane viability, with SMX having no effect. Lymphocytes were significantly (P<0.05) more sensitive to the direct cytotoxic effects of SMX-NO than neutrophils. 6. Partitioning of SMX-NHOH into red blood cells was significantly (P<0.05) lower than with the hydroxylamine of dapsone. 7. Our results suggest that the balance between oxidation of SMX to its toxic metabolites and their reduction is an important protective cellular mechanism. If an imbalance exists, haptenation of the toxic metabolites to bodily proteins including the surface of viable cells can occur, and may result in drug hypersensitivity.

omega-Oxidation of lipoxin B4 by rat liver. Identification of an omega-carboxy metabolite of lipoxin B4

Lipoxin B4 (LXB4) is metabolized to 20-hydroxy-LXB4 by rat liver microsomes. The omega-hydroxylation requires both molecular oxygen and NADPH, and is inhibited by carbon monoxide, indicating involvement of a cytochrome P-450 (P-450). This is supported by inhibition of the reaction by antibodies raised against NADPH-P-450 reductase. The P-450 appears to be the one responsible for leukotriene B4 omega-hydroxylation, because leukotriene B4 inhibits the formation of 20-hydroxy-LXB4 and LXB4 blocks the leukotriene B4 omega-hydroxylase activity in microsomes. Incubation of 20-hydroxy-LXB4 with both rat liver cytosol and NAD+ leads to formation of a more polar metabolite on high-performance liquid chromatography. The metabolite is identified as 20-carboxy-LXB4, a novel metabolite of LXB4, based on analyses by ultraviolet spectrometry and by gas chromatography/mass spectrometry. The 20-carboxy-LXB4-forming activity is localized in cytosol, with an optimal pH of 8.5. The activity is dependent on NAD+, but NADP+ can not replace NAD+. The reaction is inhibited by pyrazole and 4-methylpyrazole, inhibitors of alcohol dehydrogenase, and by substrates of the enzyme such as ethanol and 20-hydroxy-leukotriene B4. Disulfiram, an inhibitor of aldehyde dehydrogenase, also blocks the 20-carboxy-LXB4 formation. These observations suggest that both alcohol dehydrogenase and aldehyde dehydrogenase participate in the oxidation of 20-hydroxy-LXB4 to 20-carboxy-LXB4.