Hepatocytes possess three pathways for ethanol metabolism situated in different subcellular compartments [Figure 1]. The oxidation of ethanol (low ethanol concentration) to acetaldehyde by ADH is the major pathway resulting in increased the formation of NADH. The rate of NADH production exceeds its rate of oxidation leading to an increase in NADH/NAD + ratio [Figure 1]: Step 1], thereby, resulting in reduction of hepatic gluconeogenesis, the decrease in citric acid cycle activity and impairment of fatty acid oxidation.
Figure 1: Alcohol metabolism and its subsequent effects
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However, ethanol oxidation at high concentration is brought about by the cytochrome P 450 (cyt P 4502E1 ) dependent microsomal ethanol oxidizing system (MEOS). After chronic alcohol consumption there is an increase in the activity of MEOS which involves specific cytochrome P 450 (CYP 4502FI ). [21] The third pathway for ethanol metabolism utilizes catalase that is capable of oxidizing ethanol in the presence of hydrogen peroxide (H 2 O 2 ). This system involves the oxidation of hypoxanthine and NADPH by xanthine oxidase and NADPH oxidase, respectively, and makes H 2 O 2 available for oxidation of alcohol. Regardless of the pathway, acetaldehyde is the first major specific oxidation product of ethanol and under normal conditions, acetaldehyde itself is oxidized rapidly by aldehyde dehydrogenase [22] to acetate [Figure 1]: Step 2]. Increased blood acetate in the presence of ethanol indicates metabolic tolerance to alcohol and can be used as a laboratory marker of alcoholism and heavy drinking.