Bradford BU; Rusyn I. Swift increase in alcohol metabolism (SIAM): Understanding the phenomenon of hypermetabolism in liver. Alcohol 35(1): 13-17, 2005. (35 refs.)
The swift increase in alcohol metabolism (SIAM) is a phenomenon defined as a rapid increase in hepatic respiration and alcohol metabolism after administration of a bolus dose of alcohol. Continuous exposure to alcohol is known to produce adaptive changes in liver alcohol and oxygen metabolism. A considerable burst of hepatic respiration can also occur after administration of a single large dose of alcohol and results in a near doubling of alcohol metabolism, a high demand for oxygen, and downstream or pericentral hypoxia. These dramatic changes in rates of alcohol metabolism and tissue concentrations of oxygen are not due to induced enzyme activity in liver. This phenomenon depends on activation of mitochondrial function, an increase in co-factor supply for nicotinamide adenine dinucleotide-dependent alcohol metabolism, depletion of glycogen reserves, liberation of fatty acids through activation of an adrenergic response to alcohol providing substrate for catalase, and activation of Kupffer cells, the hepatic resident macrophages responsible for production of cytokines and prostaglandins. An understanding of the mechanisms of hypermetabolism in liver can have vital ramifications for knowledge of both alcohol-related and alcohol-unrelated liver injury because hypoxia that is a result of hypermetabolism can compound effects of pharmaceuticals and environmental agents on the liver. Swift increase in alcohol metabolism is an excellent example of the complexity of cell-cell interactions in liver and extrahepatic regulation of biochemical and molecular events in this organ, and this important phenomenon shall be considered in studies of liver disease and biochemistry.
Copyright 2005, Elsevier Science
Chai YG; Oh DY; Chung EK; Kim GS; Kim L; Lee YS et al. Alcohol and aldehyde dehydrogenase polymorphisms in men with type I and type II alcoholism. American Journal of Psychiatry 162(5): 1003+, 2005. (7 refs.)
Objective: The authors examined the genetic polymorphisms of alcohol dehydrogenase 2 and 3 (ADH2 and ADH3) and aldehyde dehydrogenase (ALDH2) in patients diagnosed as having Cloninger's type I or type II alcoholism. Method: Seventy-two alcoholic men and 38 nonalcoholic, healthy men were tested for the distribution of genotypes and alleles of ADH2, ADH3, and ALDH2. Forty-eight of the alcoholic men had type I alcoholism, and 24 had type II alcoholism. Results: The frequencies of ADH2* 1 and ADH3* 2 alleles were significantly higher in men with type II alcoholism than in men with type I alcoholism and healthy men. The frequency of the ALDH2* 1 allele was significantly higher in men with alcohol dependence than in healthy men. Conclusions: The genetic characteristics of alcohol dehydrogenases in men with type I alcoholism were similar to those of healthy men, and the genetic characteristics of aldehyde dehydrogenase in men with type I alcoholism were similar to those of men with type II alcoholism. These findings suggest that the genetic characteristics of alcohol metabolism in type I alcoholism fall between nonalcoholism and type II alcoholism.
Copyright 2005, American Psychiatric Association. Used with permission
Lieber CS. The discovery of the microsomal ethanol oxidizing system and its physiologic and pathologic role. (review). Drug Metabolism Reviews 36(3-4): 511-529, 2004. (117 refs.)
Oxidation of ethanol via alcohol dehydrogenase (ADH) explains various metabolic effects of ethanol but does not account for the tolerance. This fact, as well as the discovery of the proliferation of the smooth endoplasmic reticulum (SER) after chronic alcohol consumption, suggested the existence of an additional pathway which was then described by Lieber and DeCarli, namely the microsomal ethanol oxidizing system (MEOS), involving cytochrome P450. The existence of this system was initially challenged but the effect of ethanol on liver microsomes was confirmed by Remmer and his group. After chronic ethanol consumption, the activity of the MEOS increases, with an associated rise in cytochrome P450, especially CYP2E1, most conclusively shown in alcohol dehydrogenase negative deer mice. There is also cross-induction of the metabolism of other drugs, resulting in drug tolerance. Furthermore, the conversion of hepatotoxic agents to toxic metabolites increases, which explains the enhanced susceptibility of alcoholics to the adverse effects of various xenobiotics, including industrial solvents. CYP2E1 also activates some commonly used drugs (such as acetaminophen) to their toxic metabolites, and promotes carcinogenesis. In addition, catabolism of retinol is accelerated resulting in its depletion. Contrasting with the stimulating effects of chronic consumption, acute ethanol intake inhibits the metabolism of other drugs. Moreover, metabolism by CYP2E1 results in a significant release of free radicals which, in turn, diminishes reduced glutathione (GSH) and other defense systems against oxidative stress which plays a major pathogenic role in alcoholic liver disease. CYP1A2 and CYP3A4, two other perivenular P450s, also sustain the metabolism of ethanol, thereby contributing to MEOS activity and possibly liver injury. CYP2E1 has also a physiologic role which comprises gluconeogenesis from ketones, oxidation of fatty acids, and detoxification of xenobiotics other than ethanol. Excess of these physiological substrates (such as seen in obesity and diabetes) also leads to CYP2E1 induction and nonalcoholic fatty liver disease (NAFLD), which includes nonalcoholic fatty liver and nonalcoholic steatohepatitis (NASH), with pathological lesions similar to those observed in alcoholic steatohepatitis. Increases of CYP2E1 and its mRNA prevail in the perivenular zone, the area of maximal liver damage. CYP2E1 up-regulation was also demonstrated in obese patients as well as in rat models of obesity and NASH. Furthermore, NASH is increasingly recognized as a precursor to more severe liver disease, sometimes evolving into "cryptogenic" cirrhosis. The prevalence of NAFLD averages 20% and that of NASH 2% to 3% in the general population, making these conditions the most common liver diseases in the United States. Considering the pathogenic role that up-regulation of CYP2E1 also plays in alcoholic liver disease (vide supra), it is apparent that a major therapeutic challenge is now to find a way to control this toxic process. CYP2E1 inhibitors oppose alcohol-induced liver damage, but heretofore available compounds are too toxic for clinical use. Recently, however, polyenylphosphatidylcholine (PPC), an innocuous mixture of polyunsaturated phosphatidylcholines extracted from soybeans (and its active component dilinoleoylphosphatidylcholine), were discovered to decrease CYP2E1 activity. PPC also opposes hepatic oxidative stress and fibrosis. It is now being tested clinically.
Copyright 2004, Marcel Dekker Inc.