Cellular and Enzymatic Basis for Carnitine-Mediated Attenuation of Ethanol Metabolism

(From the Introduction) Alcohol (ethanol) is the most widely used of psychoactive drug and has been known in almost all civilizations since ancient times (Keller 1979). There are millions of people throughout the world who responsibly enjoy consuming alcoholic beverages served as food, medicines, and euphoriants. However, alcohol abuse, alcohol intolerance, alcohol dependence and other alcohol-related disabilities are some of the most challenging social, economical and public health problems facing our modern-day society. In the United states alone, the annual cost of lost productivitity and health expenses related to alcohol is estimated to be $117 billion (Hoek et al. 1992). Ethanol is known to cause a wide variety of metabolic abnormalities. Some of these abnormalities are the direct effect of ethanol while others are the result of ethanol induced malnutrition (Lieber 1991). Ethanol administration both in humans and laboratory animals, results in hyperlipidemia, fatty liver, and ultimately the most severe stage of alcoholic liver disease: scarring or cirrhosis of the liver. It has been recognized that the metabolism of ethanol plays a major pathogenic role in the etiology of various diseases associated with alcohol abuse. Approximately 90% of ingested ethanol is metabolized in the liver. The liver enzyme systems able to metabolize ethanol are; alcohol dehydrogenase (ADH), the microsomal ethanol oxidizing system (MEOS), and catalase (Larsen 195.9). Carnitine (3-hydroxy-4N-trimet hylammonium butyrate), an essential cofactor for facilitating fatty acid transport into the mitochondrial matrix is synthesized in all mammals, and consequently is present in diets containing meat products (Bieber 1988). Sachan et al. found that carnitine supplementation lowered ethanol-induced increases of various lipid fractions in rat\u27s liver (Sachan and Rhew 1983, Sachan et al. 1984) in a dose related manner (Rhew and Sachan 1986). They hypothesized a state of functional carnitine deficiency in chronic alcoholism and proposed that dietary carnitine supplementation may prevent ethanol-induced hepatic diseases. Further, they demonstrated that dietary carnitine supplementation elevated blood ethanol concentration (BEC) following both acute and chronic doses of ethanol and the increase in BEC was dependent on the level of carnitine supplementation (Berger and Sachan 1986, Sachan and Berger 1987, Berger and Sachan 1991, Sachan 1992). This effect of carnitine is specific to carnitine since it could not be reproduced by choline (Sachan and Berger 1993). It was also shown that at 6 hours post ethanol administration on the third day of carnitine and ethanol treatment, there was significantly higher (96%) hepatic ethanol concentration (HEC) in the non-supplemented rats than in the carnitine supplemented rats without changes in activities of ethanol metabolizing systems (Mynatt and Sachan 1992, Sachan 1992). Those results suggested that carnitine may modulate enzyme systems via some type of mediator or may be affecting membrane permeability to ethanol. The main objective of the first study, therefore, was to explore mechanism of carnitine-mediated attenuation of ethanol metabolism by: 1. Determining the effects of carnitine on ethanol metabolism in isolated hepatocytes. 2. Establishing specificity of carnitine inhibition on ethanol metabolism in the hepatocytes. The results of first study showed that, acetylcarnitine, a metabolite of carnitine, was a far more potent inhibitor of ethanol oxidation in hepatocytes than was carnitine. These results were encouraging in that it explained the lag time in carnitine response in the intact animal (Sachan 1992) and the lack of inhibition of ethanol metabolizing enzymes by carnitine (Mynatt and Sachan 1992). The main purpose of the second study, therefore, was to reinvestigate mechanism of carnitine-mediated attenuation of ethanol metabolism at the enzymatic level by: 1. Determining the inhibitory effect of acetylcarnitine on ethanol metabolizing enzyme(s), ADH. 2. Investigating the mechanism of acetylcarnitgwe interaction with ADH