is probably determined by the dose and duration of alcohol consumption. The effects of excess ethanol and acetaldehyde on fatty acid and cholesterol metabolism, cocaine metabolism, adduct formation, free radical systems, and acetic acid and lactic acid production are discussed.

Fatty acid and cholesterol metabolism

Excessive alcohol intake may lead to the formation of ethanol-fatty acid compounds known as fatty acid ethyl esters. These abnormal compounds are produced by several enzymes called fatty acid ethyl ester synthases. It is hypothesized that tissues containing large amounts of these enzymes, particularly heart tissue, may be damaged by these fatty acid ethyl esters by any one of several mechanisms (Sobel and Lange 1983). For example, these compounds may compromise protein synthesis; disrupt cell membranes; and reduce the ability of the mitochondria, which house the cells' energy-producing machinery, to make energy (Laposata and Lange 1986).

Another enzyme, cholesterol esterase, can link a fatty acid to cholesterol to form fatty acid cholesterol esters for transport in the circulatory system. When this enzyme uses ethanol instead of cholesterol, the circulating cholesterol levels drop as fatty acid ethyl esters accumulate (Lange 1982). Also, through the action of an enzyme called phospholipase D (Alling et al. 1983), alcohol may chemically modify components of cell membranes called phospholipids. Despite the evidence for ethanol-induced changes in fatty acid, cholesterol, and phospholipid metabolism, the clinical importance of these changes remains uncertain.

Cocaine metabolism

The euphoric "high" associated with cocaine use is potentiated by alcohol (Masur et al. 1989) and may contribute to the common coabuse of alcohol and cocaine (Walsh et al. 1991). Ethylcocaine, which is made from cocaine and ethanol, has been found in individuals using both drugs simultaneously. This unnatural compound, which appears to be pharmacologically active, may enhance the toxicity of cocaine and alcohol used together (Hearn et al. 1991). A liver enzyme, cocaine esterase, normally inactivates cocaine, but in the presence of ethanol the enzyme induces ethylcocaine formation (Dean et al. 1991).

Excess acetaldehyde may combine with proteins to form acetaldehyde adducts that can disrupt normal protein functioning. Acetaldehyde can combine with the amino groups of proteins without the action of an enzyme (figure 6) (Tuma et al. 1987). Several adducts have been identified. For example, acetaldehyde adducts with a human liver protein with a molecular weight of 37 kilodaltons (Lin and Lumeng 1991) and the P450 IIE1 isoenzyme (Behrens et al. 1988) have also been isolated. Acetaldehyde adduct formation can be blocked by inhibiting the activity of alcohol dehydrogenase isoenzymes (Lin and Lumeng 1990), which lowers the amount of acetaldehyde available for adduct formation.

Adducts of rat membrane phospholipids have also been found, and with chronic ethanol intake, these surface-exposed adducts may initiate antibody production (Hoerner et al. 1986; Niremelä et al. 1987; Trudell et al. 1990). The antibodies may trigger immunological attack against

acetaldehyde adducts that could eventually lead to the death of cells containing the exposed adducts (Trudell et al. 1990, 1991). Such immunemediated destruction may participate in tissue injury (see chapter 8). Acetaldehyde may also form an adduct with the amino acid derivatives cysteine and glutathione (for a review see Lieber 1991b).

Acetic acid and lactic acid

Acetic acid, the product of the oxidation of ethanol by alcohol dehydrogenase and aldehyde dehydrogenase isoenzymes, is a pivotal metabolic intermediate. The metabolite can be oxidized to yield energy, carbon dioxide, and water; channeled into biosynthetic pathways; or released into the circulation. With each molecule of ethanol converted to acetic acid, two molecules of NAD* are converted to NADH (see figure 1). The relative concentrations of NADH and NAD+ have a significant bearing on whether the acetic acid will be directed into an energy-generating or biosynthesis pathway (Halperin et al. 1983). If the NADH/NAD* ratio is high, acetic acid is directed toward biosynthetic pathways, and if the ratio is low, acetic acid is channeled toward oxidative pathways.

As acetic acid accumulates it may be released into the circulation, thus making the blood more acidic. A moderate dose of ethanol can temporarily increase the circulating concentrations of acetic acid to 0.4 to 0.6 mM (Hannak et al. 1985; Lundquist 1962).

Other Metabolic Processes Affected by Ethanol Ingestion

Intake of ethanol affects not only the metabolism of ethanol and other alcohols, but also other metabolic processes. The alteration of the NADH/NAD* ratio influences protein, lipid, and vitamin metabolism; membrane composition and function; and energy production. These ethanolinduced changes may lead to malnutrition, weight loss, and cell injury among alcoholic individuals.

Protein Metabolism

Muscle contains specialized cells called type I and type II fibers, which are grouped by the type of work they can perform. For example, type II fibers provide the capacity for continuous

muscle activity such as distance running. With chronic alcohol intake, this type of muscle fiber is depleted (figure 7) (Preedy and Peters 1990a). This alteration may lead to muscle pain and weakness and eventually to muscle damage (Martin et al. 1985). The decrease in type II muscle fibers is not believed to be due to liver dysfunction (Preedy and Peters 19906), but may be the result of decreased total RNA levels (Preedy et al. 1990). The decrease appears to be an indication of reduced protein synthesis, which may also arrest normal cell growth (Cook et al. 1990).

Proteins are normally moved out of and into cells along tracks of microfilaments called tubulin (figure 8), a process called protein trafficking. Transport of substances out of cells is called exocytosis, and transport into cells is called endocytosis. Plasma proteins such as albumin, which are synthesized in the liver and transported out of it by exocytosis, accumulate in liver cells of individuals with alcoholic liver disease; the effect may be due to decreased protein trafficking in the cells (Matsuda et al. 1985). More specifically, acetaldehyde may form adducts with isolated tubulin (Smith et al. 1989), and these adducts in liver cells may hamper protein trafficking and lead to protein accumulation. Alcohol intake may also affect as many as three steps of endocytosis (Tuma et al. 1990). The hormone insulin is normally transported into liver cells by endocytosis; chronic alcohol treatment in rats appears to diminish the process (Tuma et al. 1991). Thus, excess alcohol may profoundly influence normal cell functioning through its adverse effects on protein transport systems.

Lipid Metabolism

Alcohol can affect lipid metabolism and function by several diverse mechanisms. For example, the high NADH/NAD* ratio produced by ethanol oxidation inhibits lipid degradation in humans. Carnitine, a substance essential to this degradative process, then accumulates (Orpana et al. 1990). Other unoxidized fats also accumulate, producing the clinical fatty liver characteristic of chronic drinkers. A high NADH/NAD* ratio in rats may also switch metabolism away from lipid biosynthesis (Carmona and Freedland 1989), although it is unclear whether this metabolic switch occurs with alcohol ingestion.

Synthesis of another class of lipids, steroid hormones, is inhibited by a high NADH/NAD* ratio (Orpana et al. 1990). Using isolated rat liver cells, researchers have demonstrated that at least

two steps in testosterone synthesis, the conversion of pregnenolone to progesterone and the metabolism of 17-hydroxy-progesterone to androstenedione, are inhibited by the high NADH/NAD* ratio (Akane et al. 1988). Further research is needed to determine whether these pathways are similarly inhibited by alcohol in humans and whether they are responsible for the hypogonadism seen in some alcoholic men.

An enzyme complex responsible for cleaving off long acyl chains from fat molecules, lipoprotein lipase, also appears to be impaired by chronic ethanol intake in rats (Sévilla et al. 1991). The enzyme complex, found in heart tissue, is believed to produce the acyl chains for energy use by the heart. The mechanism of impairment is unknown, but a loss in the enzyme activity may contribute to heart damage.

The class of lipids known as lipoproteins is affected by alcohol intake. The compounds are complexes of lipids and proteins that function as carriers of lipids, including cholesterol, in the blood. Lipoproteins are generally classified as either high or low density. Low-density lipoproteins (LDLs) are considered "bad" because of their association with increased risk for cardiovascular disease; in contrast, high levels of the "good," high-density lipoproteins (HDLs) appear to reduce risk. Alcohol intake leads to an increase in both types of lipoproteins in rats (Lin et al. 1989). The increase in HDLS may lower the incidence of atherosclerosis among moderate drinkers and alcoholics. (See chapter 8.)

Membrane Composition and Function

Lipids are essential components of membranes, and membranes are affected by alcohol both directly and indirectly. A direct effect of alcohol on membrane lipids may be observed following its entrance into cell membranes, where it disrupts the structural arrangement of lipids, thus making the membranes more fluid (Chin and Goldstein 1981; Hitzemann et al. 1991; Nie et al. 1989). Indirectly, chronic alcohol intake may also affect the lipid species found within. the membrane (Hoch 1992). For example, the relative amounts of two types of lipids, phosphatidyl inositol and phosphatidyl choline, were found to decrease, whereas other lipids, such as sphingomyelin and cholesterol, increased in the brain stem of the rat after alcohol consumption (Lalitha et al. 1990). The phosphatidyl inositol composition of liver cell membranes may also

be altered by alcohol administration (Ellingson et al. 1991). These adaptive changes in membrane lipid composition may confer resistance (tolerance) to the fluidizing effect of alcohol on the membrane (Rottenberg et al. 1992). Other changes in fatty acid species of membranes have been observed, including decreases in arachidonic acid and increases in linoleic acid (for a review see Hoch 1992).

Hormones and other substances circulating outside cells normally rely on a form of communication called signal transduction to coordinate the activity of individual cells with body functions. Signal transduction occurs through membranes, and the membrane compositional changes seen with ethanol intake may influence this and other membrane-dependent processes. Mechanisms of signal transduction include activation of the enzymes phospholipase C and phospholipase D. Ethanol may mimic hormones to activate phospholipase C or phospholipase D (Hoek and Rubin 1990; Rooney et al. 1989). The effect of ethanol on phospholipase C is observed in isolated human platelets, in which ethanol activates phospholipase C in the membrane to promote platelet aggregation (Rubin and Hoek 1990). The degree to which platelet function in the body is affected by chronic alcohol use is uncertain (Mikhailidis et al. 1990; Veenstra et al. 1990).

Proper electrolyte balance in cells is maintained by Na*/K* pumps. These ion transporters are in membranes, and the number of pumps decreases with ethanol intake in a way that may cause cells to accumulate water and swell (McCall et al. 1989).

Vitamin Metabolism

Vitamins are essential cofactors in enzymatic reactions, and many are metabolized in the liver. Enzymes are produced in the liver either to activate or to inactivate vitamin metabolites. These processes may be profoundly affected by chronic alcohol intake, leading to vitamin deficiencies, malnutrition, and impairment of cell functions. Even well-fed alcoholics may be deficient in vitamins B1 (thiamine), A, folate, D, B6, and E, in part because ethanol interferes with the absorption of these nutrients from the gastrointestinal tract, and in part because ethanol disrupts vitamin metabolism (Green and Tall 1979). Thiamine, or vitamin B1, is actively transported across the rat intestinal lining (Lumeng et al. 1979) and is subsequently metabolized in

the liver by three enzymes. Thiamine pyrophosphokinase links a pyrophosphate group on thiamine to produce the active thiamine pyrophosphate cofactor needed for normal metabolism. The cofactor is inactivated by two other enzymes that increase in humans and rats after chronic alcohol feeding, whereas the thiamine pyrophosphokinase decreases (Laforenza et al. 1990; Rindi et al. 1991). The result is a net decrease in thiamine pyrophosphate (Poupon et al. 1990). The activity of other enzymes requiring thiamine pyrophosphate is inhibited when alcohol depletes this cofactor (Rooprai et al. 1990).

Retinol, or vitamin A, is also metabolized in the liver. It is synthesized from beta-carotene and further processed to the active retinal form, which is required for eyesight. Retinol is oxidized to retinal in the cytosol by retinol dehydrogenase, which is believed to be identical to alcohol dehydrogenases (Mezey and Holt 1971). Other enzymes oxidize vitamin A (Posch et al. 1989), including a microsomal enzyme that has been purified from rat liver (Shih and Hill 1991). The cytosolic and the microsomal enzymes are both inhibited by acute ethanol intake. Low amounts of retinal may lead to night blindness among alcoholics (Leo and Lieber 1983). Inhibition of retinol oxidation may also decrease the number of lipocytes, special liver cells that store excess vitamin A (Tanaka et al. 1991).

The product of retinal oxidation, retinoic acid, may function as a messenger to promote the production of alcohol dehydrogenase isoenzymes (Duester et al. 1991). The substance is also important to a developing fetus for proper skeletal, muscular, and central nervous system development. By inhibiting retinol oxidation, ethanol may also impede further oxidation of retinal to retinoic acid. The depletion of retinoic acid may have serious consequences for cell functioning and fetal development (Pullarkat 1991).

Chronic or acute alcohol ingestion alters other vitamin metabolic pathways. Folate metabolism in rats is perturbed by ethanol, which causes increased urinary excretion of several folate metabolites (Eisenga et al. 1989; McMartin et al. 1989). Disturbances in the metabolism of vitamin D (Rico 1990), vitamin E (Meydani et al. 1991), and vitamin B6 (Lumeng and Li 1974) have also been reported. These effects could contribute to nutritional deficiencies among heavy drinkers.

Energy Production

Individuals who chronically drink alcohol may lose weight despite being well fed; weight loss among alcoholic women appears to be greater than among alcoholic men, but the reason is not clear (Colditz et al. 1991). The dramatic weight loss exhibited by alcoholics indicates that more energy is expended in these individuals than can be obtained from ethanol as a food source. The mechanisms underlying this net energy loss have not been clearly defined and may involve several events.

In rat liver mitochondria, components of the oxidative phosphorylation system are impaired by chronic ethanol intake (Cunningham et al. 1990). The oxidative phosphorylation system, housed in the mitochondria, normally produces energy for the cell. Impairment of this system, through uncoupling of oxidative phosphorylation, may reduce the efficiency of energy production.

Brown adipose tissue is a type of fat tissue that helps maintain the temperature of circulating blood. The tissue, which lines major arteries, normally displays uncoupled oxidative phosphorylation to generate heat without energy production. This type of fat tissue accumulates around the major arteries of individuals who work for long periods in cold climates. Chronic ethanol ingestion in rats induces formation of brown adipose tissue (Huttunen and Kortelainen 1988), and alcoholic individuals also appear to accumulate brown adipose tissue around major arteries (Huttunen and Kortelainen 1990). The heat generated through uncoupled oxidative phosphorylation in this tissue may lead to a net energy loss.

Adenosine and Uric Acid

As was noted earlier, the eventual product of ethanol metabolism through the enzymes ADH and ALDH is acetic acid, and a significant elevation of the level of acetic acid may occur after the consumption of alcohol (Lundquist 1962). To be used as an energy source, acetic acid must first combine chemically with one of the cell's coenzymes, known as coenzyme A, to form acetyl coenzyme A. The synthesis of acetyl coenzyme A from acetic acid requires energy, which is derived from the cell's immediate energy source, adenosine triphosphate (ATP). This process can lead to the generation of adenosine (a metabolic product of ATP) (Puig and Fox 1984). Though the significance of this pathway is yet unknown, it is possible that adenosine can cause some of

ethanol's effects on various physiological processes. Adenosine can produce some effects similar to those seen with alcohol use, including vasodilation (Liang and Lowenstein 1978), desensitization of cell receptors (Nagy et al. 1989; see Chapter 5, Neurobehavioral Aspects of Alcohol Consumption), and impairment of brain areas controlling motor coordination (Carmichael et al. 1991). The pathway could also explain the increase in the metabolic end product uric acid that is observed with alcohol consumption (Puig and Fox 1984).

Individuals who chronically drink alcohol may lose weight despite being well fed; weight loss among alcoholic women appears to be greater than among alcoholic men, but the reason is not clear.

Drug Metabolism

The cytochrome P450 IIE1 isoenzyme, which is increased by chronic alcohol use, has oxidizing activity toward not only ethanol (Lasker et al. 1987; Wrighton et al. 1987) but also other substances (Lieber 1991b). For instance, acetaminophen, a nonaspirin analgesic, is oxidized by the P450 IIE1 isoenzyme, particularly after chronic ethanol ingestion (Sato et al. 1981). Another cytochrome P450 isoenzyme may act on cocaine (Boelsterli et al. 1991). The oxidative products of P450 IIE1 may be even more toxic than the original substances after ethanol ingestion, and they may enhance liver injury (Lieber 1990).

Summary

The term alcohol refers to compounds that have a hydroxyl group, that is, an oxygen and a hydrogen (-OH) bonded to a carbon molecule. The primary enzymes that metabolize alcohols, the alcohol dehydrogenases, are found in a variety of living organisms, including humans.

Beverage alcohol, which is known chemically as ethanol, is absorbed from the stomach and intestinal tract and transported to the liver for metabolism; it also may be metabolized in the stomach. The first step in metabolism of ethanol involves its oxidation to acetaldehyde by alcohol dehydrogenase. The stomach and liver contain distinctly different forms of alcohol dehydrogenase, called isoenzymes. The number of