A Caucasian woman, height 1.60 m and body weight 65 kg, noticed that an alcohol test with a breatha

Fråga: A Caucasian woman, height 1.60 m and body weight 65 kg, noticed that an alcohol test with a breathalyser, performed by her, was still positive 12 hours after the intake of two glasses of wine. She wondered whether this could be due to a deviation in her metabolism of alcohol and whether her genotype could be tested. Further details, for example concerning the exact results of the alcohol test or whether she experiences adverse reactions after intake of alcohol, were not given.

Sammanfattning: Differences in the pharmacokinetics of ethanol elimination are in part genetically determined. There are polymorphic variants of the two main enzymes responsible for ethanol oxidation in the liver, alcohol dehydrogenase and aldehyde dehydrogenase. The most well-known example is the deficient ALDH2 phenotype which is extremely rare among Caucasians. It seems also likely that polymorphisms in ADH isoenzymes contribute to the variability in ethanol elimination. However, it is difficult to estimate to what extent this could have contributed to the detectable ethanol levels in the questioner, 12 hours after intake since the blood ethanol concentration was not known and also since the elimination rate of ethanol is determined by many other factors, such as other enzyme systems, history of alcohol intake, diet, concomitant use of drugs, etc. If considered relevant, it is possible to genotype the ethanol oxidizing enzymes by specific oligonucleotide probes. However, this is at the moment not readily available clinically.

Svar: It is well known that genetic differences among individuals in their capacity to metabolise ingested alcohol are at least partly responsible for the large interindividual and inter-ethnic variations observed after alcohol intake. However, the rate of alcohol elimination is also influenced by other factors.

After oral intake, ethanol is readily absorbed from the gastrointestinal tract and diffuses rapidly and uniformly throughout the total body water (1). Only small amounts of ethanol are excreted unchanged in expired air (5 per cent), urine (0.5-2 per cent) and sweat (0.5 per cent), the majority being completely oxidised (2). The total amount of ethanol eliminated by the human body per hour ranges from 100 to 300 mg/kg, which is equivalent to 6-9 g of ethanol per hour for a healthy subject with an average body weight. Since two glasses of wine contain about 24 g of ethanol, this would therefore take between 2.5 and 4 hours to eliminate.

The major pathway for the disposition of ethanol is its oxidation in the liver into acetaldehyde, a toxic metabolite, by way of two major enzyme systems (1): the cytosolic alcohol dehydrogenase (ADH), responsible for 80-85 per cent of ethanol metabolism, and the microsomal ethanol oxidation system (MEOS) in the endoplasmatic reticulum, responsible for 10-15 per cent of the metabolism. Acetaldehyde is further oxidised into acetate, which is then converted to carbon dioxide and other compounds. The fact that the blood acetaldehyde concentration is maintained about 1000-fold lower than the blood ethanol concentration in most individuals indicates that the ADH-catalysed step is rate limiting (3).

The elimination process obeys Michaelis-Menten kinetics (3), but the initial rate is nearly linear, due to the saturation of the oxidizing enzymes during the initial phase. There is an approximately 3-fold variation in this linear elimination rate among individuals and about half of this variability is genetically determined. The MEOS system is induced after chronic alcohol intake, and its contribution to ethanol metabolism is probably substantial under these circumstances (3).

An important first-pass effect of ethanol is due to local oxidation by the gastric mucosa, which also expresses the ADH enzyme. Gastric ADH activity is reported to be less in women, leading to a higher bioavailability of ethanol in women compared with men (1). Gastric ADH activity is reduced by chronic alcohol intake.

The pharmacokinetics of ethanol can be influenced by several drugs (4). For example, cimetidine, ranitidine and acetylsalicylic acid have been reported to inhibit gastric ADH activity. In one study (5), the AUC after intake of a small dose of ethanol doubled after pretreatment with 400 mg of cimetidine for 7 days. Disulfiram is well known for its inhibitory effect on the metabolism of acetaldehyde.

Genetically determined isoenzymes of both ADH and ALDH have been identified, which exhibit substantially different kinetic properties. This is a probable contributing factor to individual differences in the rate of alcohol metabolism.

About 20 different forms of human alcohol dehydrogenase have been identified, which have been divided into three classes, with recent evidence for two more (6). The so-called class I ADHs, the best studied, have the highest affinity for ethanol and are responsible for most ethanol oxidation. These enzymes are composed of homo- and heterodimers of three closely related 40 kDa subunits, alpha, beta and gamma; for example, alpha-alpha, alpha-beta, beta-beta, etc. The genes encoding for these class I subunits, ADH1, ADH2 and ADH3, are expressed at high levels in the liver and also to a lower extent in other tissues, such as the stomach (6). Genetic polymorphism exists at two loci, ADH2 and ADH3. Three different subunits (beta1, beta2 and beta3) are encoded by ADH2(1), ADH2(2) and ADH2(3) respectively, and two different subunits (gamma1 and gamma2) correspond to ADH3(1) and ADH3(2). There is a profound variation in kinetic constants in these isoenzymes (3). For example beta2-beta2 is about 20 times more active in ethanol oxidation than beta1-beta1. This may account for the greater elimination rate of ethanol into acetaldehyde observed in Orientals as compared with Caucasians, since the frequency of the variant ADH2(2) allele is about 80 per cent in Orientals compared with about only 10 per cent in Caucasians (2,3).

The major human liver ALDH isoenzymes are the cytosolic ALDH1 and the mitochondrial ALDH2, of which ALDH2 is the most important. A widely prevalent genetic polymorphism has been observed for the ALDH2 isoenzyme (2,3). About 50 per cent of Japanese and Chinese lack the active form of ALDH2, whereas this is extremely rare in Caucasians. Individuals with the deficient ALDH2 phenotype do not have altered ethanol elimination rates, but they do exhibit high blood acetaldehyde levels and dysphoric symptoms, such as facial flushing, nausea and tachycardia after drinking alcohol (3).

Inherited defects in the ALDH1 isoenzyme in Caucasian subjects who reported alcohol-related flushing (about 5 per cent) have also been described (7).

Direct genotyping is now possible by amplifying leukocyte DNA in vitro, using the polymerase chain reaction and distinguishing the different alleles by use of specific oligonucleotide probes (3,8). However, this is not readily available as a clinical test (8). 1 Drugline nr 07362 (year 1990) 2 Agarwal DP, Goedde HW: Pharmacogenetics of alcohol metabolism and alcoholism. Pharmacogenetics 1992; 2: 48-62 3 Bosron WF, Lumeng L, Li T-K: Genetic polymorphism of enzymes of alcohol metabolism and susceptibility to alcoholic liver disease. Molec Aspects Med 1988; 10: 147-158

4 FASS: Drugs and alcohol. 1993; page 1074-1076
5 Hansten & Horn, Drug interactions and updates. page 358
6 Edenberg HJ, Brown CJ: Regulation of human alcohol dehydrogenase genes. Pharmacogenetics 1992; 2: 185-196
7 Yoshida A: Molecular genetics of human aldehyde dehydrogenase. Pharmacogenetics 1992; 2: 129-147

8 Professor Hans Jörnvall, Department of Medical Chemistry, Karolinska Institute, personal communication