Characteristics of Aldehyde Dehydrogenases of Certain Aerobic Bacteria Representing Human Colonic Flora

We have proposed the existence of a bacteriocolonic pathway for ethanol oxidation resulting in high intracolonic levels of toxic and carcinogenic acetaldehyde. This study was aimed at determining the ability of the aldehyde dehydrogenases (ALDH) of aerobic bacteria representing human colonic flora to metabolize intracolonically derived acetaldehyde. The apparent Michaelis constant (Km) values for acetaldehyde were determined in crude extracts of five aerobic bacterial strains, alcohol dehydrogenase (ADH) and ALDH activities of these bacteria at conditions prevailing in the human large intestine after moderate drinking were then compared. The effect of cyanamide, a potent inhibitor of mammalian ALDH, on bacterial ALDH activity was also studied. The apparent Km for acetaldehyde varied from 6.8 (NADP-linked ALDH of Escherichia coli IH 13369) to 205 uM (NAD-linked ALDH of Pseudomonas aeruginosa IH 35342), and maximal velocity varied from 6nmol/min/mg (NADlinked ALDH of Klebsiella pneumoniae IH 35385) to 39 nmol/min/mg (NAD-linked ALDH of Pseudomonas aeruginosa IH 35342). At pH 7.4, and at ethanol and acetaldehyde concentrations that may be prevalent in the human colon after moderate drinking, ADH activity in four out of five bacterial strains were 10-50 times higher than their ALDH activity. Cyanamide inhibited only NAD-linked ALDH activity of Pseudomonas aeruginosa IH 35342 at concentrations starting from 0.1 mM. We conclude that ALDHs of the colonic aerobic bacteria are able to metabolize endogenic acetaldehyde. However, the ability of ALDHs to metabolize intracolonic acetaldehyde levels associated with alcohol drinking is rather low. Large differences between ADH and ALDH activities of the bacteria found in this study may contribute to the accumulation of acetaldehyde in the large intestine after moderate drinking. ALDH activities of colonic bacteria were poorly inhibited by cyanamide. This study supports the crucial role of intestinal bacteria in the accumulation of intracolonic acetaldehyde after drinking alcohol. Individual variations in human colonic flora may contribute to the risk of alcohol-related gastrointestinal morbidity. INTRODUCTION however, are not properly understood. Under anaerobic conditions, bacteria are capaExcessive alcohol consumption is frequently ble of producing energy through fermentation associated with flatulence and diarrhoea (Fields (Zeikus, 1980). The end product of alcoholic et al., 1994). Furthermore, marked pathological fermentation is ethanol, which is derived from changes have been observed in the rectal mucosa acetaldehyde in a reductive reaction mediated by of heavy drinkers (Brozinsky et al., 1978). A bacterial alcohol dehydrogenase (ADH) (Neale et positive association between alcohol intake and al., 1986). Where there is an excess of ethanol, the the development of colorectal polyps and cancer reaction catalysed by microbial ADH can run in has been found in epidemiological studies (Pollak the opposite direction with acetaldehyde as an end et al., 1984; Cope et al., 1991; Kune and Vitetta, product. Accordingly, the incubation of human 1992; Blot, 1992; Giovannucci and Willett, 1994; colonic contents with increasing ethanol concenKearney et al., 1995). The mechanisms respontrations in vitro results in a marked accumulation sible for this alcohol-related intestinal morbidity, of acetaldehyde (Jokelainen et al., 1994). Variable ADH activity and acetaldehyde•Author to whom correspondence should be addressed at: P^ducing Capacity have been found in vitro Research Unit of Alcohol Diseases, University of Helsinki, among the aerobic bacteria representing the Tukholmankatu 8 F, 00290 Helsinki, Finland. normal human colonic flora. The highly signifi-