CHARACTERISTICS OF ALDEHYDE DEHYDROGENASES OF CERTAIN AEROBIC-BACTERIA REPRESENTING HUMAN COLONIC FLORA

We have proposed the existence of a bacteriocolonic pathway for ethano l oxidation resulting in high intracolonic levels of toxic and carcino genic acetaldehyde. This study was aimed at determining the ability of the aldehyde dehydrogenases (ALDH) of aerobic bacteria representing h uman colonic flora to metabolize intracolonically derived acetaldehyde . The apparent Michaelis constant (K-m) values for acetaldehyde were d etermined in crude extracts of five aerobic bacterial strains, alcohol dehydrogenase (ADH) and ALDH activities of these bacteria at conditio ns prevailing in the human large intestine after moderate drinking wer e then compared. The effect of cyanamide, a potent inhibitor of mammal ian ALDH, on bacterial ALDH activity was also studied. The apparent K- m for acetaldehyde varied from 6.8 (NADP(+)-linked ALDH of Escherichia coli IH 13369) to 205 mu M (NAD(+)-linked ALDH of Pseudomonas aerugin osa IH 35342), and maximal velocity varied from 6 nmol/min/mg (NAD(+)- linked ALDH of Klebsiella pneumoniae IH 33385) 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 h uman colon after moderate drinking, ADH activity in four out of five b acterial strains were 10-50 times higher than their ALDH activity. Cya namide inhibited only NAD(+)-linked ALDH activity of Pseudomonas aerug inosa IH 35342 ar concentrations starting from 0.1 mM. We conclude tha t ALDHs of the colonic aerobic bacteria are able to metabolize endogen ic acetaldehyde. However, the ability of ALDHs to metabolize intracolo nic acetaldehyde levels associated with alcohol drinking is rather low . Large differences between ADH and ALDH activities of the bacteria fo und in this study may contribute to the accumulation of acetaldehyde i n the large intestine after moderate drinking. ALDH activities of colo nic bacteria were poorly inhibited by cyanamide. This study supports t he crucial role of intestinal bacteria in the accumulation of intracol onic acetaldehyde after drinking alcohol. Individual variations in hum an colonic flora may contribute to the risk of alcohol-related gastroi ntestinal morbidity.