CHARACTERISTICS OF ALCOHOL DEHYDROGENASES OF CERTAIN AEROBIC-BACTERIAREPRESENTING HUMAN COLONIC FLORA

We have recently proposed the existence of a bacteriocolonic pathway f or ethanol oxidation [i.e., ethanol is oxidized by alcohol dehydrogena ses (ADHs) of intestinal bacteria resulting in high intracolonic level s of reactive and toxic acetaldehyde]. The aim of this in vitro study was to characterize further ADH activity of some aerobic bacteria, rep resenting the normal human colonic flora. These bacteria were earlier shown to possess high cytosolic ADH activities (Escherichia coli IH 13 3369, Klebsiella pneumoniae IH 35385, Klebsiella oxytoca IH 35339, Pse udomonas aeruginosa IH 35342, and Hafnia alvei IH 53227). ADHs of the tested bacteria strongly preferred NAD as a cofactor. Marked ADH activ ities were found in all bacteria, even at low ethanol concentrations ( 1.5 mM) that may occur in the colon due to bacterial fermentation. The K-m for ethanol varied from 29.9 mM for K. pneumoniae to 0.06 mM for Hafnia alvei. The inhibition of ADH by 4-methylpyrazole was found to b e of the competitive type in 4 of 5 bacteria, and K, varied from 18.26 +/- 3.3 mM for Escherichia coli to 0.47 +/- 0.13 mM for K. pneumoniae . At pH 7.4, ADH activity was significantly lower than at pH 9.6 in fo ur bacterial strains. ADH of K. oxytoca, however, showed almost equal activities at neutral pH and at 9.6, In conclusion, NAD-linked alcohol dehydrogenases of aerobic colonic bacteria possess low apparent K-m's for ethanol. Accordingly, they may oxidize moderate amounts of ethano l ingested during social drinking with nearly maximal velocity. This m ay result in the marked production of intracolonic acetaldehyde. Kinet ic characteristics of the bacterial enzymes may enable some of them to produce acetaldehyde even from endogenous ethanol formed by other bac teria via alcoholic fermentation. The microbial ADHs were inhibited by 4-methylpyrazole by the same competitive inhibition as hepatic ADH, h owever, with nearly 1000 times lower susceptibility. Individual variat ions in human colonic flora may thus contribute to the risk of alcohol -related gastrointestinal morbidity, such as diarrhea, colon polyps an d cancer, and liver injury.