Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the principal enzymes responsible for the oxidation of ingested ethanol in humans. To study these two enzymes in surgical specimens of attached gingiva and tongue, we have examined the isozyme patterns by agarose isoelectric focusing and determined the enzyme activities. Class IV mu-ADH, class III chi-ADH, and class III ALDH3 were detected in the oral mucosa tissues. Gingival mu-ADH exhibited a pH optimum for ethanol oxidation at 10 and the K(m) value for ethanol (pH 7.5) was estimated to be 27 mM. At pH 7.5 and 30 degrees C, the ADH activities in the gingiva and tongue samples were determined to be 90.0 +/- 5.8 (mean +/- SE; n = 24) and 50.6 +/- 5.1 (n = 3) nmol/min/g tissue (at 33 mM ethanol), and 138 +/- 11 and 55.1 +/- 4.7 nmol/min/g tissue (at 500 mM ethanol), respectively. The ALDH activities at 20 mM acetaldehyde were determined to be 169 +/- 19 and 50.3 +/- 8.1 nmol/min/g tissue for the gingiva and tongue, respectively. We conclude that ethanol can be significantly metabolized in human attached gingiva and lingual mucosa by mu-ADH. The result also suggests that, due to lacking activity of low K(m) ALDH2 and ALDH1, cytotoxic metabolite acetaldehyde may be involved in the etiology of alcohol-related oral injury.