Comparison of rat pulmonary and hepatic cytosolic alcohol dehydrogenase activities

The liver is the major organ responsible for ethanol oxidation, and alcohol dehydrogenase (ADH) is the main enzyme involved. There is limited evidence suggesting the involvement of the lung in ethanol metabolism. To determine the degree to which pulmonary ADH plays a role in ethanol metabolism, ADH activity was measured spectrophotometrically using hepatic and pulmonary cytosolic fractions prepared by differential centrifugation and Sephadex G-50 column chromatography. Apparent Km values for hepatic and pulmonary ADHs were determined. Inhibition constants were calculated using 4-methylpyrazole. The ADHs were characterized by examining the influence of pH on enzyme activity. Pulmonary ADH activity was much lower at near neutral pH than at pH 9.0 or 10, whereas hepatic ADH activity was also pH dependent but was significantly higher. Pulmonary ADH is less sensitive to inhibition by 4-methylpyrazole than is hepatic ADH, as evidenced by a 1000-fold higher Ki. Pulmonary ADH would be expected to make only a minor contribution to ethanol metabolism in vivo.