Isotopic probes into pathways of ethanol metabolism

Abstract

The relative extent of tritium labeling in glucose and water was determined when l-[2- 3H]lactate or [(1 R)1- 3H]ethanol were the labeled substrates for rat liver parenchymal cells, incubated with 20 m m ethanol and 10 m ml-lactate. From the relatively lower specific yield in glucose from the tritiated ethanol one can calculate a percentage contribution of a non-alcohol dehydrogenase-mediated pathway to total ethanol metabolism. This calculated value (about 35%) is somewhat higher than that determined by the use of pyrazole at 5 m m to inhibit alcohol dehydrogenase. The utilization of [(1 R)1- 3H]ethanol is slower than that of unlabeled ethanol, both in the absence and presence of 5 m m pyrazole, indicating isotope discrimination against tritium in both the alcohol dehydrogenase and non-alcohol dehydrogenase pathways. There was only a slight difference in the rate of utilization of normal and fully deuterated ethanol by rat liver cells in the absence of pyrazole. However, in the presence of 5 m m pyrazole, where essentially only the non-alcohol dehydrogenase pathway operates, deuterated ethanol was utilized at only about half the rate of nondeuterated ethanol. These findings are difficult to reconcile with a catalase-mediated pathway of ethanol metabolism in which the rate-limiting factor is the rate of H 2O 2 generation.