Indomethacin and glucocorticoid metabolism in rat liver cytosol

3 alpha-Hydroxysteroid dehydrogenase (EC 1.1.1.50) of rat liver cytosol catalyzes the second step in glucocorticoid metabolism, namely the NADPH-dependent reduction of 5 beta-dihydrocortisol to tetrahydrocortisol. The purified enzyme is potently inhibited by the nonsteroidal anti-inflammatory drugs [Penning and Talalay, Proc. natn. Acad. Sci. U.S.A. 80, 4504 (1983)], and the consequences of this inhibition on hepatic glucocorticoid metabolism are now examined. Analyses of the specific activities for the reduction of 5 beta-dihydro-cortisone catalyzed by homogeneous preparations of 3 alpha- and 3 beta-hydroxysteroid dehydrogenases of rat liver cytosol indicated that this steroid was predominantly reduced to the 3 alpha-hydroxysteroid (tetrahydrocortisone). Whether this reaction was catalyzed by the purified 3 alpha-hydroxysteroid dehydrogenase or unprocessed cytosol, it was potently inhibited by indomethacin (IC50 = 0.6 microM). In the presence of 30 microM indomethacin, the rate of reduction of 5 beta-dihydro-glucocorticoids was 16 times smaller than in the absence of drug. Measurement of 5 beta-reductase activity in rat liver cytosol indicated that it was 25-30 times less active than the 3 alpha-hydroxysteroid dehydrogenase. In the presence of 30 microM indomethacin, enough residual dehydrogenase may exist to reduce 5 beta-dihydro-glucocorticoids as they are formed. This was confirmed by incubating the supernatant fraction obtained from the 10,000 g centrifugation of a rat liver homogenate, with [1,2-3H]cortisol plus NADPH in the presence and absence of drug. At the end of the incubation, cortisol metabolites were analyzed by TLC and the results expressed as CL:CM ratio (cortisol left:cortisol metabolized). It was found that no change in this ratio occurred in the presence of the drug. Moreover, the addition of trapping quantities of 5 beta-dihydrocortisol to incubations containing 30 microM indomethacin were insufficient to promote an increase in cortisol levels as reflected by a rather small increase in the CL:CM ratio. Thus, nonsteroidal anti-inflammatory drugs cannot effect a rise in hepatic glucocorticoid levels in vitro, and do not promote a "glucocorticoid saving effect" in this organ.