DEXAMETHASONE AND 11-DEHYDRODEXAMETHASONE AS TOOLS TO INVESTIGATE THEISOZYMES OF 11-BETA-HYDROXYSTEROID DEHYDROGENASE IN-VITRO AND IN-VIVO

Dexamethasone is used in the clinic to test the sensitivity of the hyp othalamic-pituitary-adrenal axis to negative feedback. It has also bee n proposed that metabolism of dexamethasone might differentiate betwee n the activities of the two isozymes of 11 beta-hydroxysteroid dehydro genase (11 beta HSD1 and 11 beta HSD2). We have developed a gas chroma tographic mass spectrometric assay for dexamethasone and 11-dehydrodex amethasone and have confirmed in vitro that dexamethasone is a substra te for 11 beta-HSD2 but not 11 beta-HSD1 (conversion to 11-dehydrodexa methasone 0.6 +/- 0.3% in homogenates of rat liver with NADP(+) for 11 beta-HSD1, and 29.4 +/- 10.3% and 40.0 +/- 2.0% in homogenates of rat and human kidney respectively with NAD(+) for 11 beta-HSD2). However, we have also made the novel observation that 11-dehydrodexamethasone is a substrate for both isozymes (conversion to dexamethasone 65.0 +/- 20.4% for 11 beta HSD1 and 53.5 +/- 20.8% and 69.0 +/- 4.5% for 11 be ta HSD2, rat and human respectively). In healthy humans, the concentra tions of 11-dehydrodexamethasone in plasma after an intravenous bolus of dexamethasone were less than 10% of those of dexamethasone, and 11- dehydrodexamethasone was detected (at 0.8-65.0 nM) in plasma from only 11 of 20 subjects at 0900 h on the morning after oral dexamethasone ( 0.1-1 mg taken at 2400h). Concentrations of 11-dehydrodexamethasone di d not correlate with the degree of suppression of plasma cortisol. Thu s dexamethasone is not useful in differentiating the activities of the isozymes of 11 beta-HSD in vivo and variations in 11 beta-HSD activit y do not explain the interindividual variability in suppression of pla sma cortisol by low doses of dexamethasone.