title: Dexamethasone and 11-dehydrodexamethasone as tools to investigate the isozymes of 11 beta-hydroxysteroid dehydrogenase in vitro and in vivo.
abstract: Dexamethasone is used in the clinic to test the sensitivity of the hypothalamic-pituitary-adrenal axis to negative feedback. It has also been proposed that metabolism of dexamethasone might differentiate between the activities of the two isozymes of 11 beta-hydroxysteroid dehydrogenase (11 beta HSD1 and 11 beta HSD2). We have developed a gas chromatographic mass spectrometric assay for dexamethasone and 11-dehydrodexamethasone and have confirmed in vitro that dexamethasone is a substrate for 11 beta-HSD2 but not 11 beta-HSD1 (conversion to 11-dehydrodexamethasone 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 beta HSD2, rat and human respectively). In healthy humans, the concentrations 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 2400 h). Concentrations of 11-dehydrodexamethasone did not correlate with the degree of suppression of plasma cortisol. Thus dexamethasone is not useful in differentiating the activities of the isozymes of 11 beta-HSD in vivo and variations in 11 beta-HSD activity do not explain the interindividual variability in suppression of plasma cortisol by low doses of dexamethasone.
authors.affiliation: University of Edinburgh, Department of Medicine, Western General Hospital, UK.
authors.forename: R
authors.initials: R
authors.lastname: Best
authors.forename: S M
authors.initials: SM
authors.lastname: Nelson
authors.forename: B R
authors.initials: BR
authors.lastname: Walker
mesh_terms: D042842:11-beta-Hydroxysteroid Dehydrogenases; D000328:Adult; D000818:Animals; D001681:Biological Assay; D003907:Dexamethasone / Q000031:analogs & derivatives / Q000097:blood / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006854:Hydrocortisone / Q000097:blood; D006913:Hydroxysteroid Dehydrogenases / Q000032:analysis / Q000378:metabolism*; D007527:Isoenzymes / Q000032:analysis / Q000378:metabolism*; D007668:Kidney / Q000201:enzymology; D008099:Liver / Q000201:enzymology; D008297:Male; D051381:Rats; D017208:Rats, Wistar
chemical_list: D007527:Isoenzymes; D003907:Dexamethasone; D006913:Hydroxysteroid Dehydrogenases; D042842:11-beta-Hydroxysteroid Dehydrogenases; D006854:Hydrocortisone