11β-Hydroxysteroid dehydrogenase1(11β-HSD1) can serve either as an oxo-reductase or dehydrogenase determined by the redox state in the endoplasmic reticulum (ER). This bidirectional enzyme governs paracrine glucocorticoid production. Recent in vitro studies have underscored the key role of cytoplasmic glucose-6-phosphate (G6P) in controlling the flux direction of 11βHSD-1 by altering the intraluminal ER NADPH/NADP ratio. The hypothesis that other hexose phosphoesters or the plentiful cellular oxidative protector glutathione could also regulate microsomal 11βHSD-1 activity was tested. Fructose-6-phosphate increased the activity of 11β-HSD1 reductase in isolated rat and porcine liver microsomes but not porcine fat microsomes. Moreover, oxidized glutathione (GSSG) attenuated 11β-HSD1 reductase activity by 40% while reduced glutathione (GSH) activated the reductase in liver. Fat microsomes were unaffected because they lack glutathione reductase. Nonetheless, another oxidizing agent, hydrogen peroxide (0.5 mM), inhibited both fat and liver 11β-HSD1 reductase. Consistent with the major role of the redox state, 2.5 mM GSSG and hydrogen peroxide augmented the 11β-HSD1 dehydrogenase, antithetical to the reductase, by 20-30% in liver microsomes. Given the key role of reactive oxygen species and hexose phosphate accumulation in the pathoetiology of obesity and diabetes, these compounds might also modify 11β-HSD1 in these conditions. © 2008 Elsevier Ltd. All rights reserved.