Evidence for independent 11-oxidase and 11-reductase activities of 11 beta-hydroxysteroid dehydrogenase: enzyme latency, phase transitions, and lipid requirements.

Experimental modification of the membrane structure of rat liver microsomes affected the behavior of the 11-oxidase and 11-reductase components of 11 beta-hydroxysteroid dehydrogenase in different ways. 1) The latency of 11-oxidase was released by detergents, phospholipases, or elevated temperature; 11-reductase activity was not increased by these manipulations. 2) 11-Reductase was rapidly inactivated at 25 C and 37 C; 11-oxidase was stable at these temperatures. 3) Arrhenius plots of microsome bound 11-reductase between 5 C and 40 C showed discontinuity at 23 C. Activation energies above and below the critical temperature were 2 kcal and 16 kcal, respectively. Solubilized 11-reductase showed no discontinuity [activation energy (Ea) = 15 kcal]. Ea for 11-oxidase was 15 kcal at all temperatures for membrane bound or solubilized enzyme, with no discontinuities. 4) Phospholipases A2 and C rapidly inactivated 11-reductase. Triton DF-18 regenerated 50% of the reductase activity of phospholipase C-treated microsomes, but had no effect on phospholipase A2-treated microsomes. Phospholipases increased 11-oxidase activity. The independent behavior of corticosteroid 11-oxidase and 11-reductase are consistent with the properties of closely associated, independent enzymes.