Activation of Cholesteryl Esterase in Bovine Adrenal Cortex

The influence of adenosine 3′:5′‐monophosphate (cyclic AMP) and ATP on the activity of cholesteryl esterase has been studied in bovine adrenal cortex. It has been shown that cyclic AMP and ATP stimulated the activity of cholesteryl esterase in vitro. The increase in cholesteryl esterase activity was dependent upon the presence of cyclic‐AMP‐dependent protein kinase. After removal of protein kinase from cholesteryl esterase preparation no stimulation upon the addition of cyclic AMP and ATP was observed. The addition of purified protein kinase, which was virtually free from cholesteryl esterase activity resulted in approx. 3‐fold stimulation of cholesteryl esterase activity in the presence of cyclic AMP and ATP. Without cyclic AMP and ATP no stimulation of partially purified cholesterol esterase by protein kinase was observed. Preincubation of crude cholesteryl esterase in the presence of 5 mM MgCl 2 caused a marked decrease in cholesteryl esterase activity. Under these conditions cyclic AMP and ATP protected the enzyme against inactivation Preincubation of partially purified cholesteryl esterase with [γ‐ 32 P]ATP, cyclic AMP and cyclic AMP‐dependent protein kinase resulted in an increase in cholesteryl esterase activity with a concomitant transfer of 32 P‐labelled phosphate into the protein fraction. Incubation of the phosphorylated protein fraction in the presence of 5 mM MgCl 2 revealed a decrease in cholesteryl esterase activity with a concomitant release of 32 P‐labelled phosphate from phosphorylated protein. The decrease in cholesteryl esterase activity and the release of 32 P‐labelled phosphate from phosphorylated protein exhibited similar kinetics. It is postulated that the bovine adrenal cortex cholesteryl esterase is activated by cyclic AMP‐dependent protein kinase and that the activation involves a transfer of a terminal phosphate group from ATP onto the cholesteryl esterase molecule.