Energy-dependent activation and magnesium--dependent inactivation of hepatocyte hormone-sensitive phosphodiesterase.

Incubation of solubilized hormone-activated phosphodiesterase from isolated hepatocytes, under conditions likely to favour a dephosphorylation reaction, did not cause a loss of the hormone activation. If, however, the enzyme was incubated with Mg2+ (10 mM) while still associated with its membrane, and subsequently solubilized, the activity of the hormone-stimulated enzyme declined to the level seen in control cells. Diminution of hepatocyte ATP levels to about 20% of control values, by incubation with fructose, blunted the effect of glucagon and abolished the effect of insulin on phosphodiesterase. More severe ATP depletion caused by dinitrophenol abolished the stimulation of the enzyme by both hormones. These effects were not considered likely to be due to altered hormone-binding and are consistent with the involvement of an energy-dependent step in the hormonal activation of phosphodiesterase.