Inhibition of Protein Kinase and Cyclic AMP Phosphodiesterase by Eritadenine Isoamyl Ester

Abstract

The effects of eritadenine isoamyl ester (EiA) on cyclic AMP (cAMP)-dependent biochemical systems were investigated in order to elucidate the mechanism of its hypolipidemic action. EiA exhibited a twofold effect on stimulated lipolysis in rat epididymal fat cells; i.e., EiA (5×lO⁻⁴M) inhibited theophylline-induced glycerol release, but potentiatedthe lipolytic action of epinephrine. EiA (5 ×10⁻⁴M) had no effect on the theophylline-induced augmentation of cAMP levels in fat cells. EiA inhibited cAMP-dependent protein kinase [EC 2.7.1.37] in the fat pad infranatant and liver supernatant fractions. Kinetic studies using partially purified liver enzyme showed that EiA inhibited protein kinase at the phosphorylation step, the inhibition being competitive with respect to ATP. EiA inhibited the cAMP phosphodiesterase [EC 3.1.4.17] of fat cells. It increased the rate of cAMP accumulation in fat cells induced by epinephrine on incubation for one minute and it also potentiated cAMP-induced lipolysis. EiA enhanced the action of lower concentrations (5×l0⁻⁵M–5×l0⁻⁴M) of dibutyryl-cAMP (DBcAMP) in DBcAMP-induced lipolysis, but suppressed the action of higher concentrations (10⁻³M) of DBcAMP. This biphasic pattern was also observed in DBcAMP-induced increase in a-aminoisobutyrate uptake by isolated perfused rat liver and in cAMP-induced steroidogenesis in rat adrenal cells. It is suggested that the biphasic effects of EiA may be due to its inhibition of both protein kinase and cAMP phosphodiesterase.