Effects of cAMP-dependent protein kinase and ATP on N 1-oxidation of 9-benzyladenine by animal hepatic microsomes

Abstract

N 1-Oxidation is a major metabolic pathway for 9-benzyladenine (BA) catalyzed by the cytochrome P450 system in animal hepatic microsomes. After normal hamster hepatic microsomes or phenobarbital induced rabbit hepatic microsomes were preincubated in the presence of cyclic AMP-dependent protein kinase catalytic subunit (PKA), MgCl 2 and ATP, BA-N 1-oxidation was significantly decreased. However, further investigation indicated that the decrease of BA-N 1-oxidation seemed to be a combination of the effects of PKA and ATP, as ATP alone showed a biphasic regulatory effect on BA-N 1-oxidation when microsomes were preincubated in the presence of various concentrations of ATP. In the lower ATP concentration range (0.5–2.5mM), BA-N 1-oxidation increased along with the increase of ATP concentration; whereas BA-N 1-oxidation decreased when the ATP concentration was higher (> 5mM). The biphasic regulatory effects of ATP on BA-N 1-oxidation seem dependent on the incubation process, as preincubation markedly strengthened the effects. When microsomes were incubated at 37 °C for different time lengths in the absence or presence of ATP (2.5 or 20mM), the activity of BA-N 1-oxidase decreased at similar rates in all groups, but the activity levels of BA-N 1-oxidase were different among the groups. The cytochrome P450 content was not changed parallel to the variation of BA-N 1-oxidation when microsomes were incubated in the presence of ATP, indicating that the effects of ATP on BA-N 1-oxidation were not mediated by affecting CYP stability. In addition, the activity of NADPH-cytochrome P450 reductase was not markedly affected by ATP without incubation. The result implied that ATP did not inhibit the reductase directly. After microsomes were incubated in the presence of low ATP concentration (2.5mM), the reductase was slightly inhibited, whilst high ATP concentration (20mM) showed marked inhibition (83% of control). This may partially contribute to the down-regulatory effect of ATP on BA-N 1-oxidation. Furthermore, it was found that the presence of magnesium ions during preincubation weakened the upregulatory effect of ATP (2.5mM) on BA-N 1-oxidation, but showed no effect on the down-regulatory effect of ATP (20mM). Since these observed phenomena are not readily explained, a possible mechanism, i.e. phosphorylation and dephosphorylation of cytochrome P450, is suggested.