In vitro inhibition of Ca 2+/calmodulin-dependent kinase II activity by melatonin

Abstract

Recent evidence suggests that a melatonin (MEL) mechanism of action may be through modulation of Ca 2+-activated calmodulin (CaM). MEL binds to CaM with a high affinity, and has been shown to act as a CaM antagonist. Among the CaM-dependent enzymes, Ca 2+/calmodulin-dependent protein kinase II (CaM-kinase II) is a particularly abundant enzyme in the nervous system. In the brain it phosphorylates a broad spectrum of substrates, thus modulating important neuronal functions. We describe the MEL effect on CaM-kinase II activity in vitro. CaM-kinase II was purified from rat brain by column chromatography, and identified by Western immunoblotting. CaM-kinase II activity was assessed in the presence of Ca 22+/CaM by the kinase's ability to phosphorylate the synthetic substrate syntide-2 and by enzyme autophosphorylation. MEL inhibited CaM-kinase II activity, and enzyme autophosphorylation. Inhibition of the enzyme by 10 −9 M MEL was nearly of 30%. Trifluoperazine (10 μM), W7 (10 μM), and compound 48 80 (30 μg/ml), inhibited CaM-kinase II activity by 40%, 42%, and 93%, respectively. Both EGTA (5 mM) and MEL (10 −5 M) abolished autophosphorylation. The effect of MEL on CaM-kinase II activity was specific, since neither serotonin, N-acetylserotonin, nor 6-hydroxymelatonin inhibited its activity. Our results support the hypothesis that MEL acts as a CaM antagonist and cellular functions may be rhythmically regulated by MEL modulation of CaM-dependent protein phosphorylation.