A derivative of bisbenzylisoquinoline alkaloid is a new and potential calmodulin antagonist

A new derivative of bisbenzylisoquinoline (berbamine type): 0-(4-ethoxylbutyl) berbamine (EBB) was found to possess powerful and specific calmodulin (CaM) inhibitory properties. It inhibited CaM-stimulated Ca2+-Mg2+-ATPase in human erythrocyte membrane with IC50 value of 0.35 microM compared to that of 60 microM of berbamine. CaM-independent basal Ca2+-Mg2+-ATPase, Na+-K+-ATPase and Mg2+-ATPase were not effect at 1.0 microM of EBB at which CaM-dependent Ca2+-Mg2+-ATPase was already potently inhibited. The inhibition of CaM-dependent Ca2+-Mg2+-ATPase was competitive with respect to CaM. Higher amount of CaM reversed the inhibition caused by higher concentration of EBB. Using dansyl-CaM (D-CaM), it was shown that EBB binds directly to CaM and caused a conformational change of CaM polypeptide chain. From fluorescence titration curve we obtained evidence that in the presence of Ca2+, CaM has two specific binding sites for EBB and additional unspecific binding sites. The Ca2+-dependent binding sites of EBB on CaM were novel region different from the binding sites for TFP.