Differentiation of the drug‐binding sites of calmodulin

Calmodulin contains several binding sites for hydrophobic compounds. The apparent specificity of various ‘calmodulin antagonists’ for these sites was investigated. The Ki values for the inhibition of calmodulin‐activated cyclic‐nucleotide phosphodiesterase and myosin light‐chain kinase was determined. In addition, the Kd values of the same compounds for binding to calmodulin were measured. The compounds could be separated into four groups. Group I and II compounds inhibited competitively the activation of the phosphodiesterase and myosin light‐chain kinase by calmodulin. Group I compounds inhibited the activation of the phosphodiesterase and myosin light‐chain kinase at identical concentrations. In contrast, group II compounds inhibited the activation of the phosphodiesterase at 5–10‐fold lower concentrations than that of myosin light‐chain kinase. Group III compounds inhibited the activation of these enzymes by an uncompetitive mechanism. Group IV compounds inhibited the activation of the phosphodiesterase with Ki values above 10 μM and did not affect the activation of myosin light‐chain kinase. Binding of [3H]bepridil to calmodulin under equilibrium conditions yielded one high‐affinity site (apparent Kd 0.4 μM) and four low affinity sites (apparent Kd 44 μM). Group I compounds interfered with the binding of bepridil to the high and low‐affinity sites in a competitive manner. Group II compounds interfered in a noncompetitive manner with the high‐affinity site and apparently competed only with one of the low‐affinity sites. Group III compounds did not compete with any of the bepridil‐binding sites. Nimodipine, a group III compound, bound to one site on calmodulin with a Kd value of 1.1 μM. Other dihydropyridines competed with [3H]nimodipine for this site. The group I and II compounds, trifluoperazine and prenylamine, did not affect the binding of [3H]nimodipine. These data show that ‘calmodulin antagonists’ can be differentiated into at least three distinct groups. Kinetic and binding data suggest that the three groups bind to at least three different sites on calmodulin. Selective occupation of these sites may inhibit specifically the activation of distinct enzymes.