Calmodulin antagonists inhibit activity of myosin light‐chain kinase independent of calmodulin

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Abstract
The calmodulin antagonists W‐7, trifluoperazine and R24571 in vitro inhibited calmodulin‐dependent and independent myosin light chain kinase activity with IC50 values of about 300 μM, 140 μM and 18 μM in the presence of 8 mg/ml myosin light chains. These IC50 values decreased to 15 μM. 6 μM and 2.5 μM when the concentration of myosin light chains was decreased to 0.4 mg/ml in the presence of saturating concentrations of calmodulin. Endogeneous tyrosine fluorescence of myosin light chain measured at 334 nm was quenched concentration dependently by trifluoperazine and R24571. In addition, fluorescence of W‐7 measured at 370 nm was quenched concentration dependently by myosin light chains. The quenching of fluorescence which was independent of calcium, suggested that all three compounds bound to myosin light chain. The IC50 values for trifluoperazine obtained from fluorescence quench curves at different concentrations of myosin light chain were almost identical, with those obtained under similar conditions from inhibition curves of myosin light chain kinase. These results indicate that ‘calmodulin antagonists’ inhibit the activity of myosin light chain kinase independent of calmodulin by binding to myosin light chain. The implication of this finding for the interpretation of results obtained in vivo by the use of ‘calmodulin antagonists’ is discussed.