Abstract

Chlorpromazine (CPZ) and thioproperazine (TPZ) were titrated fluorometrically with horseshoe crab myosin B. Resulting alterations in protein structure were indicated by discrete changes in the quenching and polarization of protein fluorescence, and in protein light scattering. These effects were demonstrated at drug concentrations from 0.23 to 13 µM with 46-263 µg of protein per milliliter. We observed cooperative effects of protein concentration upon the drug binding. At CPZ and TPZ concentrations less than 10 µM/mg of protein, drug binding was only 2.7-2.8 nmoles/mg of protein per milliliter. At drug concentrations above 100 µ/mg of protein, binding increased to 77-78 nmoles/mg of protein per milliliter. Equivalent minimal combining weights for the myosin B per mole of drug are 237,700 ± 2,000 g and 12,830 ± 710 g at drug concentrations below 10 µM and above 100 µM, respectively. Addition of 22 nmoles of TPZ per milligram of protein per milliliter to the ATPase assay medium increased enzymatic activity more than 100%. TPZ and trifluoperazine were also used as fluorescence microscopy stains for myosin B in intact sarcomeres. This procedure revealed the translocation of this protein within the lateral A band after contraction.