MgATPase activity of myosin subfragment 1. The dimer is more active than the monomer

The MgATPase activity of the rabbit skeletal myosin subfragment 1 (S1), in the steady state, was measured by means of the intrinsic fluorescence of tryptophan. This technique gave results similar to those obtained by other methods (linked or radioactive assays). The activity was measured under conditions that effect the monomer/dimer ratio. It is shown that there is a close correlation between MgATPase activity and the proportion of dimer. At 20 degrees C, for pH 6.9 to 8.1 and for [KCl] less than or equal to 1 M, the observed activity (kobs) can be linearly related to the proportion of dimer (Ed/Eo) by: kobs(s-1) = 0.016-7 X 10(-3)[KCl] + 0.031(Ed/Eo), where [KCl] is expressed in M. We deduce that, at 20 degrees C and for [KCl] = 0 M, the activity of the monomer is kmobs = 0.016 s-1 (Ed/Eo = 0) and that of the dimer kdobs = 0.047 s-1 (Ed/Eo = 1), i.e. a ratio kdobs/kmobs approximately equal to 3. Beyond pH approximately equal to 8.3, the activities of both the monomer and the dimer increased steeply with increasing pH value. In the standard conditions (pH 8.0, [KCl] = 0 to 100 mM), S1 is mainly in the form of a dimer, and such conditions are not appropriate for study of the S1 monomer. For studying the pure monomer, the conditions required at 20 degrees C and in bis-Tris-propane are: S1 concentration approximately equal to 0.2 mg/ml, pH 6.9 to 7.8, [KCl] approximately equal to 300 mM. For studying the pure dimer, the conditions required are: S1 concentration greater than or equal to 0.2 mg/ml, pH 7.8 to 8.1 and [KCl] approximately equal to 0. In both cases the MgATP concentration is about 50 microM. Finally, if great care is taken concerning the age of the S1 solutions and the evaluation of the proportion of dimer, the values of kobs are extremely precise: the uncertainty regarding the values of kobs, as determined by means of intrinsic fluorescence, does not exceed +/- 0.001 s-1. Beyond this error bar conditions are uncontrolled.