Fluorometric Studies on the Light Chains of Skeletal Muscle Myosin: III. Effect of Ca2+ on the Reactivity of Two SH Groups of DTNB Light Chain in Myosin and in the Isolated State1

We studied the effect of Ca2+ on the reactivities of two SH groups of DTNB light chain (Cys 128 and Cys 157) using a fluorogenic thiol reagent. It was found that a Ca2+-induced change in reactivity occurred only with Cys 128 when the light chain was in an isolated state, whereas it occurred with both Cys 128 and Cys 157 when the light chain was incorporated in myosin. These results indicate that the Ca2+-induced change in the conformation of DTNB light chain in the isolated state was different from that of the light chain in myosin. It may therefore be difficult to relate the Ca2+-induced conformational change observed in the isolated DTNB light chain to the molecular mechanism of myosin-linked Ca2+ regulation.