Dissociation of Light Chains from Cardiac Myosin

The substrate, ATP, protects the active site of cardiac myosin during a 10‐min treatment at 37°C and neutral pH in the absence of divalent cations; under these conditions there is an approximate 20% dissociation of light chain C1 and 60% loss of light chain C2 with no corresponding decrease in myosin ATPase activity. Higher temperatures, the absence of divalent cations, increased treatment time, and low protein concentration were conducive to light chain dissociation; loss of light chains did not appear to be influenced by increments of pH from 7.0 to 9.0. On the other hand, a decrease in protein concentration and an increase in both duration of exposure at 37°C and an alkaline pH caused a decrease in myosin ATPase activity. Concomitant with the dissociation of cardiac myosin light chains, particularly light chain C2, there was a corresponding loss in the number of high‐affinity calcium binding sites. When the dissociated light chains were recombined with light‐chain‐deficient myosin, reassociation took place and the number of high‐affinity calcium binding sites were regained. Reassociation of light chains with light‐chain‐deficient myosin did not restore diminished light‐chain‐deficient myosin ATPase activity. There is a shift in the pH optimum of myosin at 37°C in the absence of divalent cations, where conditions used for light chain dissociation are employed. The presence of calcium prevents light chain dissociation, helps retain myosin ATPase activity at 37°C and prevents a shift in the pH optimum.