ADP binds similarly to rigor muscle myofibrils and to actomyosin-subfragment one

The binding of Mg2+ ADP to both rabbit skeletal and bovine cardiac myofibrils has been studied at two different temperatures. In each case a single class of binding sites was observed with a binding constant very close to that reported for the analogous actomyosin-subfragment one but much weaker than that seen with the analogous myosin subfragment one alone. These findings are discussed in terms of the constraints on the myosin cross-bridges imposed by the regular array of thick and thin filaments found in myofibrils.