J. Biol. Chem., Vol. 260, Issue 10, 6047-6053, May, 1985
Kinetic rates of tryptic digestion of bovine cardiac myofibrils. An improved measurement of cross-bridge dissociation
AV Azarcon, D Applegate and E Reisler
The rates of tryptic digestion of the 50/20-kDa junction in myosin in
cardiac myofibrils were determined under various solvent conditions. This
cleavage reaction is slow in the rigor solvent and proceeds at a fast rate
in the presence of MgATP. When the reaction solvent contains 50% ethylene
glycol, the digestion of myosin in the presence of MgATP occurs at the same
rate as in myofibrils relaxed by Mg adenyl-5'-yl imidodiphosphate
(AMP-PNP). It is shown that with the help of two reference rates of
digestion, for attached and dissociated myosin heads, the initial cleavage
rates of myosin in the presence of nucleotides accurately measure the
dissociation of cross-bridges from actin in myofibrils. Under physiological
salt conditions and at 24 degrees C, MgADP, MgPPi, and MgAMP-PNP cause only
small cross-bridge detachment (less than or equal to 15%) in cardiac
myofibrils. The dissociation of myosin from actin is greatly increased by
lowering the solvent temperature to 4 degrees C. Lowering the salt
concentration of the solvent from 0.1 to 0.01 M NaCl has the most
pronounced effect on the rates of myosin digestion in the presence of
MgATP. In the low salt medium a substantial fraction of myosin heads (at
least 30%) appears to be attached to actin in the presence of 5 mM MgATP.