EXCHANGE OF ATP FOR ADP ON HIGH-FORCE CROSS-BRIDGES OF SKINNED RABBITMUSCLE-FIBERS

The contractile properties of rabbit skinned muscle fibers were studie d at 1-2 degrees C in different concentrations of MgATP and MgADP. Dou ble-reciprocal plots of maximum velocity against MgATP concentration a t different MgADP concentrations all extrapolated to the same value. T his finding suggests that MgATP and MgADP compete for the same site on the cross-bridge, and that the exchange of MgATP for MgADP occurs wit hout a detectable step intervening. The K-m for ATP was 0.32 mM. The K -i for MgADP was 0.33 mM. Control experiments suggested that the tortu osity of diffusion paths within the fibers reduced the radial diffusio n coefficients for reactants about sixfold. Increasing MgADP from 0.18 to 2 mM at 5 mM ATP or lowering MgATP from 10 to 2 mM at 0.18 mM MgAD P, respectively, increased isometric force by 25% and 23%, increased s tiffness by 10% and 20%, and decreased maximum velocity by 35% and 31% . Two mechanisms appeared to be responsible. One detained bridges in h igh-force states, where they recovered from a length step with a slowe r time course. The other increased the fraction of attached bridges wi thout altering the kinetics of their responses, possibly by an increas ed activation resulting from cooperative effects of the detained, high -force bridges. The rigor bridge was more effective than the ADP-bound bridge in increasing the number of attached bridges with unaltered ki netics.