Differing ADP release rates from myosin heavy chain isoforms define the shortening velocity of skeletal muscle fibers

To understand mammalian skeletal myosin isoform diversity, pure myosin isof orms of the four major skeletal muscle myosin types (myosin heavy chains I, IIA, IIX, and IIB) were extracted from single rat muscle fibers. The extra cted myosin (1-2 mug/15-mm length) was sufficient to define the actomyosin dissociation reaction in flash photolysis using caged-ATP (Weiss, S., Chizh ov, I., and Geeves, M. A. (2000) J. Muscle Res. Cell Motil. 21, 423-432). T he ADP inhibition of the dissociation reaction was also studied to give the ADP affinity for actomyosin (K-AD). The apparent second order rate constan t of actomyosin dissociation gets faster (K(1)k(+2) = 0.17 -0.26 mum(-1).s( -1)), whereas the affinity for ADP is weakened (250-930 mum) in the isoform order I, IIA, IIX, IIB. Both sets of values correlate well with the measur ed maximum shortening velocity (V-0) of the parent fibers. If the value of KAD is controlled largely by the rate constant of ADP release (k(-AD)), the n the estimated value of k(-AD) is sufficiently low to limit V.. In contras ts [ATP]K(1)k(+2) at a physiological concentration of 5 mm ATP would be 2.5 -6 times faster than k(-AD).