Biochemical kinetics of porcine cardiac subfragment-1. II. Pre-steady-state studies of the initial phosphate burst.

The actin dependence of the rate and magnitude of the initial phosphate burst was measured using both quench-flow and stopped-flow kinetic techniques. These studies revealed that even at high actin concentrations the magnitude of the phosphate burst was a significant fraction of the magnitude that exists in the absence of actin. Furthermore, it was shown that the rate of the burst rises rapidly as a function of the actin concentration. Detailed modeling with the four-state model revealed that if the predicted Vmax is constrained to be approximately equal to the extrapolated value (double reciprocal plot) and if the apparent dissociation constant of subfragment-1 to actin divided by the apparent activation constant of the actin-activated myosin ATPase activity (Kbinding/KATPase) is constrained to be considerably different from one, then the model is unable to simultaneously account for the ATPase activity and the rate and magnitude of the initial inorganic phosphate burst.