Biochemical kinetics of skeletal actosubfragment-1 at high subfragment-1 concentrations.

The actomyosin ATPase activity of skeletal myosin subfragment-1 (S-1) is typically studied by keeping the S-1 concentration low and varying the actin concentration. General agreement exists over the kinetic data observed. Another way of studying the ATPase activity is to keep the actin concentration low and vary the S-1 concentration. The picture that has emerged is that the maximal ATPase rate (per micromolar actin), Vamax, is several fold greater than the Vsmax measured at fixed S-1. Likewise, the apparent activation constant Kam is several fold weaker than KATPase. In addition it is found that Kam, henceforth Kam(At), varies with the total actin concentration At, but controversy continues over the actin dependence of Vamax. Of particular interest is the fact that the Lymn-Taylor and refractory state models could not account for the data. Here we have repeated studies on the ATPase activity at fixed actin concentration in an attempt to determine if the current models for the actin activated myosin ATPase activity can account for both the constant actin and constant S-1 data simultaneously, or if these data imply that new kinetic models need be postulated. We conclude that the current kinetic models can account for the data.