Aim: 

It has been suggested that the rate of ADP release from acto-myosin and the rate of acto-myosin dissociation by ATP may play different roles to define unloaded shortening velocity (Vo) of slow and fast myosins and their role might change with temperature (Nytray et al. 2006; Iorga et al. 2007).

Methods: 

To clarify the relative role in defining Vo, the affinity of the acto-myosin complex for MgATP and MgADP was studied using the "in vitro motility assay" (IVMA) approach. The velocity of actin filaments sliding on pure slow (myosin 1) and pure fast (myosin 2B) myosin isoform was determined in a range of MgATP concentration (0.01-2mM) and in the presence or absence of 2mM MgADP. Moreover, experiments were performed at the temperatures of 20-25 and 35 °C.

Results: 

(i) both MgATP and MgADP affinity was higher in slow than fast myosin at all temperatures (ii) the temperature dependence of MgATP affinity was similar for both the isoforms decreasing from 20 to 35° C (iii) the temperature dependence of MgADP affinity was different for the two isoforms: decreasing in slow and almost temperature-independent in fast isoform.

Conclusion: 

These results confirm the hypothesis of a different kinetics at the basis of the functional difference between slow and fast skeletal isoforms