AJP - Cell Physiology, Vol 267, Issue 4 C1160-C1166, Copyright © 1994 by American Physiological Society
Rapid turnover of myosin light chain phosphate during cross-bridge cycling in smooth muscle
T. M. Butler, S. R. Narayan, S. U. Mooers and M. J. Siegman
Department of Physiology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107.
The rate of phosphatase-mediated dephosphorylation of the regulatory light
chain of smooth muscle myosin was determined under nearly steady-state
conditions in permeabilized muscles, from the time course of incorporation
of 33P-labeled phosphate into the light chain after the photolytic release
of [gamma-33P]ATP from high specific activity caged [gamma-33P]ATP. The
extent of myosin light chain phosphorylation is unchanged, and, if the
kinase and phosphatase reactions are irreversible, the rate constant for
the exponential increase in 33P in the light chain is equal to the rate
constant for the phosphatase reaction. Under activated conditions (pCa 4.5)
at 20 degrees C, the incorporation of 33P into approximately 80% of the
phosphorylated light chain is fit by a single exponential with a rate
constant of 0.37 s-1. ATP usage due to phosphorylation and
dephosphorylation of the light chain is about one-third of the suprabasal
energy requirement. The high phosphatase rate constant suggests that
dephosphorylation of the light chain is rapid enough to interact with and
potentially modify the completion of the cross-bridge cycle.
