Four- [(1984), J. Biol. Chem. 259, 11908] and six- [(1985) Science 227, 999] state models have been proposed for actomyosin ATPase. A key experiment in deciding between these is whether or not there is a transient Pi burst at high actin. In the first, the cleavage and release of products rates are similar and the Pi burst is low; in the second, there are additional product complexes and the Pi burst is large. We reinvestigated the problem by carrying out burst experiments under the conditions in [(1985) Science 227, 999]. Since we find that the Pi burst at high actin is low, we conclude that the four-state model is sufficient to describe actomyosin ATPase.
