Beschreibung:
<jats:p>The actin‐activated scallop heavy meromyosin (HMM) ATPase was monitored turbidometrically during a limited number of turnovers. At 4 μM actin, the turnover rate in the presence of Ca<jats:sup>2+</jats:sup> (1.2 s<jats:sup>−1</jats:sup> per head) was 650‐fold higher than in its absence (1.8 × 10<jats:sup>−3</jats:sup> s<jats:sup>−1</jats:sup>), a Ca<jats:sup>2+</jats:sup> sensitivity which approaches that expected in vivo. The extent of Ca<jats:sup>2+</jats:sup> activation was much larger than the observed by steady‐state measurements, where the rate, in the absence of Ca<jats:sup>2+</jats:sup>, is dominated by a small proportion of unregulated molecules. Acto‐HMM formation, and its dissociation by ATP, were Ca<jats:sup>2+</jats:sup> insensitive.</jats:p>