Key result
The turnover rate of actin-activated scallop heavy meromyosin ATPase in the presence of Ca2+ was 650-fold higher than in its absence.
The study demonstrates a 650-fold higher turnover rate of actin-activated scallop HMM ATPase in the presence of Ca2+ compared to its absence, approaching expected in vivo sensitivity.
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No clinical implications; leaves open whether this Ca2+ sensitivity extends to mammalian cardiac myosins.
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 2+ (1.2 s −1 per head) was 650‐fold higher than in its absence (1.8 × 10 −3 s −1 ), a Ca 2+ sensitivity which approaches that expected in vivo. The extent of Ca 2+ activation was much larger than the observed by steady‐state measurements, where the rate, in the absence of Ca 2+ , is dominated by a small proportion of unregulated molecules. Acto‐HMM formation, and its dissociation by ATP, were Ca 2+ insensitive.
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Wells et al. (1984) studied this question. Ca2+ vs. Absence of Ca2+ was evaluated on Turnover rate of actin-activated scallop heavy meromyosin (HMM) ATPase (650-fold higher). The turnover rate of actin-activated scallop heavy meromyosin ATPase in the presence of Ca2+ was 650-fold higher than in its absence.
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