Summary
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1.
Artificial actomyosin (reconstituted from its isolated components myosin and actin without natural tropomyosin) takes up ∼3 μmoles calcium/g actomyosin if one increases the concentration of free Ca++ ions from ∼10−9 M to 10−6 M.
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2.
In the presence of magnesium, however, no calcium is taken up by artificial actomyosin.
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3.
The results 1. and 2. are independent of the actin content of actomyosin; accordingly, they hold true for pure myosin too.
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4.
Natural tropomyosin increases the calcium content of actomyosin independent of the free Ca++ at a constant amount; the concentration dependent increment remains in the same order as without tropomyosin.
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5.
In the presence of natural tropomyosin the calcium increment is independent of the magnesium concentration. Magnesium, however, decreases the total amount of bound calcium at all concentrations of free Ca++.
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6.
Bound calcium and bound magnesium are negatively correlated: The sum of bound calcium plus magnesium is 8 μmoles/g actomyosin.
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7.
Consequence: it is myosin that binds the calcium that regulates muscular activity.
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Dancker, P. The binding of calcium and magnesium to actomyosin and its modification by natural troposmyosin. Pflugers Arch. 315, 198–211 (1970). https://doi.org/10.1007/BF00586413
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DOI: https://doi.org/10.1007/BF00586413