The binding of calcium and magnesium to actomyosin and its modification by natural troposmyosin

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. 2. In the presence of magnesium, however, no calcium is taken up by artificial actomyosin. 3. The results 1. and 2. are independent of the actin content of actomyosin; accordingly, they hold true for pure myosin too. 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. 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++. 6. Bound calcium and bound magnesium are negatively correlated: The sum of bound calcium plus magnesium is 8 μmoles/g actomyosin. 7. Consequence: it is myosin that binds the calcium that regulates muscular activity