Summary
The contractile ATPase of artificial actomyosin (reconstituted from its purified components actin and myosin) is fully active even at low concentrations (∼10−9 M) of ionized calcium. Natural tropomyosin, the physiological inhibitor, however, inhibits the contractile ATPase under these conditions. Natural tropomyosin which is not further purified by ammonium sulfate precipitation inhibits also at higher concentrations (10−6 M) of Ca++.
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2.
Natural tropomyosin which is prepared by ammonium sulfate precipitation inhibits at high concentrations of Ca++ if it is briefly digested by trypsin.
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3.
Natural tropomyosin influences the interaction between actomyosin and MgATP in such a way that the affinity of actomyosin for MgATP increases, the maximal splitting rate, however, decreases. Hence, substrate inhibition can already be seen at concentrations that are otherwise optimal.
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4.
Natural tropomyosin affects the ITPase of actomyosin in a similar manner if the inhibitory activity of tropomyosin is mediated by the concentration of free Ca++. The ITPase activity of the actomyosin-tropomyosin complex, however, is even at high concentrations of free Ca++ lower than the ITPase activity of pure actomyosin.
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Dancker, P. Natural tropomyosin as a physiological inhibitor of the contractile activity of actomyosin. Pflugers Arch. 315, 187–197 (1970). https://doi.org/10.1007/BF00586412
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DOI: https://doi.org/10.1007/BF00586412