Muscle actin has been found to polymerize reversibly upon addition of low concentrations of polyamines. This polymerization, studied by centrifugation, has shown a linear relationship between the actin polymerization yield and the chain lingth of the polyamine. Aming the biological polyamines tested, spermidine and spermine are the most efficient.The polymerization of actin can also be induced by the corresponding mono of diguanidine derivatives of these polyamines but monoamines of amino acids are inactive at the same concencentration.The transformation of actin from a globular form upon addition of spermidine is also demonstrated by the changes in the near‐ultraviolet circular dichroic spectrum of this protein.Moreover, the polyamine‐induced F‐actin exhibits the same properties as the salt‐induced F‐actin: it strongly activates the Mg2+‐ATPase of myosin, its specific viscosity is enhanced to the same extent and electron micrographs show homogeneous thin filaments.
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