Adenosine Triphosphate Cleavage during the G-actin to F-actin Transformation and the Binding of Adenosine Diphosphate to F-actin.

Since the discovery by Straub and Feuer (1) and by Laki, Bowen, and Clark (2) that adenosine triphosphate bound to Gactin is transformed to adenosine diphosphate and inorganic phosphate during polymerization of actin, it has become increasingly clear that the chemical changes in the nucleotide are related to the change in the physical state of the protein. Barany, 13iro, Molnar, and Straub have shown that highly purified actin preparations, free of any enzyme which would use ATP, ADP, or AMP as a substrate, still hydrolyze ATP during polymerization, and thus support the original idea that the ATP to ADP transformation is related to the globular to fibrous transformation of the actin protein itself (1). Mommaerts (4) was the first to show that the AD1 formed during polymerization remains bound to F-actin, and Ulbrecht et al. (5), while extending Mommaert’s finding on exhaustively purified actin preparations, have shown that the Pi formed during polymerization is not bound to F-actin. The stoichiometry of the cleavage and the tightness of binding of the ADP lead inevitably to questions regarding the position of bond breaking during the hydrolysis and the nature of the forces involved in the tight binding of ADP to F-actin. To aid in clarifying these problems, this study with 180 isotope was initiated.