The influence of adenosine triphosphate, adenosine diphosphate and cytochalasin B on nucleotide exchange of F-actin. Evidence that treadmilling is not involved

14C]ATP-containing G-actin was polymerized to 14C]ADP-containing F-actin. The exchange of the filament-bound nucleotide with nucleotides of the medium was investigated by measuring the loss of radioactivity from the filaments under various conditions. Nucleotide exchange was faster in the presence of ATP than of ADP (this could be observed in the presence of Mg2+ as well as in the presence of Ca2+). Cytochalasin B had a small accelerating effect in the presence of ATP but had no effect in the presence of ADP. The kinetics of exchange remained unchanged when the filaments contained a 'cap' of actin with non-radioactive nucleotides, suggesting that nucleotide exchange was not a property of the filament ends.