This much is widely accepted but it is sometimes suggested that the observed rapid disassembly kinetics of aged filaments results from "high energy" ADP-actin being trapped in the bulk of the filament. Firstly, ATP-actin hydrolysis does not lead directly to ADP-actin; the initial hydrolysis product is ADP-Piactin, whose measured association constant to the filament is very similar to that of ATP-actinthe hydrolysis of ATP does not significantly alter the binding of the monomer unit to the filament. Secondly, Pi dissociation is an extremely slow process (t1/2 ~ 6 min) and so ADP-Pi-actin units in the bulk of the filament are kinetically long lived polymerization intermediates. Thirdly, the eventual dissociation of Pi produces an ADPactin unit embedded in the filament but it should be noted that ADP-actin itself assembles into filaments, albeit with a lower critical concentrationthis is not an unstable arrangement.