Polymerization of actin in the absence and presence of cytochalasin B: problems of determining "critical concentration".

Various concentrations of actin (0.3 or 1 mM MgCl2, 1 mM ATP, 1 mM EGTA) reached their final degree of polymerization (measured by a pyrene dye attached to actin) earlier in the presence of cytochalasin B than in its absence. The curves relating concentrations of polymeric F-actin to total actin concentration were under these conditions highly nonlinear making an unambiguous extrapolation to zero F-actin concentration (to deduce the "critical concentration" of actin polymerization) impossible. The concentration of actin, above which polymerization occurred, was unaltered by cytochalasin B (although for reasons not yet understood the specific fluorescence intensity of polymerized actin was lower in the presence of cytochalasin B than in its absence). The results show that a distinct "critical concentration" of actin polymerization must not always be well defined.