Behavior of divalent cations and nucleotides bound to F-actin

Abstract

1. 1. Nucleotides and divalent cations bound to F-actin were found to be slowly exchanged in the absence of any agitation such as sonic vibration; the half-life time, τ 1 2 , was about 5 h at 37°. 2. 2. The rate of exchange of divalent cation (Ca 2+) was always a little faster than that of nucleotide (ADP); however, the general manner of the exchange of divalent cations was similar to that of nucleotides. 3. 3. The rate of exchange was almost independent of protein concentration. High temperature or high pH increased the rate of exchange. Activation enthalpies for the exchange of ADP and Ca 2+ were both about 25 kcal/mole at neutral pH. 4. 4. The release of divalent cations or nucleotides occurred in divalent-cation- or nucleotide-free solvent. The incorporation of divalent cations and nucleotides into divalent-cation- and nucleotide-free F-actin was examined, and their binding constants were estimated. 5. 5. In the absence of ATP, the rate of exchange was decreased by myosin, H-meromyosin and tropomyosin. A large increase of Ca 2+ and ADP exchange was observed with superprecipitation. 6. 6. The mechanism of exchange or release is discussed on the basis of two models: the cycle of partial destruction of the F-actin structure, and the G-F equilibrium cycle at the peripheral region of F-actin.