The relative affinities of nucleotides to G-actin and their effects

1. If, in solutions of stable G-actin, ATP is replaced by ITP, UTP, CTP, GTP, ADP, or IDP in concentrations ⩾10−4 M, polymerization takes place and is complete in about 5 min. If the nucleotides are present in the very low concentration of about 2·10−5 M (together with a G-actin concentration of 2·10−5 M), the polymerization of G-actin-ADP and G-actin-IDP is not quite complete in contrast to the polymerization of G-actin-ATP and G-actin-ITP. 2. 2. During the polymerization 1 mole phosphate per mole G-actin is released from all nucleoside triphosphates concerned. 3. 3. The intrinsic viscosities of the various F-actin-nucleotide compounds are identical. Furthermore all actin-nucleotide compounds concerned associated with L-myosin to actomyosins of normal intrinsic viscosity and of normal AtP sensitivity. 4. 4. If the affinities of various nucleotides to G-actin are compred quantitatively, the following relative affinity constants are observed: ATP  1 > ITP  0.1 > UTP and ADP approx. 0.017–0.020 > CTP, GTP, and IDP approx. 0.0017–0.0020. 5. 5. The affinities of ATP as well as of ITP to G-actin are about 50 times higher than the affinities of the corresponding diphosphates. The affinity of purine-nucleotide triphosphates is highest with the NH2-group in the 6-position, 10 times lower in the absence of the NH2-group, and lowest with the NH2-group in the 2-position. The affinity of the pyrimidine-nucleotide triphosphaets is small and almost independent of the positino of the amino group