Effects of Salts of Monovalent Ions on the Adenosine Triphosphatase Activities of Myosin

Abstract In the absence of divalent cations, the adenosine triphosphatase activity of myosin is activated by salts of K+, Rb+, or NH4+ and the extent of activation depends both on the cation and the anion present. In contrast to the specific requirement for the monovalent cations K+, Rb+, or NH4+, all monovalent anions tested increase activity and differ only in the concentration needed to produce a given degree of activation. The effectiveness of various anions in stimulating activity follows the Hofmeister series. Salts of nonactivating cations such as tetramethylammonium chloride increase ATPase activity if an activating monovalent cation is present. The effect of salts on activity is the same in the presence or absence of EDTA, and this compound does not alter the loss of K+-activated ATPase produced by N-ethylmaleimide, indicating that the ATPase activity in the presence of EDTA is actually a K+-activated ATPase. Addition of Ca++ reverses both the effect of salts and the effect of thiol reagents on ATPase activity. Thus the effects of salts and the effects of thiol reagents on enzymic activity depend qualitatively on the nature of the activating cation. The effect of salts on the Ca++-activated ATPase of sulfhydryl-modified myosin depends on the reagent used for modification. The activity of Ca++-activated ATPase of N-ethylmaleimide-modified myosin is increased by monovalent salts, the degree of activation varying with the anion but being the same with all monovalent cations. Modification with 5,5'-dithiobis(2-nitrobenzoate) or p-mercuribenzoate results in a myosin which is inhibited by salts. In either case, whether salts inhibit or activate, their effectiveness follows the Hofmeister anionic series.