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The Effect of Structure-disrupting Ions on the Activity of Myosin and Other Enzymes

From the ¹ From the Cardiovascular Research Institute, University of California Medical Center, San Francisco, California 94122

Neutral salts in high concentrations (0.3 to 3.0 m) inhibit the activity of myosin nucleoside triphosphatase, trypsin, lactate dehydrogenase, estradiol-17ß dehydrogenase, and fumarase in an order of increasing effectiveness for anions: CH₃COO^- < Cl^- < NO₃^- < Br^- < I^- < SCN^- < ClO₄^-, and in an order of increasing effectiveness for cations: (CH₃)₄N⁺ < Cs⁺ < K⁺ < Na⁺ < Li⁺. These orders, which are maintained in all of the salt concentrations tested, and with the widely different enzymes employed, are also the orders in which these anions and cations disrupt the organized structure of diverse macromolecules as established by physical methods. Furthermore, the anions listed above disrupt the structure of myosin with a similar order of effectiveness.

Because of the parallelism between general structure-disrupting effectiveness and activity-inhibiting effectiveness, it is considered that the various ions inhibit activity by disrupting structure. Given such a conclusion, it follows that the salt sensitivity of structurally different forms of the same enzyme should differ. This is the case when native and p-chloromercuribenzoate-modified myosin are compared. The adenosine triphosphatase activity of native and p-chloromercuribenzoate-modified myosin are similar when assays are carried out in the absence of salts. The p-chloromercuribenzoate-modified form, being less sensitive to salt inhibition, thus appears to be "activated" when assayed in 0.6 m KCl (or similar concentrations of the several salts studied) and compared with native myosin in 0.6 m KCl.

These observations provide additional evidence that the and ß forms of myosin correspond to different conformations despite the fact that optical rotatory dispersion cannot clearly indicate a difference in structure. They further suggest that salt sensitivity as evaluated by the activity parameter can be used as an extremely sensitive if only semiquantitative method for the evaluation of conformational change.

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