The optical rotatory dispersion of myosin A. I. Effect of inorganic salt.

iMany reports have been published on the effect of inorganic salts, especially LiBr, on the secondary structure of protein and synthetic polypeptide. Harrington and Scheilman (1) worked with solutions of silk fibroin, serum albumin, clupein, and ribonuclease and deduced that LiBr increases the stability of intramolecular hydrogen bonds. Perlmann (2) observed the change in dispersion constant, X,, and the decrease in enzymic activity of pepsin in LiBr solution. Mandelkern and Roberts (3) have recently reported that the thermodynamic stability of the a-helix in ribonuclease decreases with increase in concentration of LiBr. Haly and Snaith (4) observed that the a-helix in wool fiber keratin is destroyed in a narrow range of concentration of LiBr. Furthermore, Laki and Bowen (5) reported shortening of glycerol-washed muscle and myosin B fibers on adding KI or KSCN, which suggests that the secondary structure of muscle structural protein changes on the addition of these salts. The changes in the molecular structure of myosin A and in its enzymic activity caused by salt, however, have not been reported yet. Therefore, we have made a detailed investigation of the effect of inorganic salts, such as KCI, LiBr, LiCI, KSCN, and KI, on adenosine triphosphatase (ATPase) activity, optical rotatory power, and viscosity of myosin A and have obtained the following results. (a) The a-helical content and reduced viscosity of myosin A decrease sharply and reversibly in a narrow range of concentration of LiBr, LiCl, KI, or KSCN. The effect of temperature and concentration of LiBr on the helical content obeys the Schellman equation (6). (b) ATPase activity decreases irreversibly in a range of low concentration of these salts, although the helical content and reduced viscosity remain almost unaffected. (c) In KC1 solution, however, the helical content and reduced viscosity are almost unchanged, and ATPase activity is decreased reversibly by increasing the concentration.