pH titrations of molluscan paramyosin at two different ionic strengths.

Paramyosin extracted from the adductor muscle of Mercenaria mercenaria, the chowder clam, was titrated both in 0.3 M KCl and in 1 mM KCl. Both the presumed native form of the molecule, acid-R-paramyosin, and a slightly degraded form, beta-paramyosin, were studied. Titrations of both types of paramyosin were similar in 1 mM k+, except that the native paramyosin is more highly charged at pH 3.2 than beta-paramyosin, as postulated previously (DeLaney and Krause, 1976, Macromolecules, 9:455), and that more groups titrate on the native molecule than on beta-paramyosin, both between pH 3.2 and 3.3 and between pH 3.2 and 10. Titrations in 0.30 M KCl, unlike those in 1 mM K, depended on starting pH; long term exposure to alkali solutions during dialysis, previously shown to cause partial dephosphorylation of paramyosin (Cooley et al., 1979, J. Biol. Chem., 254:2195), apparently also leads to a change in intermolecular interactions sufficient to cause changes in the titration curves in 0.30 M KCl but not in 1 mM K+.