Solubility properties of alpha-reduced paramyosin.

It is now believed that the reduced form of alpha-paramyosin is that found in living adductor muscles of molluscs. We have studied the solubility of a preparation of alpha-paramyosin obtained under reducing conditions. In contrast to the solubility profile of beta-paramyosin, the alpha-paramyosin, the alpha-preparation showed a rapid, almost linear decrease in solubility over the ionic strength range 0.35-0.25 at neutral pH. Solubility in this range was further decreased by the presence of physiologically small amounts of calcium ion. Lactate ion, which can accumulate during anaerobic glycolysis in molluscan muscles, also decreases the solubility at a level of 50 mM. In addition, the type of paracrystal formed by alpha-paramyosin differs greatly from those of beta-paramyosin and paracrystal formed in the presence of lactate differs from those formed in buffer solutions. Reduced alpha-paramyosin is more sensitive to the above parameters than the preparations made without reducing agents. Moreover, the pH and ionic strength ranges in which greatest change in solubility behaviour occurs are physiologic, as are the calcium and lactate ion levels effective in increasing intermolecular interactions. A model is proposed for alpha-paramyosin in which the extra 5% presumably removed in beta preparations is a "sticky head" which protrudes from one end of the molecule and confers on it an increased tendency for interaction, particularly at physiological ionic strength. Such molecules would be capable of promoting interactions between thick filaments which contain them, providing a means of accounting for the pH dependent stiffness observed in glycerinated preparations of molluscan catch muscles.