Solubility properties of α-reduced paramyosin

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Abstract
The reduced form of .alpha.-paramyosin is that found in the living adductor muscles of mollusks. The solubility of a preparation of .alpha.-paramyosin obtained under reducing conditions, [from adductor muscles of Mercenaria mercenaria] was studied. In contrast to the solubility profile of .beta.-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 Ca2+ ion. Lactate ion, which can accumulate during anaerobic glycolysis in molluscan muscles, also decreases the solubility at a level of 50 mM. The type of paracrystal formed by .alpha.-paramyosin differed greatly from those of .beta.-paramyosin, and paracrystal formed in the presence of lactate differs from those formed in buffer solutions. Reduced .alpha.-paramyosin was more sensitive to these parameters than preparations made without reducing agents. The pH and ionic strength ranges in which greatest change in solubility behavior occurred were physiological as were the Ca2+ and lactate ion levels effective in increasing intermolecular interactions. A model was proposed for .alpha.-paramyosin in which the extra 5% presumably removed in .beta. preparations was a "sticky head" which protruded from one end of the molecule and conferred on it an increased tendency for interaction, particularly at physiological ionic strengths. Such molecules would be capable of promoting interactions between thick filaments which contained them, providing a means of accounting for the pH dependent stiffness observed in glycerinated preparations of molluscan catch muscles.