Bivalve Tropomyosins Exhibit Strong Inhibition of Actomyosin Mg-ATPase and High Viscosity

Tropomyosins were isolated from striated and smooth adductor muscles of akazara scallop Chlamys nipponensis akazara and ezo-giant scallop Patinopecten yessoensis and smooth adductor muscle of surf clam Spisula sacharinensis. These bivalve tropomyosins inhibited Mg-ATPase activity of rabbit reconstituted actomyosin to 15% of original activity, i.e., approximately 1.4 times stronger than rabbit and carp tropomyosins did at a weight ratio of 2:1 for F-actin:tropomyosin. On the other hand, the relative viscosities in the absence of KCl were 3.5-6 times higher than those ofrabbit and carp tropomyosins. The intrinsic viscosities were, however, comparable to that of rabbit tropomyosin, i.e., 0.5 dl/g for bivalve tropomyosins and 0.4 dl/g for rabbit tropomyosin. These viscosities suggest that the bivalve tropomyosins tend to form a highly polymerized form compared with rabbit tropomyosin. Circular dichroism spectra and amino acid compositions of the bivalve tropomyosins are similar to one another and also to rabbit and carp tropomyosins. C-terminal three amino acids of bivalve tropomyosins were sequenced as-Ala-Gly-Tyr (scallops) and-Gly-Tyr-Thr (surf clam), whichare quite different from-Thr-Ser-Ile and Thr-Ser-Leu of rabbit α- and β-tropomyosin, respectively.