| Abstract: | Lanthanides are often considered to act as calcium analogues in biological systems. Activation profiles have been obtained as a function of terbium (Tb(III)), gadolinium (Gd(III)), and europium (Eu(III)) concentration, for the actin-activated ATPase of fully regulated myosins from both thick and thin filament regulated muscles. Scallop adductor myosin, a regulatory myosin, shows full apparent activation by free lanthanide. Activity declines rapidly with increasing lanthanide concentration reaching basal levels at 100 microM lanthanide. Rabbit skeletal muscle myosin, in the presence of rabbit skeletal troponin:tropomyosin, also shows full apparent activation by free lanthanide. However, contrary to expectation, lanthanides do not compete with calcium for the calcium-specific site of scallop myosin and therefore do not function as calcium analogues in this system. The activation curve is shown to be an artifact arising from the release of trace calcium from ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N-tetraacetic acid during lanthanide titration. Although the results with troponin:tropomyosin are necessarily more complex, the ability of lanthanides to turn on troponin:tropomyosin is brought into question. |
