Effects of removal of light chain 2 on the ATPase activities of cardiac myosin from normal and thyrotoxic rabbits

Abstract

Steady-state ATPase activities of cardiac myosin from thyroxine-treated rabbit hearts have been determined before and after removal of the 18-kDa light-chain subunit (LC 2) of myosin. LC 2 was selectively removed from myosin by treatment with a myofibrillar protease according to the method of Kuo and Bhan (Biochem. Biophys. Res. Commun. 92, 570–576 (1980)). The effects of removal of LC 2 on the enzymatic properties of thyrotoxic myosin were compared with the results obtained for cardiac myosin from normal rabbits by parallel studies. It has been found that removal of LC 2 does not affect the Ca 2+- and K + (EDTA)-ATPase activities of these myosins. The actin-activated myosin Mg 2+-ATPase activities of intact and LC 2-deficient thyrotoxic myosin were 0.18 ± 0.03 and 0.36 ± 0.03 μmol P i/mg per min, respectively, whereas the actinactivated myosin Mg 2+-ATPase activities of intact and LC 2-deficient normal myosin were 0.12 ± 0.02 and 0.18 ± 0.03 μmol P i/mg per min, respectively. Thus, removal of LC 2 increases the actin-activated myosin Mg 2+-ATPase activity of thyrotoxic myosin by 100%, and the same activity is increased about 50% for normal myosin, indicating that the degree of potentiation of actin-activated myosin Mg 2+-ATPase activity as a result of LC 2 removal is 2-fold greater in thyrotoxic myosin than that obtained for normal myosin. These results suggest that LC 2 does not influence the increased actomyosin ATPase activity of thyrotoxic myosin and that potentiation of actomyosin ATPase following LC 2 removal may depend on the variations of the heavy-chain domain where LC 2 interacts.