| 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 (LC2) of myosin. LC2 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 LC2 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 LC2 does not affect the Ca2+- and K+ (EDTA)-ATPase activities of these myosins. The actin-activated myosin Mg2+-ATPase activities of intact and LC2-deficient thyrotoxic myosin were 0.18 +/- 0.03 and 0.36 +/- 0.03 mumol Pi/mg per min, respectively, whereas the actin-activated myosin Mg2+-ATPase activities of intact and LC2-deficient normal myosin were 0.12 +/- 0.02 and 0.18 +/- 0.03 mumol Pi/mg per min, respectively. Thus, removal of LC2 increases the actin-activated myosin Mg2+-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 Mg2+-ATPase activity as a result of LC2 removal is 2-fold greater in thyrotoxic myosin than that obtained for normal myosin. These results suggest that LC2 does not influence the increased actomyosin ATPase activity of thyrotoxic myosin and that potentiation of actomyosin ATPase following LC2 removal may depend on the variations of the heavy-chain domain where LC2 interacts. |
