Reconstitution of Heavy Chain and Light Chain 1 in Cardiac Subfragment-1 from Hyperthyroid and Euthyroid Rabbit Hearts

Abstract

It is now established that cardiac myosin from hyperthyroid rabbit hearts (TXM) exhibits high Ca²⁺ ATPase activity. The high Ca²⁺ ATPase activity of TXM was completely retained in cardiac myosin subfragment-1 (S-l) (1.33 ± 0.04 (μmol Pi/mg per min; euthyroid, 0.51 ± 0.04). Cardiac S-1 from hyperthyroid and euthyroid rabbits (TXS-1 and NS-1) had the same pattern in SDS-polyacrylamide gel electrophoresis. The possible influence of heavy and light chains of TXM on increasing the ATPase activity was examined by reconstitution in the S-l preparation. Crosswise reconstitution was performed using cardiac S-l heavy chain (90,000 daltons) and light chain 1 (LCI) (27,000 daltons) from hyperthyroid and euthyroid hearts. Reconstitution was verified by using radiolabeled LCI. More than 95% of S-l was recovered with full ATPase activity. When TXS-1 was reconstituted with LCI from euthyroid hearts, the reconstituted molecule retained high ATPase activity. On the other hand, NS-1 reconstituted with LCI from hyperthyroid hearts failed to increase the ATPase activity. The ATPase activity of S-1 was determined by the source of the heavy chain. These results suggest that the high Ca²⁺ ATPase activity of cardiac myosin and S-l from hyperthyroid animals arises from the molecular alteration of the heavy chain induced by thyroxine administration.