Thyroid hormones inhibit platelet function and myosin light chain kinase

We examined the extranuclear effects of thyroid hormones on human platelets. Pretreatment with DL-thyroxine or DL-triiodothyronine inhibited collagen-induced aggregation, in a dose-dependent manner, but other derivatives of thyroid hormone had no significant effects. In contrast to collagen, 12-O-tetradecanoylphorbol-13-acetate-induced aggregation was not affected by thyroid hormones at the same concentration range. Thyroxine also inhibited the release of [14C] serotonin from collagen-stimulated platelets, with a marked reduction in the phosphorylation of 20,000-dalton protein. Thyroxine and triiodothyronine had inhibitory effects on myosin light chain kinase purified from human platelets and inhibited more markedly the myosin light chain kinase than protein kinase C (Ca2+/phospholipid-dependent enzyme) and cAMP-dependent protein kinase. In addition, L-thyroxine behaved as a competitive inhibitor of myosin light chain kinase toward calmodulin, and the Ki value was calculated to be 2.6 microM. To determine whether or not thyroxine directly binds myosin light chain kinase, we prepared an affinity column, using L-thyroxine as the ligand. Myosin light chain kinase was selectively bound to the column while calmodulin passed through. We also designed a procedure for the purification of myosin light chain kinase from human platelets, using L-thyroxine-affinity chromatography. A markedly increased purification was thus achieved, and DEAE-cellulose and L-thyroxine-affinity chromatography were made feasible. These results suggest that thyroxine can serve as a pharmacological tool for elucidating the biological significance of myosin light chain kinase-mediated reactions and is a pertinent ligand which can be used to purify myosin light chain kinase from platelets as a substitute for calmodulin.