Partial purification and characterization of myosin light chain kinase from bovine thyroid gland

Myosin light chain kinase has been purified approximately 1700-fold from bovine thyroid gland, using Affigel blue column chromatography, ammonium sulfate fractionation (0-60% saturation), Sepharose 6B gel filtration, and calmodulin-Sepharose affinity column chromatography. This partially purified kinase was Ca2+ and calmodulin dependent for its activity and specifically phosphorylated the 20,000-dalton light chain of chicken gizzard myosin. At higher CaCl2 concentration, thyroid myosin light chain kinase demonstrated a novel inhibition in the presence of calmodulin. The mechanism of this Ca2+-dependent inhibition is not clear at present. The existence of myosin light chain kinase in thyroid gland provides additional support for the role of the contractile system in the secretion of thyroid hormones.