[Cyclic-AMP dependent protein kinase activity in the soluble thyrotropin receptor complex (author's transl)]

The relationship between the thyrotropin (TSH) receptor and adenosine 3':5'-monophosphate (cyclic-AMP) dependent protein kinase activity in bovine thyroid plasma membrane fraction was investigated. After solubilization of thyroid plasma membranes, the molecular sizes of TSH binding protein and protein kinase activities were compared using the sucrose density gradient technique. Cyclic-AMP dependent protein kinase activity was present in a soluble thyrotropin receptor fraction. The Km of this enzyme was 2.2 x 10(-6) M for casein substrate in the absence or presence of 10(-5) M cyclic-AMP. A [3H]-cyclic-AMP binding protein was also found in this fraction. The Ka for cyclic-AMP binding was 0.11 x 10(6) M-1, with 3 nmoles per mg protein of total binding capacity. After fractionation using a continuous sucrose density gradient, one of the several [125I]-bovine TSH binding peaks corresponded to a [3H]-cyclic-AMP binding peak. After fractionation on a sucrose density gradient containing 0.4 M NaCl at pH 6.5, a major peak of protein kinase activity was stimulated by adding 10(-5) M cyclic-AMP. A peak of [3H]-cyclic-AMP binding activity corresponded to the same peak. Protein kinase activity in the receptor fraction was stimulated by adding 6 mg/ml bovine TSH. The soluble TSH receptor fraction also had an adenylate cyclase activity stimulated by TSH. These results suggest that some TSH receptors in thyroid plasma membranes have associated adenylate cyclase activity and cyclic-AMP dependent protein kinase activity. The receptor, cyclase, and kinase activities may exist in a functional primary receptor unit which is a component of thyroid plasma membranes.