Subunit‐specific phosphorylation of pyruvate kinase in medullary thyroid carcinomas of the rat

Pyruvate kinase from anaplastic medullary thyroid carcinomas contains predominantly K-type subunits, whereas pyruvate kinase from differentiated medullary thyroid carcinomas consist of M- and K-type subunits in about equal proportion. In order to analyse the incorporation of phosphate in the respective isozymes after endogenous phosphorylation of cytosolic extracts with [32P]ATP, homotetrameric isozymes as well as heterotetrameric hybrids of differentiated0 tumors were resolved by affinity chromatography on Blue-Sepharose CL-6B and, if necessary, further purified by immunoprecipitation. SDS-polyacrylamide gel electrophoresis of purified isozymes and subsequent autoradiography showed the incorporation of phosphate in the K4-type isozymes, but not in the other isozymes. The phosphorylation appeared to be cAMP-independent and occurred on a serine residue.