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

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Abstract
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.