Identification of circulating growth hormone-binding proteins in domestic poultry: an initial characterization

Multiple growth hormone (GH)-binding proteins (GHBPs) were identified in serum and plasma samples from domestic chickens and turkeys. Proteins were separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis on 10% acrylamide, 2.7% bis discontinuous gels under reducing conditions and electrotransferred to nitrocellulose paper. Western blots were incubated with 125I-labelled recombinant chicken GH (cGH) or bovine GH and GHBPs visualized by means of autoradiography. In fresh samples (less than 2 h from collection to gel electrophoresis), multiple minor high Mr bands were evident between approximately 72,000 and 175,000. Two major bands were observed at approximately 69,500 and 27,500. The latter is consistent with previous reports for the rat and mouse serum GHBPs based on nucleotide sequence analysis. The minor bands were essentially undetectable after storage at -25 degrees C for several months, and an additional major band at Mr approximately 52,500 appeared. The Mr-69,500 major protein contained N-linked carbohydrate, as determined by a reduction in molecular size by treatment with peptide N-glycosidase F. Binding of 125I-labelled GH was partially inhibited by co-incubation with 50 micrograms unlabelled pituitary-derived cGH/ml and excess unlabelled porcine GH as well as ovine prolactin, but not by bovine insulin. Non-specific binding of 125I-labelled GH by serum albumin was also observed. A comparison was made between these GHBPs and the hepatic GH receptor (e.g. molecular weight estimates, affinity for homologous versus heterologous GHs, cross-reactivity with prolactin, presence of N-linked carbohydrate). The origin and relationship among the various molecular weight species of GHBPs identified, and their potential role in regulation of the biological activity of GH in birds, remain to be determined.