Insulin-like growth factor binding proteins of equine serum.

Ligand blotting analysis of serum from the horse using radiolabelled IGF-I revealed a protein at 96 kDa which was not present in serum from goat, cow, sheep, deer or donkey. These latter species all displayed five labelled bands in the range 24 to 41 kDa. Conversely, these were only weakly labelled in serum from the horse. Size exclusion chromatography of horse serum pre-incubated with radiolabelled IGF-I revealed reduced binding in the 130-kDa peak compared with goat plasma, and ligand blotting analysis indicated the 96-kDa protein was present in this peak. The 96-kDa protein from horse serum binds IGF-I and IGF-II specifically and appears to be unique to this species. The nature of this protein is at present unknown.