High molecular weight forms of insulin-like growth factor II and its binding protein identified by protein immunoblotting

Insulin-like growth factor (IGF)II is a mitogenic polypeptide which circulates in association with a binding protein(s). Immunoblotting studies were performed in human serum and indicate that:(1)a approximately 200 kDa covalently-linked IGF-II/binding protein complex is antigenically related to the 30 kDa binding protein, (2)IGF-II prohormone is associated with this complex, and (3)a major portion of the IGF-II prohormone immunoreactivity in human serum is present in fractions which would not be detected by standard radioimmunoassay methods. Our data provide insight regarding the inter-relationships of IGF-II and its binding protein, and direct evidence for the presence of IGF-II prohormone in human serum.