This investigation describes the presence of insulinlike growth factor-I (IGF-I) binding proteins in chicken serum. Whole blood was collected from broiler chickens of 7–9 weeks of age and analysed for binding proteins after gel permeation chromatography under both neutral and acidic conditions, and by polyacrylamide gel electrophoresis in the presence of 12·5% sodium dodecyl sulphate (SDS-PAGE).
When serum was chromatographed under neutral conditions, about 70% of the IGF-I immunoreactivity was associated with a large protein complex (Mr=150 000) and 20–25% was associated with an intermediate-sized protein complex (Mr = 45 000). Up to 6% of the serum IGF-I immunoreactivity was eluted in a fraction which corresponded to an Mr of about 7500 and was presumably free IGF-I. Chromatography under acidic conditions dissociated the IGF-I/protein complexes and revealed the presence of an acid-stable binding protein (Mr = 50 000–60 000).
After analysis of serum by SDS-PAGE, three monomeric IGF-I binding proteins (Mr = 28 800, 33 200 and 40 700) were detected. The largest monomer (Mr = 40 700) is probably the binding protein component of the intermediate-sized IGF-I/protein complex. The relationship between the other monomers and both the large IGF-I/protein complex and the acid-stable binding protein is not known.
Although the pattern of binding proteins in chicken serum is similar to that observed in mammals, a major difference is the presence of up to 6% of the serum IGF-I immunoreactivity in an unbound form.