Chicken insulin-like growth factor-I: amino acid sequence, radioimmunoassay, and plasma levels between strains and during growth.

Insulin-like growth factor-I (IGF-I) has been purified from chicken serum and sequenced. The peptide has eight amino acid substitutions when compared with human IGF-I: serine26, leucine38, histidine39, histidine40, lysine41, glutamine50, isoleucine64, and proline67. Chicken IGF-I (cIGF-I) has been measured using a radioimmunoassay with a human IGF-I (hIGF-I) standard and an antibody raised against hIGF-I. In this assay the cross-reactivity of cIGF-I was approximately 50% with respect to hIGF-I and the cross-reactivity of chicken IGF-II was 1.7% with respect to chicken IGF-I. To determine whether binding proteins in chicken plasma can artifactually interfere with IGF-I measurements as they do in mammals, chicken plasma was fractionated by molecular sieve chromatography at acid pH. When the fractions corresponding to the binding protein region were included in the IGF-I radioimmunoassay, essentially no apparent IGF-I was detected, indicating that the binding proteins did not interfere. This result, together with the finding that IGF-I in acid-ethanol extracts of chicken plasma produced parallel dose-response curves to pure cIGF-I and hIGF-I, allows the reliable measurement of cIGF-I in such extracts. The concentrations of IGF-I in plasma from male birds increased two- to threefold between 1 and 7 weeks after hatching to achieve 30-45 ng/ml. Smaller increases were found in female chickens from a higher value at 1 week. No diurnal pattern of IGF-I levels could be detected. In 4-week-old birds, the plasma concentration of the peptide fell from nearly 40 to 15 ng/ml after 24 hr of starvation and to 9 ng/ml 20 hr later. These effects are very similar to those described for mammals and strongly suggest that the regulation of IGF-I is conserved during evolution, notwithstanding the lower plasma concentrations of the growth factor in chickens.