Identification and characteristics of a 37,000 M(r) insulin-like growth factor binding protein in canine serum

Insulin-like growth factors (IGFs) are potent stimulators of cellular growth and their half-life and biological activity are regulated by specific IGF binding proteins (IGFBPs). Western ligand blots of non-reduced human, bovine, ovine and porcine sera reveal an IGFBP-2 band at approximately 34,000 M(r). However, canine sera appear to contain a unique 37,000 M(r) IGFBP and lack the 34,000 M(r) IGFBP-2 band. In order to identify and characterize the 37,000 M(r) IGFBP, adult canine serum was subjected to non-reducing SDS polyacrylamide gel electrophoresis (PAGE), transferred to nitrocellulose paper, followed by [125I]-IGF-1 ligand blotting or immunoblotting with commercially available IGFBP antibodies. The 37,000 M(r) canine IGFBP reacted with an anti-IGFBP-2 antibody indicating that it is a canine analogue of IGFBP-2. However, the large difference in apparent molecular size indicates that this is a unique molecular form of IGFBP-2. N- or O-glycanase treatment of canine sera did not alter the molecular size of canine IGFBP-2 indicating that it is not a glycosylated variant of the IGFBP. Subjecting canine sera to reducing SDS-PAGE followed by anti-IGFBP-2 western immunoblotting revealed that the actual molecular weight of the canine IGFBP-2 is similar to that of reduced IGFBP-2 from other species indicating similar peptide lengths. Thus, the increased non-reduced size of the canine 37,000 M(r) IGFBP-2 is possibly due to a unique secondary structure.