Characterization and identification of a novel 37 kda insulinlike 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 kDa. However, canine sera appears to contain a unique 37 kDa IGFBP and lack the 34 kDa IGFBP-2 band. In order to identify and characterize this 37 kDa IGFBP, adult canine serum was subjected to non-reducing SDS polyacrylamide gel electrophoresis (PAGE), transferred to nitrocellulose paper and followed by I2II-IGF-1 ligand blotting or imrnunoblotting with commercially available IGFBP antibodies. The 37 kDa canine IGFBP reacted with an anti-IGFBP-2 antibody indicating that it is a canine analog of IGFBP-2. However, the large difference in apparent molecular si; indicates that this is a unique molecular form of IGFBP-2. N- or 0-grycanase treatment of canine sera did not alter the molecular size of canine IGFBP-2 indicating that it is not a glycosylated variant. 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 si of the canine 37 kDa IGFBP-2 is due to a unique secondary structure involving the number or placement of cysteine residues which may also alter its biological activity.