Isolation and characterization of circulating fragments of the insulin-like growth factor binding protein-3

Abstract

Proteolysis of insulin-like growth factor binding protein-3 (IGFBP-3), the major carrier of IGFs in the circulation, is an essential mechanism to regulate IGF bioavailability. To analyze naturally occurring IGFBP-3 fragments a peptide library established from human hemofiltrate was screened. Three IGFBP-3 fragments were detected with apparent molecular masses of 34, 16, and 11 kDa. Mass spectrometric and sequence analysis identified the 16 and 11 kDa peptides as glycosylated and non-glycosylated N-terminal fragments spanning residues Gly 1–Ala 98 of IGFBP-3. Both the circulating forms and those secreted from IGFBP-3 1–98 overexpressing cells bound IGF. Additionally, two smaller fragments (IGFBP-3 139–157 and IGFBP-3 139–159) were identified in the hemofiltrate. The data indicate that proteolysis of circulating IGFBP-3 occurs in the variable domain at residues alanine 98, phenylalanine 138, glutamine 157, and tyrosine 159.