The ovine pancreatic protein which binds insulin-like growth factor binding protein-3 is procarboxypeptidase A.
Insulin-like growth factor binding protein-3 (IGFBP-3) is known to modulate the actions of insulin-like growth factors (IGF)-I and -II at the level of the cell. Proposed mechanisms include association of IGFBP-3 with cell surface proteoglycan, with cell surface binding proteins, proteolysis and/or internalization of IGFBP-3. In previous studies we have characterized a protein of 40 kDa in extracts of ovine pancreas and muscle which binds IGFBP-3 on ligand blot analyses. This paper reports the identity of the pancreatic species as procarboxypeptidase A (peptidyl-L-amino acid hydrolase, E.C. 3.4.17.1; proCPA). Identity was established by amino terminal sequence analysis, binding studies with pure bovine carboxypeptidase A (CPA) and observations that the binding activity was present in pancreatic secretions consistent with the role of proCPA as a secretory zymogen. The binding activity was inhibited by unlabelled IGFBP-3 at high doses (10 micrograms/ml) and reduced but not abolished by preincubation of 125I-IGFBP-3 with excess IGF-I. Digestion of 125I-IGFBP-3 with mature CPA produced a 26 kDa product. Modification of IGFBP-3 by CPA or binding to proCPA may provide a mechanism for modulation of IGFBP activity and hence IGF action.
Fowke
P J
PJ
AgResearch, Ruakura Agricultural Research Centre, Hamilton, New Zealand.
Hodgkinson
S C
SC
D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D002268:Carboxypeptidases / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D043422:Carboxypeptidases A; D004792:Enzyme Precursors / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D005260:Female; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000378:metabolism*; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000378:metabolism*; D010179:Pancreas / Q000378:metabolism*; D011485:Protein Binding; D012756:Sheep / Q000378:metabolism*
D004792:Enzyme Precursors; D018972:Insulin-Like Growth Factor Binding Protein 3; D002268:Carboxypeptidases; D043422:Carboxypeptidases A