Fig. 4.

Proteolytic activity of PAPP-A in humans. Relationship between PAPP-A, IGF and IGFBP-4. a Heparin sulfate substituted receptors at the target cell immobilizes PAPP-A in a position close to the IGF receptor. PAPP-A than cleaves IGF from IGFBP-4 in close proximity of IGF receptor. Free IGF than binds IGF receptor there by inducing cell proliferation, trophoblast invasion, glucose and amino acid transport into the placenta. b GAG chain of ProMBP binds to PAPP-A domain there by releasing PAPP-A from cell surface. ProMBP inactivates the proteolytic activity of PAPP-A