Pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growthfactor binding protein (IGFBP)-5 independent of IGF: implications for the mechanism of IGFBP-4 proteolysis by PAPP-A

Pregnancy-associated plasma protein-A (PAPP-A) has recently been identified as the proteinase responsible for cleavage of insulin-like growth factor b inding protein (IGFBP)-4, an inhibitor of IGF action, in several biological fluids. Cleavage of IGFBP-4 by PAPP-A is believed to occur only in the pre sence of IGF. We here report that in addition to IGFBP-4, PAPP-A also cleav es IGFBP-5. Cleavage occurs at one site, between Ser-143 and Lys-144 of IGF BP-5. In the presence of IGF, IGFBP-4 and -5 are cleaved with similar rates by PAPP-A. Interestingly, cleavage of IGFBP-5 by PAPP-A does not require t he presence of IGF, but is slightly inhibited by IGF. These findings have i mplications for the mechanism of proteolysis of IGFBP-4 by PAPP-A, suggesti ng that IGFBP-4 binds IGF, which then becomes a PAPP-A substrate. Using hig hly purified, recombinant proteins, we establish that (1) PAPP-A cleavage o f IGFBP-4 can occur in the absence of IGF, although the rate of hydrolysis is very slow, and (2) IGF is unable to bind to PAPP-A. We thus conclude tha t IGF enhances proteolysis by binding to IGFBP-4, not by interaction with P APP-A, which could not previously be ruled out. (C) 2001 Federation of Euro pean Biochemical Societies. Published by Elsevier Science B.V. All rights r eserved.