INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-3 IS FUNCTIONALLY ALTERED IN PREGNANCY PLASMA

In a variety of physio-pathological conditions, but also in the normal state, calcium-dependent serine protease(s) partially proteolyze prop ortions of circulating insulin-like growth factor binding protein-3 (I GFBP-3). This occurs without disrupting the 150-kilodalton (kDa) compl exes in which IGFBP-3 carries of most of the IGF-I and IGF-II in serum . In this work we show that the 150-kDa complex is functionally altere d during pregnancy, which is when the largest proportion of IGFBP-3 is proteolyzed. After preincubation at 4 C with [I-125]IGF-I or -II with or without 1 mu g unlabeled IGF-I or -II, pooled plasma samples were gel filtered at pH 7.4. Larger proportions of labeled IGFs were found in the 150-kDa complexes of the third trimester pregnancy plasma pool than in those of the normal plasma pool, suggesting increased binding capacity. Nevertheless, competitive binding experiments using [I-125]I GF-I and II and IGFBP-3 preparations from each of the plasma pools sho wed that the competitive potencies of IGF-II and especially IGF-I were lower in pregnancy IGFBP-3 than in normal IGFBP-3. Scatchard analysis revealed a 2-fold loss of affinity for IGF-II and a 10-fold loss for IGF-I compared with that for normal plasma IGFBP-3. In studies at 37 C of the kinetics of dissociation of [I-125]IGF-I and -II bound to IGFB P-3, IGF-II was dissociated 6 times faster, and IGF-I 10 times faster from pregnancy plasma IGFBP-3 than from normal plasma IGFBP-3. After i ncubation of individual plasma samples at 37 C and gel filtration at r oom temperature (in the presence of EDTA), IGFs were assayed in the th ree circulating pools (150-kDa and 40-kDa complexes and free IGFs), re vealing a redistribution of pregnancy plasma IGFs. The proportion of t otal IGF-I in free form was nearly three times greater in pregnancy th an in normal plasma (11.4% vs. 4.1%, P < 0.005), whereas that of free IGF-II was slightly smaller (1.5% vs. 2%). In the 150-kDa complexes, t he proportion of total IGF-I was significantly smaller in pregnancy th an in normal plasma, and that of IGF-II was greater. In the 40-kDa com plexes, the proportion of total IGF-II was significantly smaller. The mean ratios of molar concentrations of free IGF-I/IGF-II were 0.43 in normal plasma and 2.23 in pregnancy plasma (P < 0.005). All these obse rvations support the conclusion that IGFBP-3 is structurally altered b y pregnancy-associated protease(s), resulting in depressed affinity fo r the IGFs, particularly IGF-I, hence accelerated kinetics of dissocia tion and increased measurable IGF in free form in the plasma. The 150- kDa complex is therefore functionally different during pregnancy. In a ddition, the protease-induced alteration of IGFBP-3 seems to be a fund amental mechanism in regulating the bioavailability of IGFs, and espec ially IGF-I, from the bloodstream.