Insulin-like growth factor-binding protein-3 is functionally normal in pregnancy serum.

Insulin-like growth factor-binding protein-3 (IGFBP-3) carries most of the serum IGFs as a 150K ternary complex which increases during pregnancy. However, recent studies have demonstrated that IGFBP-3 is absent in pregnancy serum when measured by ligand blotting after electrophoresis. In the present study we demonstrate that, by several criteria, IGFBP-3 appears functionally intact in pregnancy serum. Serum samples were collected from pregnant women between 2-40 weeks gestation. Serum immunoreactive IGFBP-3 and acid-labile (alpha) subunit increased linearly throughout pregnancy. Ligand blotting confirmed diminution of IGFBP-3 at 2 weeks gestation and complete absence after 4 weeks gestation or when nonpregnancy serum was preincubated with amniotic fluid. When less harsh methods (neutral chromatography or transient acidification) were used, IGFBP-3 appeared functionally normal in pregnancy serum. Fractionation of pregnancy serum by Superose-12 gel permeation chromatography showed similar elution profiles for both IGFBP-3 and alpha-subunit compared to those for nonpregnancy serum. Preincubation of nonpregnancy serum with pregnancy serum or amniotic fluid had no effect on IGFBP-3 recovery. After acidification and neutralization of serum to destroy endogenous alpha-subunit, ternary complex formation, measured by radiolabeled alpha-subunit binding, was essentially identical in all nonpregnancy and pregnancy serum, except for a slight loss of activity in first trimester serum. Since the 150K complex cannot form unless IGFBP-3 binds IGFs and alpha-subunit normally, these results suggest that IGFBP-3 in native pregnancy serum is functionally normal.