Western ligand blot analysis of the different molecular forms of insulin-like growth factor-binding protein IGF-BP) in serum and plasma samples from 89 pregnant women has revealed a marked decrease, after the second month of pregnancy, in the 41.5 and 38.5K species (which are the binding units of the 150K complex) as well as in the 24K form. There was also a slight decrease in the 34K form, the 30K form was unaffected, and additional 21.5 and 20K bands appeared. Cross-linking experiments demonstrated the disapperance of a 49K band which is characteristic of the 150K complex. The alterations of the electrophoretic profile of the BPs were accompanied by a decrease in binding activity of up to 90%. Gel filtration at pH 7.4 confirmed that the decrease was essentially attributable to changes in the 150K complex BPs: 1) material eluting in the 150K zone contained only one third of the binding activity, as opposed to three quarters in reference material; 2) radiocompetition experiments illustrated the loss of affinity for IGF-I and IGF-II of the BPs extracted from the 150K complex; 3) ligand blot analysis revealed, in contrast with the virtual disappearance of the 41.5 and 38.5K forms, the appearance of a broad indistinct band at 30K and additional bands at 21.5 and 20K. With immunoblotting, the anti-IGF-BP-3 antibody, which specifically recognizes the 41.5 and 38.5K species, cross-reacted with this 30K material. The alterations of the BPs appeared to be enzymatic. When pregnancy serum was mixed with reference serum, the 41.5, 38.5, and 24K forms contributed by the reference serum were markedly reduced after 30 min of incubation at 37 C. However, these alterations could be prevented by incubation at either 0 or at 37 C in the presence of EDTA or aprotinin and could be curbed in the presence of high concentrations of phenylmethylsulfonylfluoride. Unmixed reference serum incubated at 37 C yielded an unchanged BP profile. Incubation of pregnancy serum with hypopituitary serum, which has elevated levels of the 34 and 30K BPs, resulted in a marked decrease in the 41.5 and 38.5K forms, a slight alteration of the 34K form, and no change in the 30K form. These findings suggest that during pregnancy, enzymatic (probably protease) activity either appears or is significantly increased in the circulation, which specifically degrades some of the IGF-BPs.(ABSTRACT TRUNCATED AT 400 WORDS)