Protease-induced alteration of insulin-like growth factor binding protein-3 as detected by radioimmunoassay. Agreement with ligand blotting data.

Abstract

Structural alteration of insulin-like growth factor binding protein-3 (IGFBP-3) resulting from limited proteolysis by one or more serine proteases in vivo was first described in the serum of pregnant women and in certain pathological conditions. Western immunoblotting has since been employed to detect the phenomenon in normal serum, using a polyclonal antibody raised against recombinant human IGFBP-3 and a highly sensitive technique of visualization by chemiluminescence. The major proteolytic fragment of 30 kDa, which fails to be detected in native serum by ligand blotting owing to its weak affinity for IGFs, has proved clearly visible in all serum samples tested, sometimes accompanied by smaller fragments of 20 and 16 kDa. Among the serum samples analysed, increasing proportions of proteolysed IGFBP-3 were found in the following order: acromegalic patients, normal subjects, GH-deficient patients, pregnant women. In RIAs done with the same antibody, many of the serum samples yielded dose-response curves which were not parallel with standard curves, with lower gradients. In the samples where measurements were possible, apparent IGFBP-3 levels proved lower in pregnant women (2.28 +/- 0.23 mg/l, mean +/- SEM) than in normal adults (4.26 +/- 0.33 mg/l, P < 0.001). These observations, which contradict earlier reports of higher levels in pregnant women, suggest that the 30 kDa proteolytic fragment has a weaker affinity for the antibody than the intact IGFBP-3 (which in ligand- and immunoblotting appears as a characteristic 42-39 kDa doublet and which is barely or not detectable in pregnancy serum).(ABSTRACT TRUNCATED AT 250 WORDS)