Phosphorylation of insulin-like growth factor-binding protein-1 from different sources.

During purification, insulin-like growth factor binding protein-1 from amniotic fluid was separated into five different peaks by anion exchange chromatography. These peaks represent differently phosphorylated forms of IGFBP-1. The major peak (peak 1) is non-phosphorylated. Peaks 3, 4, and 5 are more phosphorylated and, in native polyacrylamide gel electrophoresis (PAGE), they migrate faster than peaks 1 and 2. The more phosphorylated forms have higher IGF-I-binding affinity. Both dephosphorylated and phosphorylated peaks enhanced IGF-I stimulated DNA-synthesis in fetal skin fibroblast cell culture. They, however, inhibited the binding of IGF-I to the same cells. The phosphorylation of IGFBP-1 was changed during pregnancy. In decidua and in amniotic fluid the degree of phosphorylation increased from early to late pregnancy, as indicated by faster mobility of IGFBP-1 in native PAGE and increased relative amount of the more phosphorylated peaks in anion exchange chromatography. Human ovarian follicular fluid, culture media from human granulosa cells and endometrial adenocarcinoma cells (HEC-1-B) consisted mostly of the non-phosphorylated form of IGFBP-1.