Insulin-like growth factor-binding protein-3 (IGFBP-3) was digested with plasmin, and the proteolytic fragments were isolated by HPLC and tested for bioactivity as measured by stimulation of glucose uptake in microvessel endothelial cells. Two of the pooled fractions of the digest stimulated glucose uptake. The major bioactive pool, at an estimated protein concentration <50 ng/ml, stimulated glucose uptake to 150% of control with greater stimulation and 220% of control at ~250 ng/ml. Two fragments were present in the bioactive fraction, the dominant one migrating at ~20,000 and the other at ~8,000. Both fragments bound ¹²⁵I-labeled insulin-like growth factor and [³H]heparin. NH₂-terminal amino acid analysis of the bioactive peak yielded two sequences. One, representing the majority of the material, had an NH₂-terminal sequence identical to IGFBP-3; the second fragment began at amino acid 202 of IGFBP-3. In contrast to the bioactive fragments, intact IGFBP-3, at concentrations up to 130 µg/ml, had no bioactivity. These findings demonstrate that IGFBP-3 can be degraded into fragments that have potent bioactivities that are not present in the intact IGFBP-3 molecule.

endothelium; insulin-like growth factor-binding protein-3; bioactivity

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