Proteolytic Conversion of Insulin-like Growth Factor-I to des (1-3) Insulin-like Growth Factor-I: A Further Site of Regulation of Insulin-like Growth Factor-I Action

Abstract

In this review we discuss our progress in investigating the pro tease which is responsible for generating des (1-3) insulin-like growth factor-I (IGF-I), a more potent form of IGF-I devoid of the first 3 N-terminal amino acid residues. Characterization of the protease activity indicated that it was most likely to be a member of the serine protease family. Although some activity was detected at neutral pH, the activity was optimal at pH 5.5. The enzyme was purified from rat liver utilizing ion-exchange, chromatofocusing, phenylsepharose chromatography followed by high performance liquid chematography (HPLC). A single band of - 31 kD was observed on silver stained sodium dodecyl sulphate-polyachylamide gel electrophoresis (SDS-PAGE) gels. We developed a rapid assay to quantify this protease activity in serum and tissue extracts. The activity was detected in mouse, rat, human serum and rat tissue extracts. The level of protease activity in the tissue extracts demonstrated the following order: liver>testes>heart>skeletal muscle>lung>thymus>kidney>brain>spleen. Protease activity was significantly higher in sera from hypophysectomized rats than in intact rats and growth hormone (GH) replacement reduced enzyme activity. In addition, serum protease activity was significantly elevated in diabetic rats and correlated with serum glucose concentrations. These data demonstrate that the proteolytic conversion of IGF-I to a more active form is enhanced under conditions associated with low IGF-I, insulin and GH levels and suggest that this conversion may be an additional site of regulation of IGF-I action.