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Structural and Immunological Comparison of Insulin-Like Growth Factor Binding Proteins of Cerebrospinal and Amniotic Fluids^*

RON G. ROSENFELD, HUNG PHAM, CHERYL A. CONOVER, RAYMOND L. HINTZ and ROBERT C. BAXTER

Department of Pediatrics, Stanford University School of Medicine (R.G.R., H.P., C.A.C., R.L.H.) Stanford, California 94305
The Department of Endocrinology, Royal Prince Alfred Hospital (R.C.B.) Camperdown, New South Wales 2050, Australia

Address all correspondence and requests for reprints to: Dr. Ron Rosenfeld, Department of Pediatrics, Stanford University School of Medicine, Stanford, California 94305.

The insulin-like growth factors (IGFs) found in plasma and a variety of other body fluids are complexed to specific binding proteins (BPs). The cDNA for a 25K IGF-BP was recently cloned and sequenced, and the primary structure of the BP deduced. This BP is found in amniotic fluid, decidual tissues, conditioned medium from HepG2 human hepatoma cells, and fetal plasma. An additional small IGF-BP has been identified in human cerebrospinal fluid (CSF). We now demonstrate that the IGF-BP found in CSF is structurally and immunologically distinct from that found in HepG2 conditioned medium. While the latter BP has approximately equal affinities for IGF-I and -II, the CSF BP has a 10- to 20-fold greater affinity for IGF-II. In affinity cross-linking studies under reducing conditions, the CSF BP had an apparent mol wt (Mr) of 32,000, while the HepG2 BP migrated as a doublet, with apparent Mr values of 30,000 and 28,000. On Western ligand blots, CSF BPs migrate as five discrete bands, with the most prominent band at an apparent Mr of 34,000, while HepG2 medium yielded a single band at an apparent Mr of 30,000. A polyclonal antibody developed against the human amniotic fluid BP immunoprecipitated the HepG2 BP and reacted with both the HepG2 and amniotic fluid BPs on Western blots, but failed to react with the CSF BP. These data indicate that the CSF and amniotic fluid/HepG2 BP are structurally and immunologically distinct small IGF-BPs

^* This work was supported in part by NIH Grants DK-28229 (to R.G.R.), DK-36054 (to R.G.R.), CA-42106 (to R.G.R.), and DK-24085 (to R.L.H.), the Diabetes Research and Education Foundation (to R.G.R.), and the National Health and Medical Research Council, Australia (to R.C.B.).

Recipient of Research Career Development Award DK-01275 from the NIH.

Received August 12, 1988.

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