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Molecular Distribution of IGF Binding Protein-5 in Human Serum

Kolling Institute of Medical Research, University of Sydney, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia

Address all correspondence and requests for reprints to: Robert C. Baxter, Ph.D., D.Sc., Kolling Institute of Medical Research, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia. E-mail: robaxter{at}med.usyd.edu.au

IGF binding protein-5 (IGFBP-5) forms ternary complexes with IGFs and the acid-labile subunit (ALS) in vitro, but these complexes have not been demonstrated in the circulation. To examine the molecular distribution of circulating IGFBP-5 we developed an RIA with high specificity for IGFBP-5 among the IGFBPs, but wide cross-reactivity among primate and nonprimate species. The mean serum IGFBP-5 level (±SD) was 208 ± 73 ng/ml in healthy men and 206 ± 67 ng/ml in nonpregnant women, decreasing to 114 ± 38 ng/ml in pregnancy. Approximately 55% of immunoreactive IGFBP-5 was associated with ternary complexes in nonpregnant adults, whereas only 35% was in these complexes in pregnancy serum. After transient acidification, all immunoreactive IGFBP-5 corresponded in size to free or binary-complexed protein. Serum IGFBP-5 levels were significantly associated with ALS levels (r = 0.478; P = 0.008), but the association was less than that between IGFBP-3 and ALS (r = 0.743; P < 0.001), reflecting the lower percentage of IGFBP-5 complexed with ALS. As free or binary complexed IGFBP-5 is a relatively high proportion of the total, we speculate that, alone or as a carrier of IGFs, IGFBP-5 might have preferential access to the tissues, where it could act to stimulate growth.

This work was supported by Project Grant 990005 (to R.C.B. and S.M.F.) from the National Health and Medical Research Council of Australia.

Abbreviations: ALS, Acid-labile subunit; IGFBP, IGF binding protein; TBS, Tris-buffered saline.

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