title: Molecular distribution of IGF binding protein-5 in human serum.
abstract: 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.
authors.affiliation: Kolling Institute of Medical Research, University of Sydney, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia. robaxter@med.usyd.edu.au
authors.forename: Robert C
authors.initials: RC
authors.lastname: Baxter
authors.forename: Sridevi
authors.initials: S
authors.lastname: Meka
authors.forename: Sue M
authors.initials: SM
authors.lastname: Firth
mesh_terms: D000818:Animals; D003429:Cross Reactions; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000097:blood; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000097:blood* / Q000276:immunology; D008297:Male; D011247:Pregnancy; D011863:Radioimmunoassay / Q000379:methods*
chemical_list: D018972:Insulin-Like Growth Factor Binding Protein 3; D018975:Insulin-Like Growth Factor Binding Protein 5