Comparison of insulin-like growth factor I serum binding proteins in sheep and cattle: species differences in size and endogenous binding capacities.
Binding proteins (BP) for insulin-like growth factor I (IGF-I) were characterized in sheep and beef cattle serum for molecular weight (Mr) and binding characteristics. Serum was incubated with [125I] IGF-I at 37 degrees C before chromatography over a 1.6-cm X 94.0-cm column of Sephacryl S-300 (pH 7.4, 4 degrees C). Beef serum exhibited a 145 k Mr (mol. wt X 1,000) and a 35 to 39 k Mr BP. Sheep serum possessed a 170 to 190 k and a 35 to 38 k Mr protein. Binding of [125I] IGF-I was inhibited in the presence of excess unlabeled ovine somatomedin, demonstrating specific binding for each BP of both species. The high Mr component was pituitary-dependent in sheep, as evidenced by binding patterns from serum of hypophysectomized sheep. Direct binding studies of the Sephacryl-separated BP demonstrated that the native BP of high molecular weight of both species bound only minor amounts of [125I] IGF-I in a manner unrelated to BP concentration. The BP of low molecular weight of beef cows displayed a bell-shaped dose-response binding curve with maximum binding at 250 micrograms/ml BP, whereas binding to sheep BP of low molecular weight was independent of BP concentration. After chromatography on Sephadex G50 at pH 2.8, both BP from both species exhibited concentration-dependent binding that plateaued at 250 to 500 micrograms/ml of BP of low molecular weight but was curvilinear for the BP of high molecular weight.(ABSTRACT TRUNCATED AT 250 WORDS)
Hossner
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KL
Dept. of Anim. Sci., Colorado State University, Fort Collins 80523.
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Yemm
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D000818:Animals; D002352:Carrier Proteins / Q000032:analysis / Q000097:blood*; D002417:Cattle / Q000097:blood*; D002850:Chromatography, Gel; D005260:Female; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000097:blood*; D008970:Molecular Weight; D012756:Sheep / Q000097:blood*; D013002:Somatomedins / Q000097:blood*; D013045:Species Specificity
D002352:Carrier Proteins; D018969:Insulin-Like Growth Factor Binding Proteins; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I