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Comparison of Insulin-Like Growth Factor I Serum Binding Proteins in Sheep and Cattle: Species Differences in Size and Endogenous Binding Capacities^1,2,3,

Colorado State University^4,5,, Fort Collins 80523

Abstract

Binding proteins (BP) for insulin-like growth factor I (IGF-I) were characterized in sheep and beef cattle serum for molecular weight (M_r) and binding characteristics. Serum was incubated with [¹²⁵I] IGF-I at 37°C before chromatography over a 1.6-cm x 94.0-cm column of Sephacryl S-300 (pH 7.4, 4°C). Beef serum exhibited a 145 k M_r (mol. wt x 1,000) and a 35 to 39 k M_r BP. Sheep serum possessed a 170 to 190 k and a 35 to 38 k M_r 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 M_r 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 µg/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 µg/ml of BP of low molecular weight but was curvilinear for the BP of high molecular weight. After acid chromatography, both sheep BP bound a higher proportion of added label than the beef BP, suggesting that ovine BP may bind more IGF in the circulation than the beef BP. In sum, our data suggest that BP function is generally similar among species, but that species differences occur in the form and binding capacities of these mammalian serum proteins.

¹ Colorado State Univ. Agricl. Exp. Sta. Pub., Project 629.

² Supported in part by BRSG Grant 7423 and USDA Grant 86-CRCR-1-1943.

³ The authors express their gratitude to Pat Beebe for typing the manuscript.

⁴ Dept. of Anim. Sci.

⁵ An abstract of this work was presented at the Am. Soc. of Anim. Sci. annual meeting, July 1987.