HIGH MOLECULAR-WEIGHT INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEIN COMPLEX - PURIFICATION AND PROPERTIES OF THE ACID-LABILE SUBUNIT FROM HUMAN-SERUM

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  • 15 July 1989
    • journal article
    • research article
    • Vol. 264  (20) , 11843-11848
Abstract
In the human circulation, the insulin-like growth factors (IGFs) circulate as part of a growth hormone-dependent 125- to 150-kDa complex. This complex has been postulated to contain, in addition to IGFs and one or more IgF-binding proteins, an acid-labile subunit (ALS) which does not itself bind IGFs. In this study, the ALS has been purified 1600-fold from human serum, and its binding properties have been examined. Fresh serum was fractionated on DEAE-Sephadex, and active fractions (determined by radioimmunoassay) were purified by affinity chromatography on an IGF-agarose column saturated with the plasma IGF-binding protein BP-53. After further high performance anion exchange chromatography, an ALS preparation was obtained which contained only an 84-86-kDa protein doublet, converting to a single 70-kDa band on N-glycanase treatment, and having an amino-terminal sequence unrelated to IGF-binding proteins or receptors. Pure ALS formed a complex with BP-53d (Ka .apprx.5 .times. 108 M-1), immunoprecipitable by anti-BP-53 antiserum, only in the presence of IGF-I or IGF-II. This complex appeared at .apprx. 150 kDa on high performance gel chromatography. Pure ALS had no intrinsic IGF-binding activity and no effect on the binding of IGF-I or IGF-II to BP-53. These studies suggest that formation of the high molecular weight IGF-binding protein complex requires ALS, BP-53, and IGF.