Transformed alpha 2-macroglobulin as a low-affinity growth hormone-binding protein.
Human growth hormone (GH) forms complexes with the purified proteinase inhibitor, alpha 2-macroglobulin (alpha 2-M). This inhibitor occurs in two different forms in serum, known as native and transformed alpha 2-M. It has been clearly demonstrated, using chromatography and electrophoresis combined with autoradiography, that human GH binds specifically to the transformed inhibitor and not to the native protein. The binding was characterized as being mainly non-covalent and involved specific binding sites present only in the transformed inhibitor molecule. Binding analysis, using an immunoprecipitation technique, revealed that GH possesses two different types of binding sites, with dissociation constants of 0.49 +/- 0.12 mumol/l and 61 +/- 8 mumol/l for the high- and low-affinity binding site, respectively. Distribution analysis of 125I-labelled GH in whole plasma suggests that the hormone is bound to two different proteins: first, to the high-affinity GH-binding protein (GHBP) and, second, to the low-affinity GHBP, identified as transformed alpha 2-macroglobulin.
Kratzsch
J
J
Institute of Clinical Chemistry, University of Leipzig, Germany.
Selisko
T
T
Birkenmeier
G
G
D002352:Carrier Proteins / Q000097:blood / Q000502:physiology*; D013006:Growth Hormone / Q000378:metabolism*; D006801:Humans; D011480:Protease Inhibitors / Q000097:blood / Q000737:chemistry / Q000494:pharmacology; D000511:alpha-Macroglobulins / Q000032:analysis / Q000737:chemistry / Q000502:physiology*
D002352:Carrier Proteins; D011480:Protease Inhibitors; D000511:alpha-Macroglobulins; D013006:Growth Hormone; C051815:somatotropin-binding protein