Identification of transformed alpha 2-macroglobulin as a growth hormone-binding protein in human blood.

We have found that the human GH forms complexes with the purified proteinase inhibitor, alpha 2-macroglobulin. This inhibitor occurs in two different forms in human blood, known as native and transformed alpha 2-macroglobulin. It could be clearly demonstrated by chromatography and electrophoresis combined with autoradiography that the GH binds specifically to the transformed inhibitor, but not to the native protein. The binding was characterized as being mainly noncovalent to specific binding sites present only in the transformed inhibitor molecule. Binding analysis using antibody precipitation technique revealed two different classes of binding sites with dissociation constants of 0.49 +/- 0.12 and 61 +/- 8 mumol/L for high and low affinity binding sites, respectively. Distribution analysis of 125I-labeled GH in whole plasma suggested that the hormone is bound to different proteins: 1) to the high affinity GH-binding protein, and 2) to the low affinity GH-binding protein identified as transformed alpha 2-macroglobulin.