Substantial Induction of a New Serum Protein by Growth Hormone: Physiochemical and Physiological Characterization*

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Abstract
Serum from growth hormone [GH]-treated hypophysectomized rats was examined by 2-dimensional gel electrophoresis followed by Ag staining to screen a large number of serum proteins for GH responsiveness. A distinctive, highly acidic protein of 60,000 MW was clearly responsive to GH administration, increasing 7-fold over levels observed in hypophysectomized controls. Administration of thyroxine [T4], corticosterone, and 5.alpha.-dihydrotestosterone failed to induce this protein. Addition of GH to this regimen resulted in a 60-fold increase in its concentration. Examination of proteins synthesized by isolated hepatocytes in the presence of [35S]methionine showed that this protein is of hepatic origin. Physiological and physiochemical evidence suggest that this protein is unlikely to be one of the other well documented GH-responsive serum proteins: somatomedin-C, .alpha.2U-globulin, or the somatomedin-binding protein. The protein demonstrates a remarkable ontogeny, increasing 135-fold over newborn levels by 35 days of age. This is qualitatively similar to changes observed in somatomedin-C. The quantitative change is much more striking. Examination of the protein in an altered physiological state which limits growth (uremia) reveals a 30-fold diminution of levels of the protein after 3 wk of renal failure. Pair-fed animals demonstrated only a 2-fold decrease. There is a profound inhibition of GH action which is not accounted for by poor nutritional intake. This protein may be useful in the investigation of the GH-hepatic axis.

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