Structure function studies on human growth hormone: evidence that tertiary structure is essential for biological activity

The relationship between the conformation of human pituitary growth hormone (hGH), biological activity, and ligand binding activity was studied by comparing conformational details previously published on in vivo and in vitro studies of identical samples of hGH and its known derivatives. In vivo assays included the rat tibia test for somatotropic activity and the pigeon crop-sac assay for lactogenic hormone activity. Relative binding affinities were compared in radioimmuno-assays using 125I-hGH as tracer with 1) anti-human chorionic somatomammotropin (hCS) serum (low discriminatory hybrid assay), 2) anti-hGH sera (in conventional assays), 3) monospecific anti-hGH serum (absence of cross-reaction with hCS) and 4) human anti-hGH sera obtained from GH-deficient patients on replacement therapy. In addition, binding affinities were examined in two receptor-binding assays, one specific for somatotropic activity (rabbit liver membranes, 125I-hGH), and the other, for lactogenic hormones (rabbit mammary membranes, 125I-oPRL). The conformational properties of native hGH and various chemically and enzymatically modified derivatives of the hormone were evaluated primarily from circular dichroism spectra, while conformational stabilities were estimated from the relative rates of tryptic digestion. Unfragmented, but chemically modified derivatives, exhibited good parallelism between retention or loss of native conformation and the in vivo potencies and in vitro binding affinities. None of the fragments of hGH showed activity in any of the radioreceptor assays or radioimmunoassays. Two derivatives of hGH, which contain gaps of 6 or 12 residues in the polypeptide backbone produced by partial enzymatic digestion, had full or increased in vivo potencies, full activities in the radioimmunoassays, and were the most active derivatives in both radio-receptor assays. One of these, missing the hexapeptide corresponding to residues 135–140, was also found to retain nearly all the conformational properties of native hGH. These studies proved further evidence that 1) retention by modified forms of hGH of a high degree of in vivo biological potency or in vitro binding affinity is causally related to the retention of most of the conformation and conformational stability of the molecule, and 2) the biologically acitve, receptor-binding and immunoreactive sites on the hGH molecule are 3-dimensional in nature.