Binding characteristics of a biologically active variant of human growth hormone (20K) to growth hormone and lactogen receptors.

The dose-dependent displacement characteristics of a biologically active 20,000 molecular weight human pituitary growth hormone variant (20K) and of human growth hormone (hGH) were compared using hGH liver plasma membrane receptors both from 20-30 day pregnant rabbits and from normal female rats and also using mammary gland plasma membrane receptors from 5-6 day lactating rabbits. Different preparations of 20K were found to be only 3-20% as potent as hGH when compared at the dose necessary to cause 50% displacement of (125I)iodo-hGH from liver receptor and was 22-53% as effective as hGH in the mammary receptor assay system. These findings suggest that 20K and hGH may have separate receptors or that the binding characteristics of the two hormones may be quite different.