Human growth hormone: additional members of the complex.

Abstract

The use of urea during electrophoretic analysis of human GH (hGH) permitted detection of two previously undetected forms of the hormone. The newly recognized components were resolved at pH 10 and 7.8 but not at pH 4. Because they migrated more slowly than the major hGH component, the forms have been denoted as Slow-slow-GH and Slow-GH. They have molecular weights near 22,000 and are neither electrophoretic artifacts nor proteolytically cleaved forms. By RIA, Slowslow-GH and Slow-GH have been found to produce displacements similar to that observed with reference hGH (NIH). Both forms had growth-promoting activity (1.8 IU/mg) in the rat tibial line assay and both resembled hGH by peptide mapping and total amino acid composition. Slow-GH had definite lactogenic activity in the pigeon crop sac assay, with a suggestion that it was somewhat more active than the hGH from which it was removed. A third form of hGH was detected when the gel electrophoresis was done at pH 7.8 with or without urea. This component was denoted as Fast-GH because it migrated just slightly ahead of the major band. At pH 10, the component migrated with the major hGH band and, therefore, could not be detected. The molecular weight of Fast-GH was near 22,000 and there was no evidence of its being an enzymically cleaved form. RIA has indicated that Fast-GH is essentially as active as hGH (NIH). Tibial line assay gave a potency of 2.0 IU/mg. Peptide mapping indicated a great similarity to hGH but no free amino-terminal group could be detected. These studies bring to five the number of intact (proteolytically uncleaved) forms of hGH that can be detected in currently available preparations of hGH. The results support the view that hGH is a complex of substances and suggest that the multiple actions of GH may be in part a result of this heterogeneity. Proteolytic conversion of each of the new forms to two-chain forms is a further possibility.