Plasma “Big” and “Big-Big” Growth Hormone (GH) in Man: An Oligomeric Series Composed of Structurally Diverse GH Monomers*

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Abstract
Human GH (hGH) immunoreactivity in plasma can be separated into three species of different molecular size by gel filtration (little, big, and big-big hGH). In contrast to pituitary high mol wt GH forms, the molecular nature of the big hGH forms in blood is not known. Therefore, we purified these hGH size isomers from the plasma of L-dopa-stimulated normal subjects and acromegalic patients. Plasma was chromatographed on Sephadex G–100, and fractions containing big-big, big, and little hGH were generated. hGH was extracted and concentrated from these fractions by immunoadsorbent chromatography and analyzed by polyacrylamide gel electrophoresis (PAGE), sodium dodecyl sulfate-PAGE, and isoelectric focusing. The resulting gel profiles indicated that the majority (70%) of big and big-big hGH was converted to little hGH during extraction and storage. The remainder migrated as distinct species with mol wt of approximately 45, 62, 80, and 110 K in sodium dodecyl sulfate-PAGE. These forms could be converted almost completely to little hGH by sulfhydryl reduction. Little hGH (both native and converted from big forms) was composed of several monomeric hGH species, namely the 22 K form (principal), the 20 K variant, and at least one acidic form. All hGH size isomers contained the same monomeric building blocks, although in somewhat different proportions. Big hGH, e.g. was particularly rich in 20 K. No abnormal or previously unrecognized hGH forms were identified as components of big or big-big hGH. Binding of hGH to plasma proteins could not be demonstrated. We conclude that 1) plasma big and big-big hGH represent an oligomeric series composed of 22 K (major), 20 K, and one or more acidic hGH monomers, 2) the majority of these oligomers are noncovalently associated, with a smaller fraction consisting of monomers linked by disulfide bridges.