Growth Hormone Heterogeneity: Genes, Isohormones, Variants, and Binding Proteins

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Abstract
I. Introduction HETEROGENEITY of polypeptide hormones has been recognized since the early 1970s. It stems from a variety of sources, including genetic, pretranslational, posttranslational, and postsecretory events. Thus, multiple genes, multiple messenger RNA transcripts, biosynthetic precursors, aggregates, derivatized functional groups, glycosylation, fragment formation, protein-protein interactions, and degradation all contribute to molecular heterogeneity. It is important to differentiate physiological heterogeneity from that which occurs as a result of laboratory procedures, such as chemical alterations of a protein during extraction or recognition of unrelated proteins by assays that lack specificity. GH is among the more heterogeneous polypeptide hormones. The recent surge of interest in GH has also refocused attention on its heterogeneous nature and the biological significance of multiple GH forms. This review will primarily examine human GH (hGH), which is the GH best studied from the standpoint of heterogeneity. Animal GHs will be mentioned when they provide interesting parallelisms or other facets relevant to hGH. In accordance with general convention, I will define as “isohormones” GH-related proteins with different primary structure, whereas the term “variants” will denote molecular forms derived from the same primary structure. Unfortunately, this nomenclature is not universally adhered to in the literature.

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