Baumann G, Shaw M A
Department of Medicine, Northwestern University Medical School, Chicago, Illinois 60611.
J Clin Endocrinol Metab. 1990 Nov;71(5):1339-43. doi: 10.1210/jcem-71-5-1339.
The 20,000-dalton variant of human GH (20K) is known to circulate in blood, but few details are known about its plasma transport. A substantial portion of 20K appears to be protein bound despite its low affinity for the receptor-related GH-binding protein (BP). We, therefore, investigated the binding pattern of 20K in human plasma. Radioiodinated 20K was incubated with plasma or plasma fractions, and the mixture was fractionated by gel filtration. Saturation/Scatchard analysis was performed with both 20K and 22,000-dalton GH (22K). The majority (approximately 80%) of protein-bound 20K was complexed with a low affinity BP (Ka = approximately 2 x 10(5) M-1), with the remainder complexed with the high affinity2 receptor-related BP. The binding of 20K to the low affinity BP was specific for 20K; it may involve a 20K-specific binding site on the previously described low affinity plasma BP (peak I) or a separate 20K-specific BP. 22K did not interact with this binding site/BP. Analysis of the 20K-BP complex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing suggested, but did not prove, the existence of a separate BP for 20K. We conclude that 20K in plasma binds primarily to a low affinity, 20K-specific binding site (possibly a new, third GH-BP), with only a small fraction bound to the GH receptor-related BP. This binding pattern is markedly different from that of 22K, which is predominantly bound to the receptor-related BP. The existence of a 20K-specific binding site/BP suggests an as yet unrecognized role for this GH variant.
已知人类生长激素（GH）的20000道尔顿变体（20K）在血液中循环，但关于其血浆转运的细节却知之甚少。尽管20K对受体相关生长激素结合蛋白（BP）的亲和力较低，但其中很大一部分似乎与蛋白质结合。因此，我们研究了20K在人血浆中的结合模式。将放射性碘化的20K与血浆或血浆组分一起孵育，然后通过凝胶过滤对混合物进行分离。对20K和22000道尔顿的生长激素（22K）都进行了饱和/斯卡查德分析。与蛋白质结合的20K大部分（约80%）与低亲和力BP（Ka约为2×10⁵M⁻¹）形成复合物，其余部分与高亲和力的2型受体相关BP形成复合物。20K与低亲和力BP的结合对20K具有特异性；它可能涉及先前描述的低亲和力血浆BP（峰I）上的20K特异性结合位点，或者是一个单独的20K特异性BP。22K不与这个结合位点/BP相互作用。通过十二烷基硫酸钠-聚丙烯酰胺凝胶电泳和等电聚焦对20K-BP复合物进行分析，提示但未证实存在一个单独的20K特异性BP。我们得出结论，血浆中的20K主要与一个低亲和力、20K特异性结合位点（可能是一种新的、第三种生长激素结合蛋白）结合，只有一小部分与生长激素受体相关BP结合。这种结合模式与22K明显不同，22K主要与受体相关BP结合。20K特异性结合位点/BP的存在表明这种生长激素变体有一个尚未被认识的作用。
