Chatterjee M, Baliga B S, Munro H N
J Biol Chem. 1976 May 25;251(10):2945-51.
Synthesis of human placental lactogen (hPL) and of human chorionic gonadotropin (hCG) by membrane-bound and free polyribosomes from early and from full term human placentas was investigated by in vitro release of the nascent hormone peptides, followed by immunoprecipitation and electrophoresis in sodium dodecyl sulfate gels, and by specific binding of 125I-labeled hPL antibody to nascent peptide chains. In addition, messenger RNA'S Were extracted from total, free, and membrane-bound placental polyribosomes and their capacities for hPL and hCG synthesis were measured in a heterologous cell-free system prepared from wheat germ. Membrane-bound polyribosomes from full term placentas were several times more active in the synthesis of both peptide hormones than were free polyribosomes. By binding 125I-labeled hPL antibody to nascent chains on the polyribosomes, it was determined that hPL is made by clusters of seven to nine ribosomes. About 8% of the nascent peptide chains released by incubation of polyribosomes from full term placentas was accounted for by hPL, and 2% by hCG. In contrast, no chains of hPL were released by polyribosomes from 10-week placentas, whereas 11% of the total released chains were accounted for by hCG. When messenger RNAs prepared from the polyribosomes of 20- and 40-week placentas were used to stimulate protein synthesis in a wheat germ system, hPL accounted for 0.4 and 2%, respectively, of total protein synthesis, while hCG was 8 and 2%, respectively. This confirmed the relative proportions observed for nascent chains on the polyribosomes of early and late placentas. Unexpectedly, translation of mRNA from free polyribosomes yielded as much hPL and two-thirds as much hCG as did translation of mRNA from bound polyribosomes. We conclude, that the decreased blood levels of hCG and increased blood levels of hPL with advancing gestational age reflect the relative in vitro rates of synthesis of these hormones by placental polyribosomes, the abundance of which is determined by availability of their respective messenger RNAs at different times in gestation.
通过新生激素肽的体外释放，随后进行免疫沉淀和十二烷基硫酸钠凝胶电泳，以及用¹²⁵I标记的人胎盘催乳素（hPL）抗体与新生肽链的特异性结合，研究了来自早期和足月人胎盘的膜结合和游离多核糖体对人胎盘催乳素（hPL）和人绒毛膜促性腺激素（hCG）的合成。此外，从总的、游离的和膜结合的胎盘多核糖体中提取信使核糖核酸（mRNA），并在由小麦胚芽制备的异源无细胞系统中测量它们合成hPL和hCG的能力。足月胎盘的膜结合多核糖体在这两种肽类激素的合成中比游离多核糖体活跃几倍。通过将¹²⁵I标记的hPL抗体与多核糖体上的新生链结合，确定hPL是由七至九个核糖体的簇合成的。足月胎盘多核糖体孵育释放的新生肽链中约8%由hPL构成，2%由hCG构成。相反，10周胎盘的多核糖体未释放出hPL链，而总释放链的11%由hCG构成。当用20周和40周胎盘的多核糖体制备的mRNA在小麦胚芽系统中刺激蛋白质合成时，hPL分别占总蛋白质合成的0.4%和2%，而hCG分别为8%和2%。这证实了在早期和晚期胎盘多核糖体上观察到的新生链的相对比例。出乎意料的是，游离多核糖体mRNA的翻译产生的hPL与结合多核糖体mRNA的翻译产生的一样多，产生的hCG是结合多核糖体mRNA翻译产生的三分之二。我们得出结论，随着孕周增加，hCG血水平降低而hPL血水平升高反映了胎盘多核糖体合成这些激素的相对体外速率，其丰度由它们各自的信使核糖核酸在妊娠不同时间的可获得性决定。
