Tolleshaug H, Berg T, Blomhoff R
Biochem J. 1984 Oct 1;223(1):151-60. doi: 10.1042/bj2230151.
Even though most of the hepatic binding capacity for mannose-terminated glycoproteins has previously been shown to reside in the hepatocytes (not in the non-parenchymal cells), detailed evidence for the specific uptake of mannose-terminated ligands has been lacking. In the present studies, yeast invertase, a large glycoprotein (Mr 270 000) containing about 50% mannose, was shown to be taken up into hepatocytes by receptor-mediated endocytosis. The uptake was saturable and could be specifically inhibited by mannosides or by a Ca2+ chelator. The asialo-glycoprotein receptor was not involved. The low-Mr (13 000) ligand ribonuclease B, which contains a single high-mannose glycan, was not taken up by hepatocytes; however, it was taken up as fast as invertase by non-parenchymal liver cells. After injection of 131I-invertase into a rat in vivo, about one-half of the labelled protein was recovered in the hepatocytes. On a per-cell basis, each endothelial cell contained 3-4 times as much radioactivity as did the hepatocytes. On fractionation of hepatocytes in sucrose gradients, invertase showed a different intracellular distribution from that of asialo-fetuin, in that invertase moved much faster into that region of the gradient where the lysosomes were recovered. This indicates that invertase and asialo-fetuin are not transported intracellularly by identical mechanisms.
尽管先前已表明，肝脏对甘露糖末端糖蛋白的大部分结合能力存在于肝细胞中（而非非实质细胞中），但一直缺乏关于甘露糖末端配体特异性摄取的详细证据。在本研究中，酵母蔗糖酶是一种大型糖蛋白（Mr 270 000），含约50%的甘露糖，已证明它可通过受体介导的内吞作用被肝细胞摄取。摄取具有饱和性，可被甘露糖苷或Ca2+螯合剂特异性抑制。这一过程不涉及去唾液酸糖蛋白受体。低Mr（13 000）配体核糖核酸酶B含有一个高甘露糖聚糖，不能被肝细胞摄取；然而，它被非实质肝细胞摄取的速度与蔗糖酶一样快。将131I标记的蔗糖酶注入大鼠体内后，约一半的标记蛋白在肝细胞中被回收。以每个细胞计算，每个内皮细胞所含的放射性是肝细胞的3 - 4倍。在蔗糖梯度中对肝细胞进行分级分离时，蔗糖酶在细胞内的分布与去唾液酸胎球蛋白不同，蔗糖酶更快地进入梯度中回收溶酶体的区域。这表明蔗糖酶和去唾液酸胎球蛋白在细胞内的运输机制不同。
