Gliemann J, Davidsen O
Biochim Biophys Acta. 1986 Jan 23;885(1):49-57. doi: 10.1016/0167-4889(86)90037-6.
High-affinity receptors for alpha 2-macroglobulin-trypsin complex were demonstrated in rat hepatocytes at 4 degrees C. The dissociation rate constant for the labelled complex was very small at low receptor occupancies, approx. 4 X 10(-4) min-1. Dissociation was biphasic at high receptor occupancies with a rate constant for the rapid phase of about 2 X 10(-2) min-1. At near-equilibrium, half of the receptors were saturated at a complex concentration of 150 pM, and the Scatchard plot was concave upwards. Thus, the binding shows complex kinetics with the probable involvement of negative cooperativity. Binding of the labelled complex was not influenced by galactose, mannose, mannose phosphate or fucoidin, whereas it was abolished in the absence of extracellular Ca2+ and inhibited by bacitracin. Approx. 70% of the labelled complex bound at 4 degrees C was rapidly internalized (kint about 3 X 10(-1) min-1) after being warmed to 37 degrees C. Radioactivity released from the cells at 37 degrees C comprised intact labelled complex and iodide. The complex was initially released at a rapid rate (k-1 about 1 X 10(-1) min-1) from about 25% of the cell-bound pool. This probably represents dissociation from the receptors. A slow phase of release followed, so that half of the bound pool was finally released as intact complex. Iodide release followed a sigmoidal curve after a 20 min lag period. Thus, specific high-affinity receptors mediate the internalization and eventual degradation of alpha 2-macroglobulin-proteinase complex into hepatocytes.
在4℃条件下，大鼠肝细胞中发现了α2-巨球蛋白-胰蛋白酶复合物的高亲和力受体。在低受体占有率时，标记复合物的解离速率常数非常小，约为4×10⁻⁴ min⁻¹。在高受体占有率时，解离呈双相性，快速相的速率常数约为2×10⁻² min⁻¹。在接近平衡时，复合物浓度为150 pM时，一半的受体被饱和，Scatchard图向上凹陷。因此，结合表现出复杂的动力学，可能涉及负协同作用。标记复合物的结合不受半乳糖、甘露糖、甘露糖磷酸或岩藻依聚糖的影响，而在细胞外Ca²⁺缺失时结合被消除，并被杆菌肽抑制。在4℃结合的标记复合物中，约70%在升温至37℃后迅速内化（内化速率常数约为3×10⁻¹ min⁻¹）。37℃时从细胞中释放的放射性物质包括完整的标记复合物和碘化物。复合物最初以快速速率（解离速率常数约为1×10⁻¹ min⁻¹）从约25%的细胞结合池中释放。这可能代表从受体上解离。随后是一个缓慢的释放阶段，因此最终一半的结合池以完整复合物的形式释放。碘化物在20分钟的延迟期后呈S形曲线释放。因此，特异性高亲和力受体介导α2-巨球蛋白-蛋白酶复合物进入肝细胞的内化和最终降解。
