Kinetic analysis of receptor-mediated endocytosis of polypeptide hormones

体内EGF在大鼠体内分布的药代动力学分析表明，肝脏和肾脏对EGF的摄取呈现明显的饱和状态，Km值分别为7 nM和0.4 nM，两种组织均可占EGF在体内总清除量的80%以上。分析肝脏和肾脏对EGF的处理，以获得与受体介导的内吞作用(RME)相关的动力学参数。在肝脏RME的整个过程中，表皮生长因子受体的结合和解离过程较快(平均时间为1t；L分钟)，降解过程较慢(平均时间为4-5h)，内化过程为中间过程(平均时间为4-5min)。在肝脏中被下调后，受体恢复所需的平均时间约为5分钟。30min，明显短于其他组织。这种从肝脏受体下调中快速恢复的过程反映了内化的EGF受体循环到细胞表面的过程。比较了从灌流的肝脏和从分离的肝细胞获得的动力学参数，发现只有结合速率常数(Kon)有显著差异。提出了一种解释这种差异的假说，认为EGF与细胞表面受体的结合率受完整肝脏(如灌流的肝脏)间质中未搅拌水层的扩散所限制。滤过性和非滤过性灌流肾脏对EGF处理的比较表明，受体介导的EGF摄取主要通过具有很高亲和力(kd=0.1 nM)的逆腔质膜进行。

Pharmacokinetic analysis of the tissue distribution of EGF in rats performed in vivo clarified that the uptake of EGF by the liver and kidney exhibited clear saturation with Km values of 7 nM and 0.4 nM, respectively, and that both tissues could account for more than 80 % of the total plasma clearance of EGF in the distribution phase. The hepatic and renal handlings of EGF by isolated perfused organds were analyzed to obtain the kinetic parameters with respect to receptor-mediated endocytosis (RME). In the overall RME process in the liver, the EGF-receptor association and dissociation processes are rapid (mean time<l min), the degradation process is much slower (mean time 4-5 h) and the internalization process is intermediated (mean time 4-5 min). The mean time required for the recovery of receptors after being down-regulated in the liver is approx. 30 min, and was much shorter than those in other tissues. Such rapid recovery from the receptor down regulation in the liver reflects the process of recycling of internalized EGF receptors to the cell surface. The kinetic parameters obtained from the perfused livers and from isolated hepatocytes were compared, and a significant difference was observed only for the association rate constant (kon). A hypothesis accounting for this difference is proposed, in which the association rate of EGF to the cell surface receptor is considered to be limited by the diffusion in the unstirred water layer in the interstitial space of the intact liver such as the perfused liver. The comparison of EGF handling between the filtering and nonfiltering perfused kidneys demonstrated that the bulk of receptor-mediated EGF uptake takes place via the antiluminal plasma membrane with very high affinity (kd=0.1 nM).