Endocytosis processes

The 80 kDa glycoprotein transferrin (Tf) is responsible for intracellular iron transport via the transferrin receptor (TfR), using a clathrin-dependent endocytosis process. Tumor tissues frequently overexpress the TfR. While natural Tf recycles to... 

Clathrin is most likely an extrinsic membrane protein on the cytosol side. It participates in receptor-mediated endocytosis processes. 

The nanoparticles can be incorporated into cells by incubation the nanoparticies suspended in DMEM medium with the cells for 12 to 48 hours. From dark field light scattering imaging [54], it has been established that gold nanoparticies are taken intracellulariy into these cell lines. The nanoparticies are accumulated inside cytoplasm of live cells by receptor-mediated endocytosis processes. Figure 20.16 shows that for most of samples gold nanospheres quench the protein fluorescoice at 12 h incubation time. However, in some samples fluorescence enhancement is observed. 

Three processes are involved in transcellular transport across the intestinal epithelial cells simple passive trans-port, passive diffusion together with an efflux pump, and active transport and endocytosis. Simple passive transport is the diffusion of molecules across the membrane by thermodynamic driving forces and does not require direct expenditure of metabolic energy. In contrast, active transport is the movement of molecules across the mem-brane resulting directly from the expenditure of metabolic energy and transport against a concentration gradient. Endocytosis processes include three mechanisms fluid-phase endocytosis (pinocytosis), receptor-mediated endocytosis, and transcytosis (Fig. 6). Endocytosis processes are covered in detail in section Absorption of Polypeptides and Proteins, later. 

The membrane lipid-clathrin interaction at work during the formation of vesicles from the membrane is not known. It is usually assumed that clathrin reconformation drives the endocytosis process. Alternatively these conformational changes might be induced in the attached lipid bilayer. 

Using cultured mammalian sarcoma cells, it has been found that transferrin is necessary in the growth medium for galllum-67 uptake to occur (95,96.97). A "transferrin receptor" on EMT-6 sarcoma cells for 25i iabeled transferrin was characterized by Scatchard analysis to have an average association constant K = 4.54 X 10 1/mole and approximately (with variation) 500,000 receptors per cell ( ). It was proposed that tumor accumulation of galllum-67 can occur only if the metal is complexed with transferrin so that it can interact with the receptors of tumor, as well as non-mallgnant cells (.33). The complex then enters the cell via an "adsorptive endocytosis" process (95.96.97.98.99) similar to the manner in which iron is taken up by reticulocytes and bone marrow cells (100.101). These transferrin receptors are saturable (that is, a plot of 125i transferrin uptake versus extracellular transferrin concentration reaches a peak ( at about 200 u g/ml) as more carrier transferrin is added to the medium) (95). Since uptake is also proportional to the fraction of... 

Endocytosis Process in which vesicles form by invagination of the plasma membrane to move substances into eukaryotic cells. 

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