Transferrin with reticulocytes, interaction

Investigations of the interactions of immobilized 359 transferrin with reticulocytes and fibroblasts... 

The dramatic role of the anion can perhaps best be appreciated from simple quantitative considerations. In the absence of a suitable anion, specific binding of iron to transferrin does not occur at all the effective binding constant is zero. At physiologic pH and bicarbonate concentrations, however, the effective binding constant is about 5 X 1023 M"1 24, 50). This means that in 1 L of blood plasma, in which the transferrin is only about 30% saturated with iron, there will be less than one free ferric ion or that a molecule of the ferric—transferrin complex will spontaneously dissociate only about once in 10,000 years. Since iron is readily removed from the transferrin molecule during its interaction with the reticulocyte without disrupting protein structure 51, 52), a... 

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... 

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