Uptake and release of transferrin and iron by mitogen-stimulated human lymphocytes

Phytohaemagglutinin stimulation of human peripheral blood lymphocytes resulted in the expression of transferrin receptors and the uptake of iron into the cells. As assessed from the resistance of the 125I label to pronase, transferrin was rapidly bound and internalized at 37 degrees C, while at 4 degrees C 85% of the 125I label remained on the cell surface and was degraded by the pronase. Over 94% of the 125I label associated with and subsequently released from the cells was acid precipitable, indicating that transferrin was not degraded during its uptake and release. After reincubation for 1 h in fresh medium 70% of the cell associated 125I-transferrin was released. In contrast, less than 30% of the 59Fe was released, showing that iron was removed from transferrin and retained by the cells. Concentration dependent binding of 125I-transferrin estimated at 37 degrees C occurred with an apparent Ka of 5.7 +/- 1.1 X 10(7) l mol-1 (mean +/- SD, n = 4) indicating little variation between cells from different individuals, although the number of transferrin molecules associated with the cells varied greatly from 6.2 X 10(4)/cell to 1.4 X 10(5)/cell. The rate of iron uptake from 59Fe and 125I labelled transferrin at 37 degrees C by the cells from different subjects was also very variable, with a range between 0.46 and 2.27 pg Fe/min/10(6) cells (n = 6). However, iron uptake did not correlate with the amount of transferrin bound. This suggests that transferrin uptake and the release of iron from the transferrin to the interior of the cell are controlled independently.