The influence of transferrin binding to L2C guinea pig leukemic lymphocytes on the endocytosis cycle kinetics of its receptor.

The parameters regulating the internalization and recycling of transferrin-specific receptors were determined in guinea pig leukemic B lymphocytes, in the absence or presence of ligand. We show that after the cells were purified, 45-56% of the total receptors were on the cell surface. In the absence of transferrin, unoccupied receptors are quickly internalized (rate constant, 0.12 min-1) whereas their recycling is much slower (rate constant, 0.026 min-1). This difference between endocytosis and recycling rates leads to a balanced receptor distribution with only 22% of the total receptors outside after incubation of the cells for 20-30 min at 37 degrees C. The internalization rate of occupied receptors, measured in the presence of transferrin is faster (rate constant, 0.21 min-1) than that of unoccupied receptors calculated in the absence of transferrin (0.12 min-1; see above). On the other hand, mere binding of transferrin to its receptor, without internalization, arrested by cytoplasm acidification, is sufficient to induce a large increase (by a factor of seven) in the recycling rate of unoccupied internal receptors from 0.026 min-1 to 0.17 min-1. Thus, in these lymphocytes, transferrin mobilizes internal receptors by modifying the kinetic rates of internalization and recycling, leading to a new equilibrium between external and internal receptors.