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Quantification of alpha-fetoprotein and transferrin endocytosis by lymphoid cells using flow cytometry.

Alpha-fetoprotein (AFP) and transferrin (Tf) are serum proteins actively internalized by many growing cells through specific cell surface receptors. The intracellular pathways of AFP and Tf are very similar: both proteins enter the cells via coated pits and receptosomes, move to tubular elements of the transreticular portion of the Golgi and are recycled back to the cell surface and extracellular medium in native form. In the present work, the capacity of human lymphoid cells to bind AFP and Tf at 4 degrees C and to endocytose them at 37 degrees C was quantified by flow cytometry analysis on a FACS 440 using fluoresceinated derivatives of these proteins. The results obtained show that binding and internalization of AFP and Tf by lymphoid cells are saturable processes at either 4 degrees C or 37 degrees C. The method developed permits direct quantitative measurement of molecules bound to the cell surface or present within the cell.