125I-labeled human epidermal growth factor (hEGF) binds in a specific and saturable manner to human fibroblasts. At 37 degrees C, the cell-bound 125I-hEGF initially may be recovered in a native form by acid extraction; upon subsequent incubation, the cell-bound 125I-hEGF is degraded very rapidly, with the appearance in the medium of 125I-monoiodotyrosine. At 0 degrees C, cell-bound 125I-hEGF is not degraded but slowly dissociates from the cell. The data are consistent with a mechanism in which 125I-hEGF initially is bound to the cell surface and subsequently is internlized before degradation. The degradation is blocked by inhibitors of metabolic energy production (azide, cyanide, dinitrophenol), some protease inhibitors (Tos-Lys-CH2Cl, benzyl guanidobenzoate), a lysosomotropic agent (chloroquine) various local anesthetics (cocaine, lidocaine, procaine), and ammonium chloride. After the binding and degradation of 125I-hEGF the fibroblasts are no longer able to rebind fresh hormone. The binding capacity of these cells is restored by incubation in a serum-containing medium; this restoration is inhibited by cycloheximide or actinomycin D.

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