This study investigates the binding, internalization, and degradation of 125I-labeled human epidermal growth factor (hEGF) in human fibroblasts. The results show that hEGF binds specifically and saturably to human fibroblasts. At 37°C, the bound hEGF is initially recovered in a native form by acid extraction, but subsequent incubation leads to rapid degradation, with the appearance of 125I-monoiodotyrosine in the medium. At 0°C, the bound hEGF is not degraded but slowly dissociates from the cell. These findings suggest a mechanism where hEGF first binds to the cell surface and is then internalized before degradation. Degradation is blocked by various inhibitors, including metabolic energy inhibitors, protease inhibitors, lysosomotropic agents, local anesthetics, and ammonium chloride. The binding and degradation of hEGF lead to a loss of the fibroblasts' ability to rebind fresh hormone, but this capacity is restored by incubation in a serum-containing medium. The restoration is inhibited by cycloheximide or actinomycin D. hEGF is a polypeptide with a molecular weight of approximately 5,400, purified from human pregnancy urine. It exhibits biological activities similar to mouse-derived EGF (mEGF) and competes with 125I-labeled mEGF for binding to human fibroblasts. However, the amino acid compositions and immunological and electrophoretic properties of human and mEGF are not identical, suggesting that hEGF is an evolved form of the mouse-derived polypeptide. The study also shows that hEGF is a potent mitogen for human fibroblasts, stimulating DNA synthesis and cell proliferation. The binding and metabolic fate of the bound hormone were examined. The results indicate that hEGF is rapidly internalized and degraded in human fibroblasts, with degradation occurring via endocytosis and lysosomal proteases. The degradation of hEGF is temperature-dependent, with degradation at 37°C being more rapid than at 0°C. The degradation of hEGF is also inhibited by various agents, including chloroquine, which is known to inhibit lysosomal enzyme activity. The study also shows that the degradation of hEGF is energy-dependent, requiring metabolic energy for the process. The results suggest that the degradation of hEGF is a complex process involving internalization, lysosomal degradation, and the release of degradation products such as monoiodotyrosine. The study also shows that the binding capacity of fibroblasts is restored by incubation in a serum-containing medium, but this restoration is inhibited by cycloheximide or actinomycin D. The study provides important insights into the mechanisms of hEGF binding, internalization, and degradationThis study investigates the binding, internalization, and degradation of 125I-labeled human epidermal growth factor (hEGF) in human fibroblasts. The results show that hEGF binds specifically and saturably to human fibroblasts. At 37°C, the bound hEGF is initially recovered in a native form by acid extraction, but subsequent incubation leads to rapid degradation, with the appearance of 125I-monoiodotyrosine in the medium. At 0°C, the bound hEGF is not degraded but slowly dissociates from the cell. These findings suggest a mechanism where hEGF first binds to the cell surface and is then internalized before degradation. Degradation is blocked by various inhibitors, including metabolic energy inhibitors, protease inhibitors, lysosomotropic agents, local anesthetics, and ammonium chloride. The binding and degradation of hEGF lead to a loss of the fibroblasts' ability to rebind fresh hormone, but this capacity is restored by incubation in a serum-containing medium. The restoration is inhibited by cycloheximide or actinomycin D. hEGF is a polypeptide with a molecular weight of approximately 5,400, purified from human pregnancy urine. It exhibits biological activities similar to mouse-derived EGF (mEGF) and competes with 125I-labeled mEGF for binding to human fibroblasts. However, the amino acid compositions and immunological and electrophoretic properties of human and mEGF are not identical, suggesting that hEGF is an evolved form of the mouse-derived polypeptide. The study also shows that hEGF is a potent mitogen for human fibroblasts, stimulating DNA synthesis and cell proliferation. The binding and metabolic fate of the bound hormone were examined. The results indicate that hEGF is rapidly internalized and degraded in human fibroblasts, with degradation occurring via endocytosis and lysosomal proteases. The degradation of hEGF is temperature-dependent, with degradation at 37°C being more rapid than at 0°C. The degradation of hEGF is also inhibited by various agents, including chloroquine, which is known to inhibit lysosomal enzyme activity. The study also shows that the degradation of hEGF is energy-dependent, requiring metabolic energy for the process. The results suggest that the degradation of hEGF is a complex process involving internalization, lysosomal degradation, and the release of degradation products such as monoiodotyrosine. The study also shows that the binding capacity of fibroblasts is restored by incubation in a serum-containing medium, but this restoration is inhibited by cycloheximide or actinomycin D. The study provides important insights into the mechanisms of hEGF binding, internalization, and degradation