Binding of Human Epidermal Growth Factor to Tissue Homogenates of the Rat

A study on binding of human epidermal growth factor (hEGF) to various rat tissue homogenates showed the existence of specific binding in the liver, kidney, small intestines (duodenum, jejunum and ileum), stomach, lung and heart. The extent of the specific binding in the liver was at least ten times as large as those in other tissues. However, no specific binding was observed in the spleen, muscle or brain. Binding kinetic analyses using tissue homogenates of the liver, kidney and small intestines showed that the binding capacities (n) differed remarkably among the tissues, whereas the tissue differences in the dissociation constants (Kd) were minimal (1-6 nm). In our previous report on the contributions of various tissues to hEGF removal from the systemic circulation in rats after i.v. administration, the uptakes of hEGF by the liver, kidney, small intestines, stomach and spleen showed remarkable saturation, which may represent the operation of a receptor-dependent uptake mechanism. In the present in vitro experiment, specific bindings in these tissues except the spleen were indeed identified, suggesting that specific binding plays an important role in the tissue distribution of hEGF. Some other mechanism may be involved in the hEGF distribution to the spleen, because specific binding to this tissue was not detected in vitro. Comparing hEGF binding kinetic data (at 4 °C) for isolated rat hepatocytes with those for the homogenate, the binding parameters showed little difference between the two preparations, indicating that, at least under normal conditions, EGF receptors are not down-regulated and most of them exist on the cell surface.