The cellular binding properties of a new conjugate, I-125-mEGF-dextran, in which the amino terminus on mEGF was covalently coupled by reductive amination to the reducing end of dextran D14 were analysed. The coupling molar ratio was 1:1 since dextran only contains one aldehyde group and mEGF only has one free amino group available; the amino terminus. The conjugates were I-125-labelled and tested for their receptor binding properties using cultured human glioma, U-343MGaC12:6, cells. The conjugate reached maximal binding around 1.5 or 2 h when incubated at 37 degrees C or 4 degrees C, respectively. The binding was receptor specific since it could be displaced by free mEGF. Dissociation constants were determined at 4 degrees C by using mEGF to displace I-125-mEGF and non-radioactive mEGF-dextran to displace I-125-mEGF-dextran and were 6.6 x 10(-10) and 7.1 x 10(-9) M respectively. Cellular internalisation was studied at 37 degrees C for both I-125-mEGF-dextran and I-125-mEGF and most of the radioactivity was internalized in both cases. However, there was a difference regarding the retention time pattern. It took less than 1 h for the internalised radioactivity delivered with mEGF to decrease to 50% of the initial level while it took about 2.5 h for the conjugate. The studies on this new form of EGF-containing conjugate serve as a model for the design of future dextran containing conjugates employing, for example, antibody fragments or small ligands with tumour specificity.

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