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Definition of binding of human serum transferrin to transferrin receptor Ung, Karen Chua


Intracellular iron levels affect cell division and cell existence; hence the viability of a cell is dependent on the presence of iron. In vertebrates, transferrin is responsible for the transport of iron, helping to control the available iron levels for conservation and bacteriostatic effect, and protecting cells against the toxic effects of free iron. Uptake of iron in cells occurs predominantly by the receptormediated endocytosis of diferric transferrin bound to the transferrin receptor on the cell surface. To date, limited information is available as to the nature of the interaction of transferrin with the transferrin receptor. High affinity monoclonal antibodies are useful as probes to determine regions of transferrin involved in binding to the transferrin receptor. For example, monoclonal antibodies raised against the N-lobe and C-lobe of chicken ovotransferrin have been used to demonstrate that both lobes of ovotransferrin must be present and associated for the binding to the ovotransferrin receptor to occur. As well, different affinities of the monoclonal antibodies to apoovotransferrin and holo-ovotansferrin have been used as evidence that local structural changes occur upon iron binding. Monoclonal antibodies against human serum transferrin have been raised and characterized. Some but not all of such antibodies inhibit the binding of serum transferrin to its receptor. In this study, the objective was to determine the antigenic epitope on human serum transferrin recognized by a monoclonal antibody that inhibits the binding of transferrin to its receptor. A bacterial cell culture system was used for the expression of N-lobe, Clobe and C-lobe fragments of human serum transferrin as a fusion protein with glutathione S-transferase. Using this expression system, the epitope of the antitransferrin antibody to C-lobe, designated as F-11, was identified to a small region on human serum transferrin. Comparison of the F-11 epitope with the known three-dimensional structure of human serum transferrin facilitated the

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