The N‐terminal domain I of human lactotransferrin binds specifically to phytohemagglutinin‐stimulated peripheral blood human lymphocyte receptors

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Abstract
Human lactotransferrin receptors have been recently characterized on mitogen-stimulated human lymphocytes [(1989) Eur. J. Biochem. 179, 481–487]. In order to define the lactotransferrin recognition site by these receptors, the binding to lymphocytes of several tryptic fragments, isolated from human lactotransferrin by mild tryptic hydrolysis [(1984) Biochim. Biophys. Acta 787, 90–96], has been investigated. The 30 kDa N-tryptic fragment (residues 4–281) and the re-associated N,C-tryptic complex bind to lactotansferrin lymphocyte receptor with a dissociation constant of 44 nM and 39 nM, respectively, similar to the value obtained for the native lactotransferrin (K d = 46 nM). However, neither the N-terminal domain II (residues 91–257) nor the 50 kDa C-tryptic fragment (residues 282–703) are recognized. These results suggest that the binding site of human lactotransferrin by the lymphocyte receptor is located in the N-terminal lobe and more precisely in the N-terminal domain I (residues 4–90 and/or 258–281).