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

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 (Kd = 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).