Receptor binding of lactoferrin by human monocytes

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Abstract
Summary. The binding of iron-saturated 125I-lactoferrin to human monocytes was studied at pH 7·4 and 37°C. Monocytes in suspension bound 125I-lactoferrin by a reversible, saturable and specific binding indicating the presence of a receptor. The dissociation constant (KD) of the binding was estimated at about 4·5 × 10−9 M and the number of receptors was about 1·6 × 106 per monocyte. The affinity of native lactoferrin (20% iron saturated) was only slightly below that of iron-saturated lactoferrin (KD about 7·9 × 10−9 M). Human transferrin, horse cytochrome c and human immunoglobulin G were without inhibitory effect on the binding of 125I-lactoferrin. The majority of cell-bound 125I-lactoferrin was dissociable. The dissociation rate was not affected by addition of unlabelled lactoferrin to the dissociation medium. The binding of 125I-lactoferrin to adherent mononuclear blood cells showed an about 100-fold lower affinity (KD about 2·5 × 10−7 M) than to cells in suspension, but the specificity of the binding was the same. These results are compatible with the idea that lactoferrin exerts a biological effect mediated by an interaction with cells of the monocyte/macrophage lineage.