The monocytic receptor for lactoferrin and its involvement in lactoferrin-mediated iron transport

Several studies suggest biological functions of the iron-binding neutrophilic glycoprotein lactoferrin that imply an initial interaction with cells from the monocyte/macrophage family. Among these, an important role of lactoferrin as responsible for the inflammatory-induced blood hyposideremia and accumulation of iron in the monocyte/macrophage system has been suggested mainly based on experiments in rodents. In a series of experiments we have examined the binding of human lactoferrin to human monocytes. We have demonstrated the presence of a receptor binding including a high-affinity component and a low-affinity component. The affinity of the binding is compatible with a biological significance of this receptor (KD is about 10(-8) M, and the number of receptors about 10(6) per cell). More than 90% of the lactoferrin will dissociate from the cell. The binding is not truly reversible since lactoferrin will lose its receptor-binding property after dissociation from the cell. The only observed change in the molecule is a small decrease in isoelectric point from 8.9 to 8.8. Lactoferrin is able to translocate at least 50% of its bound iron to intracellular ferritin in monocytes. These findings are compatible with the idea that lactoferrin might be involved in the pathogenesis of the disturbances in iron metabolism observed during inflammation.