Identification of ceruloplasmin receptors on the surface of human blood monocytes, granulocytes, and lymphocytes

Membrane receptors for ceruloplasmin (CP) were identified on all human blood leukocytes (granulocytes, monocytes, and lymphocytes) by a visual probe and 125I-CP binding. To synthesize the visual probe, amide-modified submicron-sized polystyrene latex minibeads, activated with glutaraldehyde, were covalently bound to CP. Incubation of this probe with human leukocytes, either fractionated or unfractionated, led to its binding, which was visualized on individual cells by using electron microscopy. At 4 degrees C, only surface binding occurred, but internalization also occurred at 37 degrees C. The binding was completely inhibited in the presence of excess nonderived CP, indicating the specificity of the binding. Incubation of fractionated leukocytes with 125I-CP also led to its specific binding to all three fractions. Scatchard analysis indicated the highest number of receptors for granulocytes and the lowest for lymphocytes. The binding affinity was lowest, however, for granulocytes, with monocytes showing the highest affinity. These data, indicating active uptake of CP by blood leukocytes, may reflect the requirement of leukocytes for copper that can be derived from CP. CP may also serve other functions within the cells.