Interaction of ¹²⁵I-labelled ceruloplasmin with human trophoblast cells in vitro

In vitro binding, internalization and release of the plasma protein ceruloplasmin was investigated using primary cultures of human placental trophoblast cells. Binding of ¹²⁵I-labelled ceruloplasmin at 4°C reached equilibrium by 5-6 h; binding was linear throughout all concentrations tested (1 nM-3.3 μM). Addition of greater than 5-10 μM unlabelled ceruloplasmin or a variety of other proteins (albumin, transferrin, IgG) were equally effective in displacing bound ceruloplasmin in a concentration-dependent manner. When cells were incubated at 37°C, the majority of surface-bound ¹²⁵I-labelled ceruloplasmin was released directly to the extracellular medium. Trypsin-resistant radioactivity increased to 18 per cent of initially bound ceruloplasmin within 1 min, declining to 5 per cent by 2 h. The acquisition of trypsin-resistant radioactivity was unaffected by the addition of a variety of metabolic inhibitors and no evidence of intracellular degradation of ceruloplasmin was found. In summary, our results suggst that the majority of ceruloplasmin binding to trophoblast cells is nonspecific, of low affinity, and easily dissociable at 4°C. Only a small amount of ceruloplasmin appeared to be internalized, by an as yet unknown mechanism.