Uptake of ¹²⁵I-labelled α₂-macroglobulin and albumin by human placental syncytiotrophoblast in vitro

We have investigated the binding and internalization of α₂- macroglobulin and serum albumin by human placental syncytiotrophoblast cells in vitro. The time course (obtained at 4°C) of α₂-macroglobulin binding indicated that an equilibrium was reached after 4 h. The binding of ¹²⁵I- labelled α₂-macroglobulin to syncytiotrophoblast cells was competitively reduced in the presence of excess unlabelled α₂-macroglobulin. When the concentration-dependence of binding was examined over a wide concentration range, non-linear regression analysis yielded a K(d) of 6.4 nM. In the case of albumin, binding was weak and ligand dissociated from the cell surface during aqueous washing making it impractical to analyze the binding reaction. In other experiments, syncytiotrophoblast cells were incubated with ¹²⁵I- labelled α₂-macroglobulin at 37°C. Under these conditions, trypsin- resistant cell-associated radioactivity increased with time consistent with ligand internalization. ¹²⁵I-Labelled-ligand was internalized with a t 1/4 of about 5 min. After a lag period some radioactivity was released back into the incubation medium. When measured at times up to 210 min, this was found to consist of mostly TCA-precipitable material that had been lost from the cell surface. However, when the incubation was extended to 24 h, almost 15% of the initial cell-associated radioactivity was released to the extracellular medium as TCA-soluble material, consistent with a slow rate of ligand degradation. The specific binding of ⁶⁵Zn-labelled α₂M was similar to that of the ¹²⁵I-labelled ligand and trypsin-resistance measurements provided evidence of α₂M-mediated ⁶⁵Zn uptake. These results support a role for syncytiotrophoblast in the metabolism of α₂-macroglobulin during pregnancy and are also consistent with a role for α₂-macroglobulin in the maternal-fetal transport of zinc.