Heme binding to Hep G2 human hepatoma cells.

Utilizing high specific activity [55Fe]hemin, the interaction of heme with monolayer cultures of human Hep G2 hepatoblastoma cells was examined. Initial characterization was performed at 4 degrees C to minimize the possibility of heme internalization. Specific binding of [55Fe]hemin at 4 degrees C reached equilibrium within 6 h and was 66% dissociable after 18 h in the presence of fresh binding buffer. Stereospecificity of the binding interaction was evidenced by variable degrees of competition with different metalloporphyrins. Scatchard analysis revealed a Ka of 0.01 nM-1 and the number of binding sites per cell was approx. 200,000. Elevation of the temperature during binding to 22 degrees C resulted in a small decrease in the affinity of the interaction and the apparent number of sites per cell was doubled, possibly due to artefactual elevation of the cell-associated radioactivity secondary to intracellular accumulation. These data are interpreted to indicate the presence of a specific heme receptor on the plasmalemma of Hep G2 cells.