2 · 4H
2O), hydroxylammonium chloride, and Eppendorf ultrafiltration tubes with a molecular weight cut-off (MWCO) of 3 kDa were purchased from Merck (Darmstadt, Germany). Iron reference standards 1 g/L were bought from Bernd Kraft GmbH (Duisburg, Germany). Iron (III) chloride hexahydrate (FeCl
3 · 6H
2O), dialysis tubes (Spectrapor 6, MWCO 8 kDa), sterile Rotilabo
® syringe filters with cellulose mixed ester membranes were supplied by Roth (Karlsruhe, Germany). Spectra/Por
® 6 dialysis tubing with an MWCO of 10 kDa and a diameter of 29 mm was purchased from Serva Electrophoresis (Heidelberg, Germany). DMEM was purchased from Biochrom (Berlin, Germany), L-glutamine from PAA Laboratories (Pasching, Austria). Endothelial Cell Growth Medium (ECGM) with supplements (C-22010, C-39215) was purchased from PromoCell (Heidelberg, Germany). Water used in all experiments was of ultrapure quality.