EVIDENCE FOR A LOW K-M TRANSPORTER FOR NON-TRANSFERRIN-BOUND IRON IN ISOLATED RAT HEPATOCYTES

Non-transferrin-bound iron (NTBI) plays an important role in the hepat ocellular injury induced by iron overload. However, the mechanism resp onsible for NTBI uptake into hepatocytes remains poorly defined. The p urpose of this study was to define the kinetics of NTBI uptake by isol ated rat hepatocytes and to characterize the uptake process. NTBI upta ke was time and temperature dependent, exhibited a Michaelis-Menten co nstant (K-m) value of 1.25 mu M and maximum uptake of 241 pmol . 10(6) cells(-1) . min(-1), and Fe-55 was incorporated in part into intracel lular ferritin. Uptake was Ca2+ dependent, exhibiting 15 and 80% of ma ximal uptake in the presence of 0.6 and 0.75 mM CaCl2, respectively. T he putative NTBI transporter was highly specific; divalent (Zn2+, Mn2, Cd2+, and Co2+) or trivalent (La3+) cations did not inhibit Fe3+ upt ake. Reduction from Fe3+ to Fe2+ was not essential for uptake or the p rocess occurred deep within the membrane bilayer, since the Fe2+ chela tor ferrozine did not influence Fe-55 uptake. These data provide evide nce for a low K-m plasma membrane transporter for NTBI, which should b e functional at physiological serum concentrations and saturated in ir on-overload diseases, such as hemochromatosis.