The mechanisms of iron uptake by fetal rat hepatocytes in culture

The mechanisms of iron accumulation by cultured hepatocytes isolated from fetal rat liver (19 days gestation) were investigated using rat transferrin labeled with 125I and 59Fe. The rates of iron and transferrin internalization by the cells were measured by incubating the hepatocytes with the labeled transferrin at 37°C followed by treatment with pronase at 4°C to remove surface‐bound transferrin and iron. Iron internalization increased linearly with time. Approximately 65% of the internalized iron was incorporated into ferritin. In contrast to iron, the rate of transferrin internalization was biphasic, with a rapid phase during the first 10 to 15 min and a second slower phase which becomes more apparent after that time. Iron and transferrin internalization were temperature‐dependent. Chase experiments showed that the internalized transferrin donated all of its iron to the cell and was then released in a biphasic manner which was dependent on the time of preincubation with radiolabeled transferrin. These experiments showed that iron uptake occurs by at least three processes. The first mechanism involves the specific receptor‐mediated endocytosis of transferrin. Each cell has an average of 7.8 ± 1.0×105 (mean ± SE, n = 5) transferrin binding sites with an apparent association constant of 2.0 ± 0.4×106 M−1. The second process is nonsaturable up to a transferrin concentration of at least 6 μM but like the specific process, also leads to accumulation of iron in excess of transferrin. It involves the endocytosis of transferrin mediated by 4.2×106 transferrin binding sites which have a K′a value of 2.6×105 M−1. Third, fluid‐phase endocytosis of transferrin occurs at a rate of 0.13×104 molecules per cell per min at 1.25 μM transferrin concentration. This rate is not sufficient to account for the additional iron accumulated by the cells by the second process.