In a previous paper, we demonstrated that cultured human fibroblasts synthesize transferrin (Tf). Two types of Tf are present
one is produced by the cells and the other is internalized from the culture medium. To study the metabolism of intracellular Tf, we investigated the subcellular localization of the two types of Tf in human fibroblasts by immunocytochemical and fluorescence-labeling techniques. The internalized Tf was found to be localized in the perinuclear area, and the synthesized Tf was associated with microtubules, forming a fibrous structure in the cytoplasm. When the cells were treated with colchicine which depolymerizes microtubules irreversibly, the synthesized Tf lost its fibrous structure and spread out in cytoplasm, but the internalized Tf remained around the nucleus. These results suggest that the two types of Tf are regulated differently in the cells.