ABSTRACT
Transferrins are a group of bilobal glycoproteins that contain two homologous metal binding sites with high affinities for ferric iron [41]. Although originally thought to be restricted to vertebrates, transferrin-like proteins have now been identified in several invertebrate species where they occur in the hemolymph [42]. In vertebrates, the transferrins are found in a variety of body fluids, including the serum transferrins found in blood; the ovotransferrins found in avian egg white; the lactoferrins found in milk, tears, saliva, and other secretions, and the melanotransferrins found anchored to the membrane surfaces of melanocytes and other cells via a glycosyl-phosphatidylinositol linkage. Each of these vertebrate transferrins consists of a single polypeptide chain with an Mr of about 80,000 containing two structurally similar binding sites for ferric iron and other metals and a binding site for a synergistic anion [41]. The proteins of the transferrin family play a crucial biological role by sequestering and solubilizing iron, thereby controlling the levels of free iron in body fluids [43]. Serum transferrin, in particular, has the role of binding ferric ions in the bloodstream and transporting this bound iron to cells where it is released by a process of receptormediated endocytosis [44]. Iron release occurs at the reduced intracellular pH (—5.5), apparently with the active participation of the receptor [45], and the iron-free apotransferrin is returned to circulation without degradation.
