Transferrin in fishes: A review article

Most forms of life require iron for growth and survival, because it is involved in many cellular metabolic pathways and enzymatic reactions (oxygen transport, electron transport, DNA synthesis, etc.). Iron does not normally exist in a free form in animals, but excess iron can catalyze the conversion of hydrogen peroxide into free radicals, via the Fenton reaction[1,2]. This can damage the cellular structure and finally kill the cell[3,4]. The toxic effect of iron is avoided under normal circumstances through mechanisms that minimize free iron in the body. Any iron not incorporated as a functional moiety of proteins is bound to transport or storage proteins in a nontoxic ferric (Fe) form[5]. The iron-complexing agents serve to solubilize iron in the media surrounding the living organism, transport it within the animal and deliver it to the cells. Iron exists as a heme complex in heme proteins (hemoglobin, myoglobin, cytochromes) or as a nonheme protein compounds (transferrin, ferritin and hemosiderin) to be transported as a redox-inactive form[1]. The nonheme ferritin and hemosiderin are mostly involved in iron storage, whereas transferrin is an iron transport protein that binds to iron atom, thus making it unavailable for catalysis of superoxide radical formation. Transferrin is a single monomeric glycoprotein of molecular weight of 80 kDa, approximately 700 amino acids in length that transports iron involved in many metabolic processes amongst the sites of absorption[6], storage and utilization, hence considered as the major iron binding protein in the plasma of vertebrate species. It is an iron-binding protein which reversibly binds iron and can create low-iron conditions and which restricts the growth of some pathogenic bacteria[7]. Members of transferrin group of proteins are evolutionarily related and include transferrin from serum (serotransferrin), ovotransferrin (conalbumin) from egg white, lactoferrin from milk, tears and leucocytes, and the membranebond, tumour-associated melanotransferrin[8].