Iron is essential for maintaining hemoglobin synthesis and the integrity of red blood cell function. Iron metabolism in developing erythroid cells is highly controversial. The purpose of the proposed research is to analyze and define the iron delivery pathway in erythropoietic cells. These studies will be performed on a unique erythropoietic model in the European newt, Triturus cristatus, and with reticulocytes of Sprague-Dawley rats. Using biochemical and electron microscopic techniques, the specific aims of this research are: (1) to determine the extent to which synthesis of heme and globin are co-ordinate in different erythroid stages (2) to determine the physiologic significance and requirement for endocytic internalization of transferrin in the iron delivery process (3) to continue ongoing studies which suggest ferritin as an alternative source of cellular iron (4) to analyze the mechanism of ferritin iron release by pyridoxal-5 feet-phosphate and the efficacy of this factor in treatment of iron overload (5) to measure the number and binding affinity of transferrin receptors in different stages of RBC development and to correlate these parameters with rates of iron uptake and (6) to determine the extent to which primary regulation of iron uptake is dependent upon transferrin receptor number and a requirement for replacement of receptor by receptor recycling or receptor synthesis. These studies form part of a long-time project to elucidate the mechanism of iron uptake and delivery in erythropoietic cells and may possess critical value not only in understanding of iron metabolic disorders in man but also in the development of a non-toxic therapeutic agent for effective treatment of iron overload.