Iron metabolism; utilization of intravenous radioactive iron.

T HE HUMAN body shows great economy in its handling of iron. Only small amounts of iron arefound in the excreta, and these amounts cannot be appreciably increased through loading body iron stores by oral ingestion,’ by parenteral iron administration2 or by multiple blood transfusions.3 These observations imply that iron is rigidly conserved by the body and that no attention need be directed toward excretion of iron in the absence of blood loss. This conservation serves to emphasize the importance of the metabolic cycle in the body where iron is reused again and again for hemoglobin formation. With the exception of the red cell iron, any estimate of the distribution of iron in man is at best an approximation because of the difficulty in separating tissue from hemoglobin iron. Circulating erythrocytes account for the largest mass of body iron, some 2. to 3 grams. This includes all erythrocytes in the body other than those developing in the bone marrow, since there appear to be no reserve depots of erythrocytes.4 From analogy with animal data,5 tissue iron available for hemoglobin production is perhaps one-fourth as much. Myoglobin, cell enzyme iron, and fixed tissue iron represent even smaller fractions whose iron turnover is not known. However, the work of Hahn and Whipple would indicate that myoglobin iron remains relatively constant despite marked changes in red cell and storage iron.6 The following studies were undertaken to determine the dynamic relationship between storage and circulating iron in normal subjects and in patients with a variety of hematologic disorders. When single tracer doses of radioactive iron (Fe55 and Fe59) are given intravenously in man, he radioiron rapidly enters the hemoglobin cycle and “tagged” erythrocytes begin to appear in the circulation within twenty-four hours. Thereafter, the radioactivity of circulating red cells rises and reaches a plateau in from two to three weeks. Similar findings have been reported by Dubach, Moore, and Minnich.7 This procedure appears to offer a method of measuring the participation of injected iron in the hemoglobin cycle. In the subsequent discussion the term utilization curve refers specifically to the utilization of the injected radioiron for hemoglobin production.