The association of the urobilin "early peak" and erythropoiesis in man.

In a normal individual the administration of isotopically labeled glycine (1, 2) results in two peaks of radioactivity in the excreted fecal bile pigment. The first or "early" labeled peak of fecal bile pigment excretion occurs within the first few days and cannot be explained on the basis of the breakdown of normal circulating red cells. The second peak occurs 100 to 130 days after administration of the isotopic precursor and coincides with the termination of the life-span of the group of red cells that contains the labeled precursor. When the conversion of intravenously administered glycine-2-14C to serum bilirubin-14C was studied, two peaks of bilirubin specific activity were found, the first at 24 hours and the second at the third to fourth day (3). It must be recognized that the single observed fecal urobilin early peak is a result of fusion of the two early labeled peaks in the gastrointestinal tract and that the changes measured in these patients could, in theory, be due to a quantitative change in either component. Several theories have been proposed to explain the first or early fecal peak. The most widely accepted is that this pigment is derived from red cells that are destroyed before or shortly after their release from the bone marrow. For this reason the early peak has been thought of as representing "ineffective" erythropoiesis. With this hypothesis, London, West, Shemin, and Rittenberg (1) and Gray, Neuberger, and Sneath (2) calculated that in a normal individual 11 to 20%o of erythropoiesis is represented in this peak. However, recent studies have been interpreted as showing that the early peak might arise by a direct synthetic pathway from a common precursor pool that does