Shunt bilirubin: evidence for two components.

Studies with C(14)-labeled glycine and delta-aminolevulinic acid as heme-bilirubin precursors in man indicate that the early labeled or shunt bilirubin consists of two fractions. Fraction 1 requires 1 to 24 hours for maximum synthesis, is not dependent on marrow erythropoietic heme synthesis, and is possibly of anabolic origin (formed by a direct pathway from heme precursors). Fraction 2 requires 3 to 4 days for maximum production, is dependent on heme synthesis, and probably has its origin in the bone marrow, as a degradation product of red-cell heme.