Erythropoietic defects in protoporphyria: a study of factors involved in labelling of porphyrins and bile pigments from ALA- 3 H and glycine- 14 C.

Abstract ALA- 3 H (tritium-labelled δ-aminolevulinic acid) and glycine- 14 C were employed simultaneously to determine the major tissue sources of porphyrins and bile pigments excreted by a patient with protoporphyria. In vitro data indicated a 30-fold increase in ALA synthetase activity in the patient's blood (compared to 2 normal controls), with a relative deficiency of heme synthetase. Most of the synthesized porphyrin appeared rapidly in the plasma with minimal mixing with preformed erythrocyte porphyrin. In vivo studies suggested the presence of 4 pools of protoporphyrin in the circulating red cells, with a "half-times" ranging from 1 hour or less to 12 days. Nearly all of the early peak 3 H and 14 C in fecal coproporphyrin, protoporphyrin, and urobilin were excreted within 4 days. Because the criteria generally used to relate isotope labelling to tissue sources may be quite misleading, new approaches to the quantitative analysis of such isotope data have been developed. Application of the proposed formulas to the present results indicates that the average 63 μM (35 mg.) of fecal protoporphyrin excreted daily represented a total formation of approximately 153 μM of excess protoporphyrin. The large majority of this excess was formed erythropoietically, apparently in older normoblasts and reticulocytes, from which it was liberated into the plasma. About 60 per cent of this excess pigment was converted to rapidly degraded (mainly nonerythropoietic) hemes, and then excreted as early peak urobilin. Several theoretical implications and practical advantages of the proposed formulas are considered.