Free erythrocyte protoporphyrin.

Porphyrins are widely distributed in living cells throughout nature and play essential roles in various metabolic processes such as photosynthesis, transportation of oxygen and cellular respiration. In man the most important of the porphyrins is protoporphyrin 9, type III, which in combination with iron and specific proteins forms such compounds as hemoglobin, myoglobin, cytochrome, peroxidase and catalase. In addition to its occurrence in these compounds, protoporphyrin has been found in apparently uncombined or free form in erythrocytes, hence the term "free erythrocyte protoporphyrin" (1-3). Cartwright and associates (4) recently have summarized many of the salient features of previous work concerning free erythrocyte protoporphyrin. Suffice it to say here that the origin, function and fate of free protoporphyrin in erythrocytes remain uncertain. Indirect evidence suggests that it serves as an intermediary in synthesis of hemoglobin. Increased free erythrocyte protoporphyrin is thought to result from an interruption in normal synthesis of hemoglobin, as in the presence of iron deficiency states, increased erythropoiesis, or a disturbance of iron metabolism (5). Decreased levels have been ascribed to a deficiency in synthesis of protoporphyrin (6). There is evidence that reticulocytes contain more free protoporphyrin than do mature erythrocytes (7, 8). This investigation has been concerned with the study of amounts of free protoporphyrin 2 in eryth-