Multiple regulatory steps in erythroid heme biosynthesis

Current models for regulation of heme synthesis during erythropoiesis propo se that the first enzyme of the pathway, 5-aminolevulinate synthase (ALAS), is the rate-limiting enzyme. We have examined cellular porphyrin excretion in differentiating murine erythroleukemia cells to determine in situ rate- limiting steps in heme biosynthesis. The data demonstrate that low levels o f coproporphyrin and protoporphyrin accumulate in the culture medium under normal growth conditions and that during erythroid differentiation the leve l of excretion of coproporphyrin increases approximately 100-fold. Iron sup plementation lowered, but did not eliminate, porphyrin accumulation. While ALAS induction is necessary for increased heme synthesis, these data indica te that other enzymes, in particular coproporphyrinogen oxidase, represent downstream rate-limiting steps. (C) 2000 Academic Press.