Abstract
Heme formation in reticulocytes from rabbits and rodents is subject to end product negative feedback regulation: intracellular "free" heme has been shown to control acquisition of transferrin iron for heme synthesis. To identify the site of control of heme biosynthesis in the human erythron, immature erythroid cells were obtained from peripheral blood and aspirated bone marrow. After incubation with human 59Fe transferrin, 2-[14C]glycine, or 4-[14C]delta-aminolevulinate, isotopic incorporation into extracted heme was determined. Addition of cycloheximide to increase endogenous free heme, reduced incorporation of labeled glycine and iron but not delta-aminolevulinate into cell heme. Incorporation of glycine and iron was also sensitive to inhibition by exogenous hematin (Ki, 30 and 45 microM, respectively) i.e. at concentrations in the range which affect cell-free protein synthesis in reticulocyte lysates. Hematin treatment rapidly diminished incorporation of intracellular 59Fe into heme by human erythroid cells but assimilation of 4-[14C]delta-aminolevulinate into heme was insensitive to inhibition by hematin (Ki greater than 100 microM). In human reticulocytes (unlike those from rabbits), addition of ferric salicylaldehyde isonicotinoylhydrazone, to increase the pre-heme iron pool independently of the transferrin cycle, failed to promote heme synthesis or modify feedback inhibition induced by hematin. In human erythroid cells (but not rabbit reticulocytes) pre-incubation with unlabeled delta-aminolevulinate or protoporphyrin IX greatly stimulated utilization of cell 59Fe for heme synthesis and also attenuated end product inhibition. In human erythroid cells heme biosynthesis is thus primarily regulated by feedback inhibition at one or more steps which lead to delta-aminolevulinate formation. Hence in man the regulatory process affects generation of the first committed precursor of porphyrin biosynthesis by delta-aminolevulinate synthetase, whereas in the rabbit separate regulatory mechanisms exist which control the incorporation of iron into protoporphyrin IX.
Highlights
Heme formation in reticulocytes from rabbits and rodents is subject to end product negative feedback regulation: intracellular “free” heme has been shown to control acquisition of transferrin iron for heme synthesis
In human reticulocytes, addition of ferric salicylaldehyde isonicotinoylhydrazone, to increase the pre-heme iron pool independently of the transferrin cycle, failed to promote heme synthesis or modify feedback inhibition induced by hematin
In human erythroid cells heme biosynthesis is primarily regulated by feedback inhibition at one or more steps which lead to d-aminolevulinate formation
Summary
Heme formation in reticulocytes from rabbits and rodents is subject to end product negative feedback regulation: intracellular “free” heme has been shown to control acquisition of transferrin iron for heme synthesis. Many experiments on negative feedback control in reticulocytes were carried out under conditions where availability of substrates may have been limiting for hemoglobin synthesis (especially amino acids and transferrin iron (5)) Under these circumstances, the disturbance of cellular metabolism would be expected to desensitize the mechanism which normally regulates heme formation. Further experiments showing that protoporphyrin synthesis is unaffected by hemin treatment and that addition of b-aminolevulinate or free protoporphyrin IX does not stimulate heme synthesis, suggested that uptake of iron from transferrin, intracellular iron delivery, or feedback inhibition of ferrochelatase regulates heme formation (12). Further support for these findings was provided by elegant experiments with iron chelates of acylhydrazones, which donate iron for heme synthesis independently of transferrin (13). It has long been known that synthesis of heme may be detected readily in peripheral blood samples obtained from individuals with sickle cell anemia (15, 16), there are few reported investigations into the control of heme formation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
