Haem synthesis in sideroblastic anaemia

Understanding Sideroblastic Anemia: An Overview of Genetics, Epidemiology, Pathophysiology and Current Therapeutic Options

Sideroblastic anemia (SA) consists of a group of inherited and acquired anemias of ineffective erythropoiesis characterized by the accumulation of ring sideroblasts in the bone marrow due to disrupted heme biosynthesis. Congenital sideroblastic anemia (CSA) is rare and has three modes of inheritance: X-linked (XLSA), autosomal recessive (ARCSA), and maternal. Acquired SA is more common and can be a result of myelodysplastic syndromes (MDS) or other, generally reversible causes. The diagnostic approach to SA includes a work-up for reversible causes and genetic testing for CSA based on clinical suspicion, family history and genetic pedigree. The treatment of SA depends on the underlying etiology but remains primarily supportive with vitamin B6 supplementation for select cases of XLSA, thiamine for thiamine-responsive megaloblastic anemia subtype, red blood cell transfusions for symptomatic patients and iron chelation therapy for iron overload. The management of anemia in MDS subtypes with ring sideroblasts remains unique and includes the recently approved erythroid maturation agent, Luspatercept. Although there is currently no curative therapy for CSA, anecdotal reports of hematopoietic stem cell transplant demonstrate remissions in selective, non-syndromic cases. This review summarizes the genetics, pathophysiology, diagnosis and treatment of SA for general practitioners and clinical hematologists.

High-dose vitamin therapy stimulates variant enzymes with decreased coenzyme binding affinity (increased K(m)): relevance to genetic disease and polymorphisms

As many as one-third of mutations in a gene result in the corresponding enzyme having an increased Michaelis constant, or K(m), (decreased binding affinity) for a coenzyme, resulting in a lower rate of reaction. About 50 human genetic dis-eases due to defective enzymes can be remedied or ameliorated by the administration of high doses of the vitamin component of the corresponding coenzyme, which at least partially restores enzymatic activity. Several single-nucleotide polymorphisms, in which the variant amino acid reduces coenzyme binding and thus enzymatic activity, are likely to be remediable by raising cellular concentrations of the cofactor through high-dose vitamin therapy. Some examples include the alanine-to-valine substitution at codon 222 (Ala222-->Val) [DNA: C-to-T substitution at nucleo-tide 677 (677C-->T)] in methylenetetrahydrofolate reductase (NADPH) and the cofactor FAD (in relation to cardiovascular disease, migraines, and rages), the Pro187-->Ser (DNA: 609C-->T) mutation in NAD(P):quinone oxidoreductase 1 [NAD(P)H dehy-drogenase (quinone)] and FAD (in relation to cancer), the Ala44-->Gly (DNA: 131C-->G) mutation in glucose-6-phosphate 1-dehydrogenase and NADP (in relation to favism and hemolytic anemia), and the Glu487-->Lys mutation (present in one-half of Asians) in aldehyde dehydrogenase (NAD + ) and NAD (in relation to alcohol intolerance, Alzheimer disease, and cancer).

Pyridoxine Refractory X-Linked Sideroblastic Anemia Caused by a Point Mutation in the Erythroid 5-Aminolevulinate Synthase Gene

To elucidate how pyridoxine-refractory X-linked sideroblastic anemia (XLSA) develops, we analyzed the erythroid-specific 5-aminolevulinate synthase (ALAS-E) gene of a patient with the anemia. The activity and amount of the enzyme in bone marrow cells of the patient were found to be approximately 5% of the normal control. We identified a point mutation, which introduces an amino acid substitution from Asp 190 to Val. In transient transfection analyses using quail fibroblasts, accumulation of aberrantly processed proteins, the sizes of which were larger than that of mature ALAS-E, was found in mitochondria. The proteins were reproducibly detected in assays combining in vitro transcription/translation of ALAS-E precursor and import of the precursor into isolated mouse mitochondria. These results suggest that the mutation causing pyridoxine-refractory XLSA affects the processing of the ALAS-E precursor, thus provoking instability of the ALAS-E protein.

Pyridoxine Refractory X-Linked Sideroblastic Anemia Caused by a Point Mutation in the Erythroid 5-Aminolevulinate Synthase Gene

To elucidate how pyridoxine-refractory X-linked sideroblastic anemia (XLSA) develops, we analyzed the erythroid-specific 5-aminolevulinate synthase (ALAS-E) gene of a patient with the anemia. The activity and amount of the enzyme in bone marrow cells of the patient were found to be approximately 5% of the normal control. We identified a point mutation, which introduces an amino acid substitution from Asp 190 to Val. In transient transfection analyses using quail fibroblasts, accumulation of aberrantly processed proteins, the sizes of which were larger than that of mature ALAS-E, was found in mitochondria. The proteins were reproducibly detected in assays combining in vitro transcription/translation of ALAS-E precursor and import of the precursor into isolated mouse mitochondria. These results suggest that the mutation causing pyridoxine-refractory XLSA affects the processing of the ALAS-E precursor, thus provoking instability of the ALAS-E protein.

Histological changes in rat embryonic blood cells as a possible mechanism for ventricular septal defects produced by an N-phenylimide herbicide

An N-phenylimide herbicide, S-53482, inhibits protoporphyrinogen oxidase, an enzyme common to chlorophyll and heme biosynthesis, and produces embryolethality, teratogenicity [mainly ventricular septal defects (VSD) and wavy ribs], and growth retardation in rats. In order to elucidate the mechanism of the developmental toxicity, in particular VSD, effects of the herbicide on rat embryonic blood cells were investigated histologically at the light and electron microscopic levels at 6, 12, 24, 36, and 48 h after oral administration of the chemical to pregnant rats on day 12 of gestation, the most sensitive day for toxicity. Electron and light microscopy demonstrated mitochondrial lesions, including abnormal iron deposits that were probably due to inhibition of heme biosynthesis, in erythroblasts derived from the yolk sac. Subsequently, degeneration of these erythroblasts occurred followed by erythrophagocytosis. Histologically hearts from exposed embryos had a thin ventricular wall, which may reflect a compensatory reaction to a loss of embryonic blood cells. Thus, the herbicide may induce VSD due to hematological dysfunction caused by the inhibition of heme biosynthesis rather than by direct injurious effects on the heart.

Molecular defects of erythroid 5-aminolevulinate synthase in X-linked sideroblastic anemia

The erythroid-specific isozyme of 5-aminolevulinate synthase (ALAS2), the first and rate-limiting enzyme of heme biosynthesis, is expressed concomitantly with the differentiation and maturation of the erythroid cell in order to accommodate generation of the large amounts of heme required for hemoglobin production. During the past few years the ALAS2 gene and its transcript have been characterized and the amino acid sequence of the enzyme deduced. The human genetic disorder X-linked sideroblastic anemia, previously postulated to be caused by defects of ALAS, has now been analyzed at the molecular and tissue-specific level. A heterogeneous group of point mutations in the catalytic domain of the ALAS2 enzyme has been found to cause the disorder. Impaired activity of recombinant mutant ALAS2 enzymes has also been demonstrated. Characterization of molecular defects in individuals with X-linked sideroblastic anemia has provided improved diagnosis for at-risk family members.

Myopathy, lactic acidosis, and sideroblastic anemia: a new syndrome

We describe 2 sibs (brother and sister) with myopathy, sideroblastic anemia, lactic acidosis, mental retardation, microcephaly, high palate, high philtrum, distichiasis, and micrognathia. Very low levels of cytochromes a, b, and c were detected in the patients' muscle mitochondria. Deposition of iron within the mitochondria of bone marrow erythroblasts was observed on electron microscopy. Irregular and enlarged mitochondria with paracrystalline inclusions were also seen on electron microscopy of the patients' muscle specimen. Examination of DNA from the affected sibs showed no deletions in the mitochondrial DNA nor the mutations identified in the syndromes of mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) or myoclonus, and epilepsy associated with rugged-red fibers (MERRF). Since the parents were first cousins and 2 of 6 sibs (male and female) were affected, we suggest that the syndrome expressed by our patients represents a previously unknown autosomal recessive disorder that includes mitochondrial myopathy, lactic acidosis, and sideroblastic anemia.

Sideroblastic anaemia

The startling morphological abnormalities of sideroblastic anaemia contrasts our uncertainty about its cause. Studies are hampered by the fact that the abnormality resides in the dividing and differentiating erythroblast which is difficult to obtain pure and in large numbers, and in which many levels of metabolic control must coexist. Recent molecular biology approaches have confirmed abnormalities of erythroid delta-aminolaevulinic acid synthase as the cause of X-linked, pyridoxine-responsive sideroblastic anaemia and mitochondrial DNA deletions as the most common cause of congenital macrocytic sideroblastic anaemia. They have also identified a second X-linked sideroblastic anaemia locus linked to phosphoglycerate kinase and associated with ataxia. An association between sideroblastic anaemia and the use of an oral copper chelating agent has highlighted unexplained links between erythroid copper and iron metabolism. Management decisions in relation to pyridoxine treatment, iron reduction, family studies, genetic counselling and antenatal diagnosis have in recent years become of practical relevance to families with known cases of congenital sideroblastic anaemia and careful documentation of the clinical outcome of these cases and of other family members is invaluable. Parallel and integrated studies on the molecular biology of erythroid differentiation are revealing the range of possible controlling influences on erythroblasts and defining the circumstances for each, allowing studies on the cause of the most prevalent form of sideroblastic anaemia (the idiopathic acquired form) and those inherited forms that are not X-linked to be approached with a much clearer perspective.

Accumulation of iron in erythroblasts of patients with erythropoietic protoporphyria

We have studied the iron metabolism in nine patients with erythropoietic protoporphyria (EPP) and three patients with sideroblastic anaemia (SA). All, except one EPP patient were iron deficient. The SA patients had a secondary haemochromatosis. The bone marrow aspirates of patients with SA and also three patients with EPP had a high incidence of ring sideroblasts. Ultrastructural examination of the bone marrow consistently showed finely dispersed electron-dense deposits localized in mitochondria of erythroblasts in all patients with EPP and SA. Mitochondrial electron energy-loss spectroscopy (EELS) indicated identical iron compounds in erythroblasts of all EPP and SA patients. These findings indicate that the mitochondrial iron utilization is disturbed in EPP and SA. The observation of mitochondrial iron deposition in erythroblasts in EPP and SA suggests that this failure is not of pathognomonic value for diagnosis of SA, but is apparently the result of an inefficient haem synthesis, in EPP due to a defective ferrochelatase. The mitochondrial iron deposition does not depend on the iron status (iron overload or iron deficiency) of the EPP patient.

Drug-induced red cell dyscrasias

Major advances have been made in recent years in our understanding of the pathogenetic mechanisms of drug-induced blood dyscrasias, particularly those involving the red cell. Among the latter, hemolytic anemia is the most common. Drug-induced red cell destruction may occur on an immune basis or through disruption by the drug of red cell metabolism. The immunological basis of drug-induced hemolysis is reviewed with emphasis on the clinical and laboratory manifestations, differential diagnosis and the major mechanisms involved. Drug-induced oxidative hemolysis both in normal individuals and in those with certain enzymopathies, notably glucose-6-phosphate dehydrogenase deficiency, is summarized. Drugs may also produce red cell dyscrasias by acting on the immature erythroid compartment. Some of these inhibit erythroid growth by as yet poorly understood mechanisms. Others exert more specific metabolic effects in erythroid precursors. These include drugs which interfere with DNA synthesis causing megaloblastic erythropoiesis and those which disrupt mitochondrial function and the synthesis of heme manifested by sideroblastic erythropoiesis. A brief consideration of heme biosynthesis and the action of drugs which are associated with sideroblastic anemia, including the antituberculous agents, lead, alcohol and chloramphenicol is presented. Finally, where pertinent, an updated listing of drugs involved in red cell dyscrasias is included.

Haem arginate treatment for hereditary sideroblastic anaemia

It has been shown that haem arginate treatment increases blood cell counts, improves the sideroblast status of the bone marrow and normalises decreased activities of haem synthesising enzymes in some patients with acquired sideroblastic anaemia, or with other types of myelodysplastic syndromes. 4 patients with hereditary sideroblastic anaemia (HSA), belonging to two families, were therefore treated with haem arginate infusions, 3 mg/kg, on 4 consecutive days, and thereafter weekly for 10 wk. No effect was observed on the mildly anaemic haemoglobin levels or on the red cell counts. However, the initially low or low-normal myeloid to erythroid ratio in the marrow increased in all patients. A consistent decrease in the percentage of ring sideroblasts and other abnormal sideroblasts was seen in 1 patient (Family A), and a temporary decrease of abnormal sideroblasts took place during the most intensive treatment period in 2 other patients (Family B). Two of three initially abnormal haem synthesising enzyme activities became normal in Family A, whereas no clearly consistent effects on the haem synthesising enzymes were observed in Family B. The present study shows that haem arginate infusions can normalise the activities of haem synthesising enzymes in some patients with HSA. Further studies are needed to evaluate the impact of haem infusions on the iron balance of these patients.

5-Aminolaevulinic acid synthase activity in developing human erythroblasts

5-Aminolaevulinic acid (ALA) synthase activity was measured in highly purified preparations of age-matched human erythroblasts. Enzyme activity in immature normoblasts was four-fold higher than that found in late orthochromatic normoblasts. ALA synthase activity in the immature erythroblasts in primary acquired sideroblastic anaemia (PASA) was reduced and remained unchanged during further erythroid differentiation. The pattern of erythroblast ALA synthase activity in two patients with congenital dyserythropoietic anaemia (CDA) and in one patient with beta-thalassaemia intermedia was similar to that found in PASA. This study has clearly demonstrated reduced erythroblast ALA synthase activity in PASA but has also found reduced enzyme activity in conditions in which ring sideroblasts are not prominent. This would suggest that haem synthesis is abnormal in PASA but that reduced erythroblast ALA synthase activity does not inevitably lead to ring sideroblast formation.

Heme arginate treatment for myelodysplastic syndromes

Heme arginate was given to 26 patients with a myelodysplastic syndrome (MDS) as infusions of 2-3 mg/kg body weight weekly for 8-12 weeks. Most of the patients first received a loading dose on four consecutive days. Six of the patients showed improvement in cytopenias during the therapy. In three of the responders severely depressed blood cell counts recovered to normal or close to normal. So far the maximum duration of a response after the cessation of the treatment is 25 months, and the two ongoing responses have lasted for 11 and 12 months, respectively. In two responders of the eight patients with more than 15% ring sideroblasts the number of ring sideroblasts decreased during the treatment but remained unchanged in six non-responders. The responders were characterized by a low or low normal heme synthase activity which increased during the treatment, whereas the non-responders showed a higher mean heme synthase activity which decreased during the treatment. In general, the responders had significantly fewer defects in heme synthetic enzyme activities than the non-responders. FAB type, karyotype or growth pattern in in vitro cultures of hematopoietic progenitors did not predict the response. Apart from one case of mild venous irritation, no other adverse effects were seen. The present study shows that heme arginate induces beneficial effects on cytopenia in some MDS patients and has very few side-effects.

Sideroachrestic anemia with iron loading: treatment with desferrioxamine

A case of nontransfusion-dependent sideroachrestic anemia (SA) with hemosiderosis is described that showed significant improvement of hemosiderosis and fibrosis of the liver following treatment with desferrioxamine. The anemia, although not transfusion dependent, did not allow continued therapy with phlebotomies. Following the removal of about 16 g iron over 4.5 years, normalization of serum ferritin and reversal of fibrosis of the liver were observed. Management problems and the prognostic implications of desferrioxamine therapy are discussed.

Measurement of 5-aminolevulinic acid synthase activity in whole and fractionated human bone marrow: effect of myeloid cell lysis by monoclonal antibody

A sensitive radiochemical assay for the measurement of bone marrow and erythroblast 5-aminolevulinic acid (ALA) synthase (EC 2.3.1.37) was developed and optimized with respect to sample preparation and reagent concentration. Succinylacetone (4,6-dioxoheptanoic acid) was used to prevent ALA utilization during the incubation period. Sample purification on a Sep-Pak cartridge (Waters Associates) followed by reverse-phase high-performance liquid chromatography (HPLC) allowed rapid isolation of pure ALA-pyrrole, free from radioactive succinate and other contaminants. ALA synthase activity was measured in unfractionated bone marrow and in samples from which myeloid cells had been removed by monoclonal antibody-mediated cell lysis. Myeloid-derived ALA synthase was calculated and found to contribute approximately half of the total unfractionated marrow enzyme activity. This suggests that results from previous studies using unfractionated bone marrow which have assumed that myeloid cells are an insignificant source of ALA synthase require reappraisal.

Hereditary sideroblastic anaemia and ataxia: an X linked recessive disorder

We report two families in which a non-progressive spinocerebellar syndrome and a sideroblastic anaemia are segregating together in an X linked recessive fashion. Four males in two generations of one family and a fifth male from an unrelated family had both conditions. Both the sideroblastic anaemia and the spinocerebellar syndrome differ from those which have previously been reported to be inherited in an X linked recessive manner. The association of these two clinically distinct disorders in two unrelated families suggests that they are either two closely linked loci which have undergone simultaneous mutation or pleiotropic effects of an altered allele at a single locus. All the heterozygous women had normal neurological examinations and normal haematocrits and red cell indices. Some had ring sideroblasts on bone marrow examination, a dimorphic peripheral blood smear, and raised serum free erythrocyte protoporphyrin, suggesting that a proportion of heterozygotes can be detected by appropriate haematological studies.

Coproporphyrinogen oxidase activity and porphyrin concentrations in peripheral red blood cells in hereditary sideroblastic anaemia

The activity of coproporphyrinogen oxidase and the concentrations of coproporphyrin and protoporphyrin (measured by HPLC) in peripheral red blood cells were established in 2 families with different types of hereditary sideroblastic anaemia. 2 males and 4 females were members of a family with an X-chromosome-linked and pyridoxine-responsive HSA, and 3 females were members of another family where the mode of inheritance is not clear and where pyridoxine did not produce a haematological response. Coproporphyrinogen oxidase activity was normal in 8 of 9 patients and slightly decreased only in 1 patient. All patients had normal red cell coproporphyrin concentrations, but red cell protoporphyrin concentration was decreased in 4 patients. These findings indicate that in vivo haem synthesis was not impaired at the step of coproporphyrinogen oxidase, hence enzymatic defects in earlier steps of haem synthesis are more evident. Earlier suggestions of impaired haem synthesis at this level, based on observed increased concentrations of coproporphyrin in peripheral red blood cells might be explained by the use of unspecific methods.

Pathologic Sideroblasts and Siderocytes Associated with Chloramphenicol Therapy in a Dog

Siderotic granules were recognized in blood erythrocytes from a male Boxer dog with suppurative prostatitis, cystitis and pyelonephritis that was being given high dosage chloramphenicol therapy. Siderotic inclusions were recognized in the cytoplasm of 96% of the rubricytes and metarubricytes in a bone marrow aspirate. Siderotic inclusions were numerous and in some cases formed a ring around the nucleus. This perinuclear location suggested that pathologic mitochondrial iron accumulation had occurred, resulting in the formation of "ringed" sideroblasts. The occurrence of pathologic sideroblasts was confirmed by electron microscopy. Blood siderocytes and bone marrow sideroblasts disappeared after cessation of chloramphenicol therapy.

Late onset pyridoxine-responsive sideroblastic anaemia after allogeneic bone marrow transplantation

We report the late development of transfusion-dependent sideroblastic anaemia in two patients who received a bone marrow transplant for chronic granulocytic leukaemia in blast crisis and T-cell acute lymphoblastic leukaemia respectively. Treatment with pyridoxine has either stopped or reduced these transfusion requirements. Red cell transfusion dependence later after marrow transplantation is rare and sideroblastic anaemia must be excluded.

Sideroblastic anaemia. A review of seven paediatric cases

Sideroblastic Anaemias are characterised by a) chronic hypochromic anemia, b) ringed sideroblasts in the bone marrow, c) an increase in total body iron, d) ineffective erythropoiesis and e) often abnormal concentrations of F.E.P. A classification of Sideroblastic Anaemia is given and the pathophysiology of Sideroblastic Anaemia is discussed. A series of seven paediatric cases with Sideroblastic Anaemia is presented and the results of studies of the iron, vitamin B6 and porphyrin metabolism are discussed. In two cases arguments for an ALA-synthetase deficiency are given. All five males were diagnosed as hereditary X-linked Sideroblastic Anaemia, one female as I.R.S.A. and the other female, who showed the features of the X-linked type, as congenital Sideroblastic Anaemia.

The laboratory evaluation of microcytic red blood cells

Microcytic red blood cell states are common clinical problems in both adult and pediatric age groups. The recent widespread availability of electronic blood cell counters for performing routine blood counts has increased the detection of microcytic red blood cells. Physicians must workup both symptomatic and asymptomatic patients with microcytic red blood cells before they can initiate proper therapy and/or counseling. The purpose of this review is threefold: (1) to discuss the causes of microcytic red blood cells in terms of disorders of decreased heme production vs. disorders of decreased globin production, (2) to review the clinical laboratory tests useful in differentiating microcytic red blood cell states, and (3) to present a practical approach for the laboratory workup of microcytic red blood cells.

Haem biosynthesis in refractory sideroblastic anaemia associated with the preleukaemic syndrome

The activities of 5 enzymes of the haem biosynthetic pathway and the protoporphyrin concentrations have been measured in peripheral red blood cells of 23 patients having a preleukaemic syndrome with refractory sideroblastic anaemia. A decreased delta-aminolaevulinic acid synthase (ALA-S) activity, an increased uroporphyrinogen I synthase activity and an increased red cell protoporphyrin concentration were consistent findings. Patients with abnormal leucocyte and/or platelet counts in the peripheral blood as well as patients with an excess of blast cells in the bone marrow had the lowest ALA-S activities. A further decrease in ALA-S activity was observed in 3 patients after leukaemic change in the disease. Patients having cytogenetic abnormalities showed no unique enzyme abnormalities. These results indicate that enzymatic disturbances of haem synthesis cannot be used as prognostic indicator of leukaemic transformation in refractory sideroblastic anaemia, but a very low ALA-S activity appears to accompany the development of a leukaemia in such patients.

Determination of delta-aminolaevulinic acid synthase activity in human bone marrow using high performance liquid chromatography

A high performance liquid chromatographic (HPLC) method is described for the rapid and specific determination of the activity of the enzyme delta-aminolaevulinic acid synthase (ALA-S) in mitochondria prepared by sonication of human bone marrow cells. After incubation with 14C-alpha-ketoglutarate the 14C-delta-aminolaevulinic acid (ALA) formed is converted to a pyrrole derivative, 2-methyl-3-carbethoxy-4-(3-propionic acid) pyrrole. This is isolated by reversed-phase ion-pair chromatography on a Hypersil-SAS column with methanol-water (45:155, v/v) in the presence of 0.005 mol/l 1-heptanesulphonic acid (PIC B-7) as the mobile phase. The radioactivity of the isolated pyrrole is determined by scintillation counting. The optimal substrate concentration and pH were 0.17 mmol/l alpha-ketoglutarate and pH 7.4, with an optimal period of sonication of 18s. Under these conditions ALA production was proportional to the concentrations of erythroblasts in the initial sample and was linear with time up to 60 min. The addition of pyridoxal phosphate (PLP) did not affect ALA-S activity in normal subjects. The mean ALA-S activity in 10 haematologically normal control subjects was found to be 318.8 pmol.10-6 erythroblasts.h-1 (S.D. 125.8, range 193-444.6).

The activities of delta-aminolaevulinic acid synthase and haem synthase in experimental sideroblastic anaemia. Effect of mitochondrial iron excess on the enzyme activity in peripheral red blood cells

Experimental sideroblastic anaemia was produced in normal and in iron loaded guinea pigs by intraperitoneal (i.p.) administration of isoniazid and cycloserine. Subsequently, the activities of delta-aminolaevulinic acid synthase (ALA-S) and of haem synthase in peripheral red blood cells were measured and in particular the relationships of enzyme activities to the iron status were examined. The ALA-S activity showed a similar decrease in all animals with sideroblastic anaemia. The haem synthase activity was increased probably due to secondary induction, but it was significantly less increased in animals with the highest values for iron status. This finding indicates that mitochondrial iron accumulation may have limited the compensatory increase of haem synthase activity. It is likely that also in human sideroblastic anaemia mitochondrial iron overload may have a secondary limiting effect on the haem synthase activity in erythroid cells.

Energy-dispersive X-ray analysis of the mitochondria of sideroblastic anaemia

Energy-dispersive X-ray analysis has been performed on marrow sideroblasts obtained from 10 patients with sideroblastic anaemias or erythroleukaemia (six primary refractory sideroblastic anaemia, two pyridoxine-responsive, one secondary sideroblastic anaemia, two erythroleukaemia). Irrespective of the nature of the disorder associated with the presence of sideroblasts, X-ray analysis of siderotic mitochondria consistently revealed the presence of iron and phosphorus with the average Fe/P intensity ratio measuring 1.4-1.5. Other elements variably detected within siderotic mitochondria included calcium, lead, potassium and zinc. Variation in the presence of these latter elements was detected not only between different patients, but also within different samples taken at different times from a single patient and even among different cells of the same sample. Despite the detection of lead in siderotic mitochondria of a significant number of patients (five out of seven), there was no clinical evidence of lead toxicity. The elemental composition of the intramitochondrial deposits in sideroblasts was distinct from that of ferritin or haemosiderin and probably consists of ferric phosphate, possibly, ferric orthophosphate (FePO4).