Heme controls expression of genes involved in the synthesis of globins and heme. The mammalian transcription factor Bach1 functions as a repressor of the Maf recognition element (MARE) by forming antagonizing hetero-oligomers with the small Maf family proteins. We show here that heme binds specifically to Bach1 and regulates its DNA-binding activity. Deletion studies demonstrated that a heme-binding region of Bach1 is confined within its C-terminal region that possesses four dipeptide cysteine-proline (CP) motifs. Mutations in all of the CP motifs of Bach1 abolished its interaction with heme. The DNA-binding activity of Bach1 as a MafK hetero-oligomer was markedly inhibited by heme in gel mobility shift assays. The repressor activity of Bach1 was lost upon addition of hemin in transfected cells. These results suggest that increased levels of heme inactivate the repressor Bach1, resulting in induction of a host of genes with MARES:

FLVCR, a member of the major facilitator superfamily of transporter proteins, is the cell surface receptor for feline leukemia virus, subgroup C. Retroviral interference with FLVCR display results in a loss of erythroid progenitors (colony-forming units-erythroid, CFU-E) and severe anemia in cats. In this report, we demonstrate that human FLVCR exports cytoplasmic heme and hypothesize that human FLVCR is required on developing erythroid cells to protect them from heme toxicity. Inhibition of FLVCR in K562 cells decreases heme export, impairs their erythroid maturation and leads to apoptosis. FLVCR is upregulated on CFU-E, indicating that heme export is important in primary cells at this stage. Studies of FLVCR expression in cell lines suggest this exporter also impacts heme trafficking in intestine and liver. To our knowledge, this is the first description of a mammalian heme transporter.

Summary Recent advances in the molecular understanding of the porphyrias now offer specific diagnosis and precise definition of the types of genetic mutations involved in the disease. Molecular diagnostic testing is powerful and very useful in kindred evaluation and genetic counselling when a disease‐responsible mutation has been identified in the family. It is also the only way to properly screen asymptomatic gene carriers, facilitating correct treatment and appropriate genetic counselling of family members at risk. However, it should be noted that DNA‐based testing is for the diagnosis of the gene carrier status, but not for the diagnosis of clinical syndrome or severity of the disease, e.g. an acute attack. For the diagnosis of clinically expressed porphyrias, a logical stepwise approach including the analysis of porphyrins and their precursors should not be underestimated, as it is still very useful, and is often the best from the cost‐effective point of view.

δ-Aminolevulinic acid (ALA) dehydratase catalyzes the synthesis of porphobilinogen (PBG) from two molecules of ALA. A semimicro method for the colorimetric determination of ALA dehydratase is presented and applied to various tissues. The enzyme activity in adult male rat liver was 2.22 and 1.94 μmol PBG formed/g liver/h for homogenates assayed with or without dithiothreitol, respectively. The assay was linear for at least 2.5 h and for up to 2.5 mg tissue per assay. The K(m) for ALA was 4.0 X 10^-4 M and the pH optimum was 6.2-6.4. The effects of activators and inhibitors on enzyme activity are discussed.

The process of erythroid differentiation in mouse Friend leukemia virus transformed cells (T3-C1-2) was examined by following changes in several enzyme activities of the heme biosynthetic pathway and in heme concentration while the cells were undergoing erythroid differentiation after treatment with dimethylsulfoxide. Untreated cells on the one hand, have a limited capacity for spontaneous differentiation. On the other hand, dimethylsulfoxide(DMSO)-treated cells showed an increase in the activities of delta-aminolevulinic acid (ALA) synthetase, ALA dehydratase, uroporphyrinogen-I synthetase, ferrochelatase, and heme concentration by days 1, 1.5, 2, and 4, respectively. The increase of the heme pathway enzymes and heme concentration followed the order of these enzymes or products as they are arranged in the heme biosynthetic pathway. These changes induced by DMSO were effectively inhibited by treatment with actinomycin D, suggesting that continued RNA synthesis is required for the differentiation process. 5-bromo-2'-deoxyuridine (BrdU) (10(-5) M) inhibited the DMSO-induced changes of the heme pathway enzymes. BrdU was most effective when it was present during the first 2 days of cell culture. It gradually lost its inhibitory effect when added after the 3rd day or later. The BrdU-mediated inhibition was completely overcome by the addition of thymidine (7 x 10(-5) M), but not by uridine (7 x 10(-5) M). All these data suggest that a sequential induction of the heme pathway enzyme takes place during erythroid differentiation of Friend leukemia cells, and that the sequential induction of the enzymes may be due to a sequential activation of genes coding for these enzyme activities.