The Biological Role of Heme in Human Nutrition

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Four out of five people in the world may be iron-deficient, making nutritional iron deficiency the most common nutritional disorder. Although considerable experimental and nutritional data support an essential role for heme (iron-protoporphyrin IX) as a bioavailable source of iron in human nutrition, the pathways for heme absorption and utilization are currently unknown. From a cell biological perspective, heme is the prosthetic group for many important biological processes, and in eukaryotes it is synthesized in the mitochondrial matrix. Because heme is a hydrophobic molecule and is cytotoxic due to its intrinsic peroxidase activity, we hypothesize that heme does not merely diffuse through lipid bilayers but is actively transported via specific intracellular pathways. We propose to use the animal model, Caenorhabditis elegans, to identify the cellular pathways for heme transport and the molecules mediating heme trafficking, because this animal is the only known genetic animal model that is unable to synthesize heme eye no^o albeit requiring heme to sustain metabolic processes. Since C. elegans lacks the ability to make heme, it provides us with a clean genetic background devoid of endogenous heme, and the capacity to externally control the metabolic flux of heme. Thus, C. elegans is an obligate heme auxotroph and will be an excellent animal model to study dietary heme absorption as well as intracellular heme trafficking for transport, sequestration and incorporation into critical hemoproteins. The cellular pathways for heme transport will be mapped in C. elegans by characterizing phenoclusters of mutants isolated from forward genetic screens by biochemically assaying for heme levels and hemoprotein activity, evaluating viability and tracking heme transport in live animals with heme analogs, and determining heme distribution at the ultrastructural level using electron microscopy. The molecular identities of the mutated genes will be determined by categorizing mutants in genetic complementation groups, mapping the mutated genes using SNPs, localizing the mutant gene by RNA interference and gene rescue, and by sequencing candidate genes to determine the molecular lesion. The heme regulated genes in C. elegans, identified from our DMA microarrays, will be characterized by quantitatively validating subsets of genes by real-time PCR and RNA blot analysis, determining the function of the candidate genes by RNA interference, and defining the temporal and spatial expression of these gene products in response to heme by synthesizing transcriptional and translational reporter gene fusions. The results from these studies will provide new mechanistic insights into heme homeostasis in eukaryotes and may aid in the development of heme-based nutritional interventions for human iron deficiency, and potential drug targets for human helminthic infections that exacerbate iron deficiency.