Extracellular glycine is necessary for optimal hemoglobinization of erythroid cells

Vertebrate heme synthesis requires three substrates: succinyl-CoA, which regenerates in the tricarboxylic acid cycle, iron and glycine. For each heme molecule synthesized, one atom of iron and eight molecules of glycine are needed. Inadequate delivery of iron to immature erythroid cells leads to a decreased production of heme, but virtually nothing is known about the consequence of an insufficient supply of extracellular glycine on the process of hemoglobinization. To address this issue, we exploited mice in which the gene encoding glycine transporter 1 (GlyT1) was disrupted. Primary erythroid cells isolated from fetal livers of GlyT1 knockout (GlyT1−/−) and GlyT1-haplodeficient (GlyT1+/−) embryos had decreased cellular uptake of [2−14C]glycine and heme synthesis as revealed by a considerable decrease in [2-14C]glycine and 59Fe incorporation into heme. Since GlyT1−/− mice die during the first postnatal day, we analyzed blood parameters of newborn pups and found that GlyT1−/− animals develop hypochromic microcytic anemia. Our finding that Glyt1-deficiency causes decreased heme synthesis in erythroblasts is unexpected, since glycine is a non-essential amino acid. It also suggests that GlyT1 represents a limiting step in heme and, consequently, hemoglobin production

Human Ind1, an Iron-Sulfur Cluster Assembly Factor for Respiratory Complex I

Respiratory complex I (NADH:ubiquinone oxidoreductase) is a large mitochondrial inner membrane enzyme consisting of 45 subunits and 8 iron-sulfur (Fe/S) clusters. While complex I dysfunction is the most common reason for mitochondrial diseases, the assembly of complex I and its Fe/S cofactors remains elusive. Here, we identify the human mitochondrial P-loop NTPase, designated huInd1, that is critically required for the assembly of complex I. huInd1 can bind an Fe/S cluster via a conserved CXXC motif in a labile fashion. Knockdown of huInd1 in HeLa cells by RNA interference technology led to strong decreases in complex I protein and activity levels, remodeling of respiratory supercomplexes, and alteration of mitochondrial morphology. In addition, huInd1 depletion resulted in massive decreases in several subunits (NDUFS1, NDUFV1, NDUFS3, and NDUFA13) of the peripheral arm of complex I, with the concomitant appearance of a 450-kDa subcomplex representing part of the membrane arm. By a novel radiolabeling technique, the amount of iron associated with complex I was also shown to reflect the dependence of this enzyme on huInd1 for assembly. Together, these data identify huInd1 as a new assembly factor for human respiratory complex I with a possible role in the delivery of one or more Fe/S clusters to complex I subunits