Iron Transport Function Research Articles

The G185R Mutation Disrupts Function of the Iron Transporter Nramp2

Microcytic anemia (mk) mice and Belgrade (b) rats have severe iron deficiency anemia due to defects in intestinal iron transport and erythroid iron utilization. Both animal mutants carry the same missense mutation in Nramp2, the first mammalian iron transporter to be identified. This mutation, in which glycine 185 is changed to arginine (G185R), occurs within predicted transmembrane domain 4 of the protein. We have performed site-directed mutagenesis of murine Nramp2, focusing on amino acids of transmembrane domain 4 that are highly conserved among Nramp-like proteins. We have expressed each mutant form in transfected cells and examined iron transport function, subcellular localization, and protein amounts. All tested forms of Nramp2 localize to the plasma membrane and to transferrin-containing endosomes. Most transmembrane domain 4 mutations affect the amount of protein detected and consequently show diminished iron transport. The G185R mutation, however, causes near total loss of Nramp2 function that cannot be fully explained by a decreased amount of protein, indicating that G185R disrupts iron transport through an alteration in the function of Nramp2, rather than degradation of the protein.© 1998 by The American Society of Hematology.

Inhibition of proliferative responses of mouse spleen lymphocytes by lacto‐ and ovotransferrins

This study evaluated the influence of three transferrin family proteins (bovine lactotransfer‐rin, bLf; chicken ovotransferrin, cOf; quail ovotransferrin, qOf) at concentrations ranging from 10 to 50 fig ml‐1 on proliferative responses of mouse spleen lymphocytes stimulated by lipopolysaccharide (LPS) and phytohemagglutinin (PHA). The transferrins significantly inhibited both the mitogen‐induced proliferation in a dose‐dependent manner. Ovotransferrins, especially qOf, had cytotoxicity for the lymphocytes. There is no relationship between iron saturation and inhibitory effect of each transferrin or cytotoxicity of ovotransferrins. All these transferrins bound to CD4+ T‐lymphocytes, CD8+ T‐lymphocytes and B‐lymphocytes, but no transferrin conjugated with PHA. PHA bound to lymphocytes previously incubated with each transferrin. These results indicate that the inhibition of proliferation by the transferrins and the cytoxicity of ovotransferrins are not attributable to a function of iron transport and to inactivation of mitogen, suggesting that the hyporesponsiveness is due to interaction of lymphocytes and/or accessory cells with each transferrin.

RNA-DNA hybridization analysis of transcription of the plasmid ColV-K30 aerobactin gene cluster.

Plasmid pABN1 contains the genetic determinants for the aerobactin iron uptake system of plasmid ColV-K30. Transposon Tn1000 mutants of pABN1 defective in synthesis of a 50,000-dalton polypeptide were found neither to secrete nor to accumulate aerobactin, but were not impaired in iron transport functions, clearly indicating a role for this polypeptide in aerobactin biosynthesis. RNA-DNA hybridization studies with probes spanning the entire aerobactin gene cluster showed that the system is regulated at the transcriptional level by the availability of iron in the external medium. When induced by low-iron stress, all five genes of the cluster were transcribed at a uniformly high level. When repressed by excess iron, transcripts of the four biosynthesis genes were some 30-fold less abundant in the case of the parental ColV-K30 plasmid and 10-fold less for the recombinant plasmid pABN1, whereas the receptor gene in either plasmid was transcribed at only about a third of the induced level.

Review of melanoma antigens recognized by monoclonal antibodies (MAbs). their functional significance and applications in diagnosis and treatment of melanoma

The introduction of monoclonal antibody techniques has led to a rapid advance in information concerning antigenic structures in melanoma cell membranes. These have been classified according to the extent of their expression on cells of other tissues, but it is evident that a more precise classification based on their biochemical nature is possible. Several monoclonal antibodies appear to define antigens restricted to melanoma cells and fetal tissues. Many antibodies recognize antigens shared with gliomas and nevi, whereas other groups can be defined which recognize antigens on melanocytes or other carcinomas. One of the commonly detected antigens was shown to be a high molecular weight (MW) proteoglycan which may be involved in reactions with other cells and the intercellular matrix. A second antigen was shown to be a ganglioside which may have receptor functions in cells. A third was shown to be a glycoprotein with iron transport functions. The latter antigen and the large MW proteoglycan have been a focus of attention for in vivo targeting studies in treatment and diagnosis. The ganglioside, large MW proteoglycan and a melanocarcinoma antigen may be detected in the circulation of patients and are being evaluated for monitoring of disease activity in patients with melanoma. Several monoclonals may be of value in histological evaluation of melanoma, e.g. diagnosis of preneoplastic lesions, metastatic lesions of unknown origin and identification of cell structures related to metastatic behaviour in the host. Further studies should help to define cellular structures recognized by the immune system in humans.

Transferrin: physiology and function in iron transport.

Since its identification about 30 years ago transferrin has attracted the interest of many investigators concerned with its structure, metal-binding properties, genetic polymorphism, and especially its role in the transport of iron within the body. Transferrin's two iron-binding sites appear to be structurally identical with equivalent iron binding after addition of iron in vitro. However, since the experiments of Fletcher and Huehns, the functional homogeneity of transferrin-bound iron has been questioned. Understanding of the precise mechanism of iron release from transferrin to receptor sites on reticulocytes and other tissues active in iron exchange is incomplete. Considerable evidence has been assembled from rats to support the Fletcher-Huehns hypothesis of selective release of A-site iron to erythrocyte precursors and placenta while B-site iron is delivered to hepatocytes. Results of experiments in rabbit and human systems remain controversial. Reasons for this controversy include: variations in the technique of iron addition to transferrin with the possibility of non-specific binding; variations in reticulocytes used in preparing selectively labelled transferrin and in its biological assay; and artifacts introduced in mixed-species experiments. Until methods are more refined and the transferrin-iron receptor interaction is better understood, the controversy about transferrins's iron transport function will persist.

Clearance of Iron from Hemochromatotic and Normal Transferrin in Vivo

Abstract The iron transport function of transferrin from normal subjects and patients with idiopathic hemachromatosis has been studied using the radioisotopes Fe59 and Fe55. It was concluded that transferrin from hemochromatotic patients functions in a normal manner in vivo.