Abnormalities of flavin monooxygenase as an etiology for sideroblastic anemia

We postulated that a deficiency of flavin monooxygenase (FMO)-a ferrireductase component of cells-could produce sideroblastic anemia. FMO is an intracellular ferrireductase which may be responsible for the obligatory reduction of ferric to ferrous iron so that reduced iron can be incorporated into heme by ferrochelatase. Abnormalities of this mechanism could result in accumulation of excess ferric iron in mitochondria of erythroid cells to produce ringed sideroblasts and impair hemoglobin synthesis. To investigate this hypothesis we obtained blood from patients with sideroblastic anemia and normal subjects. Extracts of peripheral blood lymphocytes were used to measure ferrireduction by utilization of NADPH. Lymphoid precursors are reported to accumulate iron in mitochondria similarly to erythroid precursors. Utilization of lymphoid precursors avoided the need for bone marrow aspirations. We studied three patients with sideroblastic anemia. One patient and his asymptomatic daughter had a significant decrease in ferrireductase activity. They also had markedly diminished concentrations of FMO in lymphocyte protein extracts on Western blots. This was accompanied by increased concentration of mobilferrin in the extracts. These results suggest that abnormalities of FMO and mobilferrin may cause sideroblastic anemia and erythropoietic hemochromatosis in some patients.

Separate pathways for cellular uptake of ferric and ferrous iron

Separate pathways for transport of nontransferrin ferric and ferrous iron into tissue cultured cells were demonstrated. Neither the ferric nor ferrous pathway was shared with either zinc or copper. Manganese shared the ferrous pathway but had no effect on cellular uptake of ferric iron. We postulate that ferric iron was transported into cells via beta(3)-integrin and mobilferrin (IMP), whereas ferrous iron uptake was facilitated by divalent metal transporter-1 (DMT-1; Nramp-2). These conclusions were documented by competitive inhibition studies, utilization of a beta(3)-integrin antibody that blocked uptake of ferric but not ferrous iron, development of an anti-DMT-1 antibody that blocked ferrous iron and manganese uptake but not ferric iron, transfection of DMT-1 DNA into tissue culture cells that showed enhanced uptake of ferrous iron and manganese but neither ferric iron nor zinc, hepatic metal concentrations in mk mice showing decreased iron and manganese but not zinc or copper, and data showing that the addition of reducing agents to tissue culture media altered iron binding to proteins of the IMP and DMT-1 pathways. Although these experiments show ferric and ferrous iron can enter cells via different pathways, they do not indicate which pathway is dominant in humans.

Iron absorption and transport-an update

Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Iron absorption and transport-an update

Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Iron absorption and transport-an update

Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Iron absorption and transport

Iron is vital for living organisms because it is essential for multiple metabolic processes to include oxygen transport, DNA synthesis, and electron transport. However, iron must be bound to proteins to prevent tissue damage from free radical formation. Thus, its concentrations in body organs must be regulated carefully. Intestinal absorption is the primary mechanism regulating iron concentrations in the body. Three pathways for intestinal iron uptake have been proposed and reported. These are the mobilferrin-integrin pathway, the divalent cation transporter 1 (DCT-1) [or natural resistance-associated macrophage protein (Nramp2)] pathway, and a separate pathway for uptake of heme by absorptive cells. Each of these pathways are incompletely described. However, studies with blocking antibodies, observations in rodents with disorders of iron metabolism, and studies in tissue culture cells suggest that the DCT-1 pathway is dominant in embryonic cells and is involved with cellular uptake of ferrous iron, whereas the mobilferrin-integrin pathway facilitates absorption of dietary inorganic ferric iron. Thus, there are separate pathways for cellular uptake of ferric and ferrous inorganic iron. Body iron can enter intestinal cells from plasma via basolateral membranes containing the classical transferrin receptor pathway with a high affinity for holotransferrin. This keeps the absorptive cell informed of the state of iron repletion of the host. Intestinal mucosal cell iron seems to exit the cell via a distinct apotransferrin receptor and a newly described protein named hephaestin. Unlike the absorptive surface of intestinal cells, most other cells possess transferrin receptors on their surfaces and the vast majority of iron entering these cells is transferrin associated. There seem to be 2 distinct pathways by which transferrin iron enters nonintestinal cells. In the classical clathrin-coated pitendosome pathway, iron accompanies transferrin into the cell to enter a vesicle, which releases the iron to the cytosol with acidification (high affinity, low capacity). Under physiological conditions, a second transferrin associated pathway (low affinity, high capacity) exists which has been named the transferrin receptor independent pathway (TRIP). How the TRIP delivers iron to cells is incompletely described. In addition, tissue culture studies show that nonintestinal cells can accept iron from soluble iron salts. This occurs via the mobilferrin-integrin and probably the DCT-1 pathways. Cellular uptake of iron from iron salts probably occurs in iron overloading disorders and may be responsible for free radical damage when the iron binding capacity of plasma is exceeded. Radioiron entering the cell via the heme and transferrin associated pathways can be found in isolates of mobilferrin/paraferritin and hemoglobin. This interaction probably occurs to permit NADPH dependent ferrireduction so iron can be used for synthesis of heme proteins. Production of heme from iron delivered via these routes indicates functional specificity for the pathways.

Iron absorption and cellular transport: the mobilferrin/paraferritin paradigm

Dietary inorganic iron is mostly ferric iron. This is solubilized at the acid pH level of the stomach where it chelates mucins and certain dietary constituents to keep them soluble and available for absorption in the more alkaline duodenum. Mucosal uptake of iron is facilitated by a beta 3 integrin and a 56 kDa protein known as mobilferrin. In the cytosol of the absorptive cell, iron is associated with a 520-kDa complex known as paraferritin which contains integrin, mobilferrin, and flavin monooxygenase. This complex serves as a ferrireductase to reduce iron to the ferrous state so that it is available for formation of end products such as heme proteins. The large complex has other constituents, such as beta 2 microglobulin, whose functions remain to be delineated. We postulate that the basolateral membranes of absorptive cells possess both holo-transferrin and apotransferrin receptors that regulate the ingress and egress of cellular iron, respectively. Unlike absorptive cells, nonintestinal cells appear to possess three pathways for uptake of inorganic iron: (1) the classical transferrin-transferrin receptor pathway, (2) the transferrin-associated transferrin receptor independent pathway (TRIP), and (3) the transferrin-independent mobilferrin-integrin pathway (MIP) observed in intestinal absorptive cells. The TRIP is used when transferrin receptors become saturated at physiological concentrations of iron and transferrin. The MIP may only be used efficiently for mucosal uptake of iron and iron-overloaded individuals with fully saturated transferrin. Alternatively, it may facilitate iron uptake from the TRIP after degradation of transferrin near the surface of the cell. However, both transferrin-associated pathways donate iron to a common intracellular iron pathway for ferri-reduction and probably other functions.

The alternate iron transport pathway: mobilferrin and integrin in reticulocytes

Iron transport in reticulocytes is known to occur via the well-described transferrin-receptor-endosome pathway. An alternative pathway for iron transport independent of transferrin has been postulated in reticulocytes and other cells. Transport of iron into reticulocytes from ferric citrate solutions was shown to be saturable and independent of transferrin. During transport of iron from ferric citrate, both cell surface integrins, and a soluble protein, mobilferrin, were labelled. This demonstrated that the reticulocyte transferrin independent pathway for iron transport involved integrins and mobilferrin similar to intestinal absorptive cells. This pathway would be expected to transport iron into cells under conditions of iron overload and was capable of providing iron for haemoglobin synthesis. Mobilferrin was also radiolabelled when radioiron labelled transferrin was incubated with reticulocytes and this occurred with a different time course than was observed following reticulocyte exposure to radiolabelled ferric citrate. This suggested that mobilferrin may serve as an intermediary in both pathways.

Modulation of glycosaminoglycans in mouse mammary tumors

Glycosaminoglycans were isolated from mouse mammary tumors and compared to the glycosaminoglycans isolated from normal postnatal developmental stages of the gland. Small, early tumors contained hyaluronic acid and a heparan with low sulfate content. This pattern was also characteristic of undeveloped normal glands from virgin mice. These tumors were encapsulated and not locally invasive. Large tumors contained a dermatan sulfate-nucleic acid complex previously described as the predominant glycosaminoglycan in the fully developed lactating normal gland. The large tumors were locally invasive. Mammary tumors did not utilize new patterns of extracellular matrix glycosaminoglycans, but returned to develop essentially less mature patterns in an uncontrolled manner.

Mobilferrin is an intermediate in iron transport between transferrin and hemoglobin in K562 cells

Iron is bound to transferrin in the plasma. A specific receptor on the cell surface binds transferrin and internalizes transferrin and the iron in clathrin-coated pits. These invaginate to form vesicles which release iron to the cytoplasm. Inorganic iron can be transported by an alternative pathway from iron citrate, utilizing a cell surface integrin and a cytoplasmic protein mobilferrin. This article shows that the two pathways donate iron to mobilferrin which acts as an intermediate between the iron bound to transferrin and the incorporation of iron into hemoglobin. Mobilferrin is found associated with the transferrin containing vesicles, and becomes labeled with iron released from transferrin in the vesicles. Mobilferrin is also found in the cytoplasm where pulse-chase experiments show that it, in turn, releases iron to be used for the synthesis of hemoglobin.

Proteoglycans and glycosaminoglycans during maturation of the mouse mammary gland

The mammary gland underwent morphological changes starting from a simple tubular structure, developing into a rich alveolar formation during lactation and ultimately a terminally differentiated tubular complex in retired breeders. Early stages of development, male and virgin mouse glands, were rich in hyaluronic acid but also contained smaller amounts of a low sulfate heparan. During lactation, there was a dramatic increase in sulfated glycosaminoglycan, in particular dermatan sulfate. Retired breeders were characterized by a highly sulfated heparan. These changes in glycosaminoglycan composition were analogous to changes seen in many embryologic tissues during morphogenesis and reflect the alterations in tissue modeling. The changes in glycosaminoglycans were reflective of the change observed in solubilized membrane associated proteoglycan. During lactation there was a dramatic increase in the content of dermatan sulfate proteoglycan. Both the dermatan sulfate and the proteoglycan which predominate during lactation was isolated as a complex with a small RNA, called pgRNA. The pgRNA was found only in-the lactating stage and not in virgin mice or retired breeders.

A small RNA is associated with dermatan sulfate proteoglycan

A specific RNA, called pgRNA, was bound to purified dermatan sulfate proteoglycan from porcine skin. The pgRNA was approximately 20 bases in length and contained greater than 80% guanosine. The pgRNA-proteoglycan complex was dissociated into pgRNA and apo-proteoglycan by denaturing conditions. The complex was reconstituted from apo-proteoglycan and pgRNA, but not by other RNA molecules. Both pgRNA and an additional smaller RNA, perhaps derived from pgRNA, were isolated complexed to the glycosaminoglycan dermatan sulfate derived from dermatan sulfate proteoglycan. Both the chromatographic and physical properties of the dermatan sulfate were influenced by the pgRNA binding.

Paraferritin: a protein complex with ferrireductase activity is associated with iron absorption in rats

Recent studies reported that iron salts were absorbed in the duodenum utilizing a pathway involving membrane-associated integrin and a cytosolic protein named mobilferrin. In addition, a large molecular weight cytoplasmic complex was labeled with radioiron during mucosal uptake of iron in the duodenum. The molecular mass of this protein was 520 000 daltons, slightly larger than ferritin. On denaturing SDS-PAGE, the purified protein complex appeared to consist of at least four polypeptides, closely associated with each other. This complex was called paraferritin because its hydrodynamic volume resembled ferritin. In the present work, antibody studies demonstrate the presence of integrin, mobilferrin, and flavin monooxygenase in the water-soluble complex. Biochemical studies demonstrate the presence of a NADPH-dependent flavin monooxygenase ferrireductase activity that reduces Fe(III) to Fe(II). Antibodies against either integrin or mobilferrin inhibit monooxygenase activity. Inhibition of monooxygenase activity decreases radioiron uptake by tissue culture intestinal cells. Thus, we postulated that paraferritin plays a role in the mucosal uptake and transport of inorganic iron in small intestinal absorptive cells and is a mechanism for both the internalization of integrin from membranes to cellular cytosol and the delivery of iron to cellular constituents in an appropriate redox state.

Hereditary hemochromatosis: a prevalent disorder of iron metabolism with an elusive etiology

Hereditary hemochromatosis is a prevalent inherited disorder with an estimated frequency of homozygosity of 0.2 to 0.45% in Caucasians. The disease is characterized by progressive iron overload until a massive accumulation of body iron occurs. Undetected, the disorder eventually can produce either cirrhosis, diabetes mellitus, cardiac disease, arthritis, or hepatocellular carcinoma or a combination of these manifestations. Early diagnosis and treatment prevents organ damage and normalizes life expectancy. Screening studies to detect hemochromatosis are most effectively accomplished by measurement of the serum iron and total iron binding capacity. Treatment is most effectively performed by frequent phlebotomy until body stores are empty and then 3 to 4 times yearly for life. The basic defect of hemochromatosis appears to increase iron absorption, decrease iron excretion, and produce preferential deposit of iron in hepatic parenchymal cells rather than Kupffer cells. The genetic abnormality of hemochromatosis is located on chromosome 6 in close association with the gene for HLA antigens. Recent speculation postulates that tumor necrosis factor may be involved in the etiology of this disease because of its location on chromosome 6 and its effect upon iron transport.

Alternate iron transport pathway. Mobilferrin and integrin in K562 cells

A transferrin-independent iron transport system in cells containing transferrin receptors was described previously by several investigators. Prior studies did not identify the proteins involved in this alternate iron transport pathway. Using a human-derived erythroleukemia tissue culture line, iron-binding proteins were isolated from cytosol and cell membranes. The cytosol protein was soluble in 60% ammonium sulfate, had a molecular mass similar to mobilferrin (56 kDa), and reacted with anti-mobilferrin antibodies. The water-insoluble radiolabeled protein was solubilized with Nonidet P-40 and immunoprecipitated with monoclonal antibody against beta 3 human integrin. Pulse-chase studies suggested sequential passage of iron to integrin, mobilferrin, and ferritin, respectively. Thus, the alternate iron transport pathway contained proteins similar to those observed in intestinal cells which did not possess transferrin receptors on their absorptive surface. The alternate iron transport pathway is only partially shared with zinc and cadmium. Mobilferrin bound zinc and iron competitively, but the two metals were not transported competitively into K562 cells. Immunoprecipitates of integrin containing radiozinc were obtained with a monoclonal antibody against beta 1 human integrin. This suggested iron and zinc may utilize different integrins to passage the cell membrane.

Regulation of iron absorption: proteins involved in duodenal mucosal uptake and transport

Newly identified iron (Fe)-binding proteins isolated from both rat and human duodenal mucosa permit a better understanding of Fe absorption. Mucins bind Fe at acid pH to keep it soluble and available for absorption at the more alkaline pH of the duodenum; this explains the development of Fe deficiency in achlorhydric subjects. Integrin was identified on the surface of enterocytes in association with radioiron and is believed to facilitate the transfer of Fe through the microvillous membrane. Mobilferrin, a 56 kDa Fe-binding protein, was identified in enterocyte cytosol. It coprecipitates with integrin and appears in close association with integrin in the apical cytoplasm of absorptive cells. We postulate it accepts dietary Fe from integrin and acts as the shuttle protein from Fe in the cytoplasm. Since Fe in enterocytes remains in equilibrium with body stores, we postulate mucosal Fe uptake is regulated by the number of Fe-binding sites either occupied or unoccupied by Fe on mobilferrin. Fe repletion of enterocytes from body stores is probably accomplished via transferrin receptors on the basal membranes of enterocytes. Increased transfer of Fe from blood into absorptive enterocytes occurs in Fe-replete animals to inhibit mucosal uptake of dietary Fe. Little transfer of Fe from plasma to enterocytes occurs in Fe deficiency. Enhanced mucosal transfer into the body occurs with increased body need for Fe. The exact mechanism for mucosal transfer of Fe into the plasma has not been defined but may also be mediated by an integrin.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat duodenal iron-binding protein mobilferrin is a homologue of calreticulin

BACKGROUND

Mobilferrin is a water soluble 56-kilodalton protein isolated from human and rat duodenal mucosa. It binds iron and other transitional metals in vivo and in vitro and is postulated to play a role in their absorption and intracellular metabolism. The purpose of this study was to characterize mobilferrin.

METHODS

Mobilferrin was characterized by identification of the N-terminal amino acid sequence, two-dimensional protein electrophoresis, and studies of mobilferrin and homologues using anti-mobilferrin antibody and competitive metal binding.

RESULTS

The N-terminal amino acid sequence of mobilferrin was Asp-Pro-Ala-Ile-Tyr-Phe-Lys-Glu-Gln-Phe-Leu-Asp-Gly-Asp-Ala-Ser-Thr- and is a homologue of calreticulin (calregulin). The proteins had a similar molecular mass (56 kilodalton) and isoelectric point (4.7). Anti-mobilferrin antibodies react with calreticulin. Both proteins bind iron and calcium but have a greater affinity for iron.

CONCLUSIONS

Mobilferrin and calreticulin are homologues that bind iron with greater affinity than calcium and other transitional metals. Competitive binding of metals by mobilferrin provides insight into the absorptive pathway shared by both essential and toxic transitional metals.

Function of integrin in duodenal mucosal uptake of iron

A mechanism for the absorption of inorganic iron in the small intestine is described in which integrins appear to play an important role in the passage of iron across microvillous membranes. Biochemical isolates from microvillous preparations of duodenum from rats dosed with radioiron showed radioactivity concentrated in integrins. The presence of integrins on mucosal surfaces of duodenal cells was confirmed by immunofluorescent microscopy using anti-integrin monoclonal antibodies. Immunoprecipitation methods were used to show that microvillous radioiron was precipitated with anti-integrin antibodies and that mobilferrin, a 56-Kd cytosol iron-binding protein, coprecipitated with integrins. We postulate from these data that the mucosal uptake of iron from the gut lumen is mediated via an integrin-mobilferrin pathway.

A concise review: iron absorption--the mucin-mobilferrin-integrin pathway. A competitive pathway for metal absorption

Newly identified iron binding proteins isolated from rat duodenal homogenates permit better understanding of iron absorption. Mucins bind iron at acid pH to keep iron soluble and available for absorption at the more alkaline pH of the duodenum; this explains iron deficiency following prolonged achlorhydria. Integrin (90/150 kD) was identified on the absorptive surface of enterocytes in association with radioiron and is believed to facilitate transit of iron through the microvillous membrane. Mobilferrin, a 56 kD iron binding protein, was isolated from enterocyte cytosol. It coprecipitates with integrin and appears in close association with integrins in the apical cytoplasm. We postulate it accepts dietary iron from integrin and acts as the shuttle protein for iron in the cytoplasm. Since iron in enterocytes remains in equilibrium with body stores, we postulate mucosal iron uptake is regulated by the number of iron binding sites either occupied or unoccupied by iron on mobilferrin. Iron repletion of enterocytes from body stores is accomplished via transferrin receptors on the posterolateral membranes of enterocytes. Increased transfer of iron from blood into absorptive enterocytes occurs in iron replete animals to inhibit mucosal uptake of dietary iron. Little transfer of iron from plasma to enterocytes occurs in iron deficiency. Enhanced mucosal transfer of iron into the body occurs with increased body need for iron. The exact mechanism for mucosal transfer of iron into the plasma has not been defined but may also be mediated by an integrin.

Newly identified iron-binding protein in human duodenal mucosa

Studies were undertaken using human duodenal mucosa to determine whether it contained a counterpart to a newly identified iron-binding protein recently isolated from rat duodenum and named mobilferrin. Water-soluble homogenates were prepared from duodena of patients undergoing surgery for pancreatic carcinoma. An iron-binding protein with an approximate molecular mass of 56 Kd was purified to homogeneity using 60% ammonium sulfate and serial chromatographic steps. The protein was biochemically and immunologically distinct from transferrin and ferritin, and competitively bound to zinc, cobalt, and lead. Each molecule bound one molecule of iron with a kd of 8.9 x 10(-5). Human isolates reacted in an enzyme-linked immunosorbent assay with a polyclonal antibody raised in rabbits against a similar duodenal protein isolated from rat duodenum. It is postulated that mobilferrin plays a significant role in the absorption of iron and other metals and may explain partially the competition between certain metals for absorption in the small intestine.

A role for mucin in the absorption of inorganic iron and other metal cations. A study in rats

The steps involved in iron absorption are poorly understood. Although transferrin and ferritin are water soluble, most radioiron in gut homogenates after an intraluminal dose of radioiron is recovered in water-insoluble precipitates. Most radioiron in the precipitates was insoluble in detergents and organic solvents and was characterized as mucins. These isolates bound iron in vitro with a Kd of 9.09 x 10(-5). Similar iron binding was observed with commercial mucins. Iron binding to mucin occurred at acid pH and maintained the iron available for absorption with alkalinization. Similar pH-dependent binding to mucin was observed with zinc, cobalt, and lead. Iron competitively inhibited binding of these metals to mucin. However, iron chelates of ascorbate, fructose, and histidine donated iron to mucin at neutral pH. These data provided a role for gastric HCl and intestinal mucin in absorption of iron and metal cations and partial explanation of the competition for absorption between certain metals from the gut lumen. It is postulated that intestinal mucin delivers inorganic iron to intestinal absorptive cells in an acceptable form for absorption.

The distinction of small cell and non-small cell lung cancer by growth in native-state histoculture

Histological analysis remains the primary method of distinguishing between small cell (SCLC) and non-small cell lung cancer (NSCLC). This distinction has significant impact therapeutically because of their relative difference in chemoresponsiveness (J.D. Minna et al., Principles and Practice of Oncology, pp. 396-474, 1981). Yet for at least 10% of lung tumors, pathologists will disagree upon the classification (A.R. Feinstein et al., Am. Rev. Respir. Dis., 101: 671-684, 1970). Furthermore, current neuroendocrine markers lack specificity for SCLC although the presence of these markers may help predict chemosensitivity (S.L. Graziano et al., J. Clin. Oncol., 7: 1375-1376, 1989; S.B. Baylin, J. Clin. Oncol., 7: 1375-1376, 1989; C.L. Berger et al., J. Clin. Endocrinol. Metab., 53: 422-429, 1981; A.F. Gazdar et al., Cancer Res., 45: 2924-2930, 1985). In vitro growth characteristics may more accurately reflect biological properties of aggressiveness and susceptibility to chemotherapy. In this study, 3-dimensional gel-histoculture was used to retrospectively distinguish between NSCLC and SCLC. Tumor explants from 78 patients with NSCLC and 13 patients with SCLC were grown in gel-supported histocultures with an overall success rate of 92%. These 2 tumor types were distinguishable by their 3-dimensional in vitro tissue architecture. In addition, proliferation rates were measured by histological autoradiography after 4-day incorporation of [3H]dThd. The percentage of cells labeled in the most proliferatively active regions of the autoradiograms was termed the growth fraction index (A.F. Gazdar et al., Cancer Res., 45: 2924-2930, 1985; R.A. Vescio et al., Proc. Natl. Acad. Sci. U.S.A., 84: 5029-5033, 1987; R.M. Hoffman et al., Proc. Natl. Acad. Sci. U.S.A., 86: 2013-2017, 1989). The mean growth fraction index for pure small cell lung cancer was 79 +/- 10%, differing markedly from that of 35 +/- 19% for mixed small cell/large cell tumors, adenocarcinoma (38 +/- 16%), large cell undifferentiated carcinoma (40 +/- 18%), and squamous cell carcinoma (33 +/- 15%) (P less than 0.001 in each case). We therefore conclude that 3-dimensional gel-histoculture is a useful means of distinguishing pure SCLC from NSCLC, which may improve treatment decision making.

A newly identified iron binding protein in duodenal mucosa of rats. Purification and characterization of mobilferrin

An iron binding protein with an approximate molecular mass of 56,000 daltons was purified to homogeneity from homogenates of rat duodenal mucosa. The protein was biochemically and immunologically distinct from transferrin and ferritin and competitively bound cobalt, copper, zinc, and lead. Each molecule bound one molecule of iron with a Kd of 9 X 10(-5). Dissociation of iron and the protein was accelerated at acid pH. Using an immunogold method, the protein was identified in the apical cytoplasm of proximal small intestinal cells and was not observed elsewhere in the intestinal mucosa and in other body organs. It was named mobilferrin from its city of origin and to differentiate it from other previously identified iron binding proteins.

Cancer biology for individualized therapy: correlation of growth fraction index in native-state histoculture with tumor grade and stage

There is a need for individualization of all aspects of cancer therapy. Because of significant heterogeneity within a tumor class, there is a need to develop an in vitro test to accurately gauge tumor aggressiveness. Such a measurement would greatly aid treatment decision making. Current methodologies such as flow cytometry, which lacks unambiguous interpretation of cell-proliferative data, and determination of the thymidine-labeling index, which measures nucleotide uptake in a nonphysiological state, have not reproducibly attained this goal. We have developed an in vitro native-state three-dimensional gel-supported histoculture system that allows the growth of all human solid tumor types for relatively long time periods. The native-state system was used to identify the percent of cells capable of incorporating [3H]thymidine over a 4-day period, which we term the growth fraction index (GFI). We have compared the ability of cancer tissue to proliferate in native-state culture to the stage and histological grade of four major types of human carcinomas: breast, ovarian, colon, and lung. Eighty percent of tumor explants could be evaluated, even when sent from across the country. We have determined that the GFI correlates with tumor stage and grade for breast and ovarian carcinoma. In colon carcinoma, there is a trend toward higher GFIs in tumors of more advanced stage and grade. In non-small cell lung carcinomas, GFI, stage, and grade do not correlate. These results suggest the applicability of gel-supported three-dimensional native-state histoculture for prognostic purposes in patients with breast and ovarian cancers and demonstrate the clinical relevance of the native-state histoculture system.

Transfer of tumor cells between cell aggregates as a model for adhesive changes in metastasis

A model for cell detachment which may influence metastasis in vivo is described involving the transfer of hamster melanoma cells from aggregates of those tumor cells to hamster fibroblast aggregates. The aggregates were made by the spontaneous association of cultured cells. Tumor and fibroblast aggregates were then incubated together but separated by a nylon net that allowed only single cells to pass. Melanoma cells rapidly separated from the tumor aggregates, crossed the net, and attached to the fibroblast aggregates, but fibroblast cells did not. This model of metastasis reflects the postulated role of cell-to-cell adhesion in metastasis and will allow further study of the role of cell attachments in metastasis.

Relation of detergent HLB number to solubilization and stabilization of D-alanine carboxypeptidase from Bacillus subtilis membranes

The ability of various nonionic detergents to solubilize D-alanine carboxypeptidase and other membrane-bound enzymes was correlated to a physical property of the detergent, the HLB number. Only a fairly narrow range of detergents were effective solubilizing agents. Purified carboxypeptidase required either a detergent or detergent plus a lipid fraction for stability. Only those detergents effective in solubilizing the enzyme were effective in stabilizing it.

Isolation of polyisoprenyl alcohols from Streptococcus faecalis

C(55)-isoprenyl alcohol and its derivatives have been isolated from Streptococcus faecalis and characterized. The relative amounts present as free alcohol, neutral lipid esters, and phosphate ester derivatives were determined. The chain lengths, mass spectra, and cis to trans ratio of double bonds are reported.

Isolation of the lipid intermediate in peptidoglycan biosynthesis from Escherichia coli

The lipid intermediate in peptidoglycan biosynthesis was isolated from Escherichia coli strain W and characterized as C(55)-isoprenyl-pyrophosphoryl N-acetylmuramyl(-pentapeptide)-N-acetyl-glucosamine.

Complex lipid requirements for detergent-solubilized phosphoacetylmuramyl-pentapeptide translocase from Micrococcus luteus

Phospho-MurNAc-pentapeptide translocase activity in the membrane of M. luteus was lost upon addition of the detergent, Triton X-100, but could be restored by addition of lipid fractions to the assay. By assay in the presence of lipid, the activity of the Triton-solubilized enzyme could be measured. The synthesis of C(55)-isoprenyl-P-P-MurNAc-pentapeptide from UDP-MurNAc-pentapeptide required C(55)-isoprenyl-P, and was stimulated by a neutral lipid. The exchange reaction of UDP-MurNAc-pentapeptide with UMP required a polar lipid fraction, but the reaction was not affected by C(55)-isoprenyl-P or the neutral lipid. Thus, measurement of activity of the detergent-solubilized enzyme requires addition of three lipids, the lipid substrate (C(55)-isoprenyl-P), the neutral lipid, and a polar lipid.