Biochemical and behavioural aspects of sideropenia

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 deficiency alters discrete proteins in rat caudate nucleus and nucleus accumbens

Young rats (21 days old) made nutritionally iron deficient, by feeding them a semisynthetic diet containing skimmed milk for 5 weeks, had significantly lowered hemoglobin levels (5.2 +/- 4 g/100 ml). The nonheme iron content in caudate nucleus was decreased by 47%. The behavioral response of iron-deficient rats to apomorphine (2 mg/kg) and the density of 3,4-dihydroxyphenylethylamine (dopamine) D2 receptors, as measured by [3H]spiperone binding in caudate nucleus, were significantly reduced by 70 and 53%, respectively. The possibility that nutritional iron deficiency may affect protein content in brain was investigated by measuring the apparent concentration of proteins in caudate nucleus and nucleus accumbens from iron-deficient and control animals using two-dimensional gel electrophoresis. The data indicate that iron deficiency can affect content in these two brain regions. Significant changes in the content of 10 proteins were noted in the caudate nucleus and nucleus accumbens in iron-deficient rats. The albumin level was significantly increased in both regions studied, whereas the neuron-specific enolase level was increased in the nucleus accumbens and the glial fibrillary acidic protein level was reduced in the caudate nucleus. The significance of these protein content changes, as well as a reduction in content of a 94-kilodalton protein (a molecular size similar to that of the D2 dopamine receptor), remains to be established.

Brain iron-deficiency causes reduced learning capacity in rats

Rats made nutritionally iron-deficient (ID) showed a significant deficit in water-maze learning compared with normal rats. The deficit was substantially greater the longer the rats stayed on the ID diet. The deficit in learning was established prior to any significant decrease in hemoglobin (Hb) level in the blood. Three weeks after the ID rats were placed on a control diet, the Hb level was restored to normal value, but the cognitive deficit remained. Previous studies showed that the behavioral effects of ID are mediated by a decrease in the functional activity of the dopaminergic system. The ID effects on learning and memory may be related to the irreversible diminished dopaminergic neurotransmission that results from ID.

Cognitive deficits in iron-deficient and iron-deficient anemic children

Eighteen- to 60-month-old iron-deficient anemic children given iron therapy (n = 25) and a control group matched for mother's educational level showed no significant difference in mean mental development score at baseline. The control group's mean score was increased significantly over baseline score at 3 and 6 months and was significantly higher than the experimental group's mean score at 3 months. Although the experimental group demonstrated hematologic correction over 6 months, mean mental development score showed no significant improvement. Scores for an iron deficient not anemic group given iron (n = 22), despite complete hematologic correction over the six months of observation, and for its control group, did not change significantly. Baseline scores for an iron-deficient not anemic placebo group (n = 23) and for its control group were not significantly different. At 3 months the control group score had increased significantly, whereas that for the experimental group had not. When experimental and control subjects were matched on baseline mental development score, the control subjects experienced increases in scores over time, further confirming an impaired ability to improve scores with repeated testing in the experimental groups. Behavioral rating data (responsiveness to examiner, responsiveness to environment, and emotional tone) revealed significant differences between the iron-deficient anemic group and its control group at 3 and 6 months, with the control group rated more responsive, suggesting that iron deficiency, alone or in association with anemia, may have some lasting effect on behavior and development. Group differences were also found between the mean number of occurrences of multiple stressful events. Failure to show improvement in scores in the iron-deficient anemic group may reflect the fact that those children were less testable than were children in the control group, despite repeated testings, a theory supported by the infant behavior rating data. This may be related to some irreversible behavioral deficit or to an adverse environmental milieu (e.g., stress).

Learning achievement and behavior at school of anemic and non-anemic infants

In a prospective study the learning achievement and behaviour of second grade children who were anemic in infancy (n = 20) was compared with that of non-anemic children (n = 55). All children received preventive care and were under surveillance from birth at the Community Health and Research Center of Hadassah in Jerusalem. The anemic children had a hemoglobin level of less than 10.5 g/dl whereas the comparison group had more than 11.5 g/dl at 9 months of age. The learning achievement score was significantly lower in the anemic group 9.3 S.D. 3.8, than in the non-anemic group 11.5 S.D. 2.9 (P = 0.009). The positive task orientation was also significantly lower in the anemic group, though on additional behavior scores no differences were found. Differences remained significant even after controlling for maternal education and sex of child in a multivariate analysis. It is possible that an iron deficiency state which persists during the critical period of brain development in infancy has long-lasting effects on cognitive functioning. Prevention of iron deficiency may contribute to promotion of child development and better achievement.

Reduced frequency of iron deficiency anaemia in sickle cell trait

The prevalence and causes of anaemia among the inhabitants of a Southern Lebanese village were studied. Of the 317 patients examined 75, or 24% were found to be heterozygotes for Hb S. There was a reciprocal relation between the presence of sickle cell trait and of iron deficiency anaemia. The prevalence of iron deficiency anaemia defined as the coexistence of Hb below 13 g/dl in adult males and below 12 in adult females and children with two laboratory indicators of iron deficiency, was 10.3% in the normal population as against 1.3% in the sickle cell trait group (P less than 0.008). The reduced frequency of iron deficiency anaemia in sickle cell trait may be explained by increased iron absorption, or alternatively by reduced iron requirements and a lower risk of discrepancy between iron supply and demand. Because of the central role of iron in the function of a great number of enzymes and proteins, it is possible that protection against iron deficiency anaemia by the sickle cell trait may result in improved working capacity and a lower incidence of infections.

Nutritional iron and dopamine binding sites in the rat brain

Iron-deficiency (ID) anemia in man is associated with neurological disorders and abnormal behavior. Rats made nutritionally iron-deficient have markedly diminished behavioral responses to centrally-acting drugs (amphetamine and apomorphine) which affect monoaminergic systems. ID has no effect on either the levels of monoamines or on the activities of monoamine-metabolizing enzymes in the brain. We have investigated the possibility that ID may affect postsynaptic events at the level of receptor by measuring the specific binding sites of several neurotransmitters in different brain areas. The results clearly show that ID causes a significant (40-60%) reduction of the DA D2 binding sites in the caudate. DA-sensitive adenylate cyclase, alpha- and beta-adrenergic, muscarinic cholinergic and the benzodiazepine binding sites were not affected by ID. The effects of ID on DA D2 binding sites and the behavioral responses to apomorphine can be reversed when iron-deficient rats are placed for 8 days on an iron-deficient diet supplemented with iron. Chronic hemolytic anemia produced by repeated phenylhydrazine injections caused no change in serum iron and had no effect on either apomorphine-induced hyperactivity or 3H-spiroperidol binding in the caudate. Since the highest concentration of iron is found in DA-rich brain areas, it is possible that iron may be crucial to either the synthesis or coupling of the DA D2 binding site. The possibility that the DA supersensitivity induced by neuroleptics may be related to iron metabolism in the brain has been investigated.

The laboratory evaluation of microcytic red blood cells

Microcytic red blood cell states are common clinical problems in both adult and pediatric age groups. The recent widespread availability of electronic blood cell counters for performing routine blood counts has increased the detection of microcytic red blood cells. Physicians must workup both symptomatic and asymptomatic patients with microcytic red blood cells before they can initiate proper therapy and/or counseling. The purpose of this review is threefold: (1) to discuss the causes of microcytic red blood cells in terms of disorders of decreased heme production vs. disorders of decreased globin production, (2) to review the clinical laboratory tests useful in differentiating microcytic red blood cell states, and (3) to present a practical approach for the laboratory workup of microcytic red blood cells.

Long-term effects of early iron deficiency on consummatory behavior in the rat

Two experiments were designed to investigate the effects of early iron deficiency on consummatory behavior in the adult rat. In experiment 1, animals were placed in a novel chamber, either with or without water available. Although there were no effects of iron deficiency per se, the data suggested that decreased caloric intake experienced early in life may have different long-term consequences for males and females. While ad lib control males, and females in all diet conditions, exhibited less elevation of plasma corticosterone when water was available in the novel chamber, calorically restricted males appeared unable to use the cues or reinforcement provided by consummatory behavior to reduce arousal. In Experiment 2, a conditioned taste aversion situation involving conflict, we were able to separate effects due to early iron deficiency from those due to early caloric restriction. When reexposed to milk, calorically restricted (weight control) males exhibited an attenuated plasma corticoid response, compared to that of ad lib control males, while weight control females resembled ad lib control females in their response. Thus, as in Experiment 1, early caloric restriction affected males more than females. Early iron deficiency, however, markedly altered pituitary-adrenal responsiveness in both males and females. Not only was the response to reexposure completely reversed in rehabilitated males and females, but also, the corticoid response to deprivation was increased in rehabilitated males and decreased in rehabilitated females. Taken together with previous data, these results suggest that early iron deficiency alters both behavioral and physiological arousal or responsiveness, and may do so differentially in males than females.

Factors affecting iron balance

The availability of radioisotopes of iron led to the performance of physiologic studies of the absorption, excretion, and kinetics of iron. The effects of dietary constituents and intestinal secretions upon iron absorption have been thoroughly explored and have permitted the development of tools for nutritional studies in various geographic areas of the world. The basic mechanisms for regulation of iron absorption are not fully understood. However, the isolation and characterization of newer iron-binding proteins, the existence of mucosal receptors in which there is competition for iron-binding sites with other metals, and the identification of a number of animal models with genetic abnormalities of iron absorption provide promise for development in the foreseeable future. The means by which the body informs the intestine to increase or decrease iron absorption is poorly understood, and the abnormality in hemochromatosis and iron overloading disorders remains unexplained. Much has been learned in recent years regarding the structure of iron-containing proteins and the variations that exist in different species. Similar problems remain to be solved with regard to the intermediary metabolism of iron in red blood cell precursors and other cells in the body. It is postulated that the combined usage of physiologic, biochemical, and immunologic investigations will provide the basic information required.