California Association of Chemistry Teachers
Paul Saltman
The University of Southern California LOS Angeles
The Role of Chelation in 1ron"~etabolism A
The importance of iron for the health and well-being of human beings was realized long before we were admonished by television pitchmen to rejuvenate our "tired blood'' (1). Iron is essential for many activities of cellular metabolism, including its role as a carrier of oxygen in hemoglobin and myoglobin and its function in electron transport as the various cytochrome systems and ferredoxin in respiration, as well as its essential role in metalloenzymes mediating important biochemical reactions. Our particular interest in irou metabolism began in an attempt to understand the biochemical or cellular basis of a variety of iron storage diseaws. If we could understand how too much irou was accumulated, we could perhaps more effectively introduce iron into the organism and in so doing overcome a variety of iron deficiency anemias. Dietary iron enters the body via the gastrointestinal tract. At present there are three principal theories to account for the regulat,ion and control by the intestine of iron uptake from dietary sources into the blood.
Figure 1. The mucoral block mechanism for intestinal regulation ond control of Iron uptake into the blood.
The best known is that of the "mucosal block" hypothesis originally proposed by Hahn (?2) and elaborated upon by Granick (3). The overall mechanism is presented in Figure 1. Experimentally it can be demonstrated that ferrous iron is much more effectively utilized by the organism. This finding led to the proposal that ferric iron initially is reduced to the ferrous state, diiuses into the mucosal cell, is reoxidized, and then is bound to an iron carrying protein, ferritin. The ferritin-iron is carried across the cell where it is released by a subsequent reduction and moves passively Presented at the CACT Conference, California State College at San Dimas, April 1965.
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/ Journal of Chemical Education
across the serosal membrane. Once in the blood, the ferrous ion is again reoxidized and bound to a second iron-binding protein, transferrin. The regulation and control aspect of this process is linked to the availability of apoferritin to bind the iron and carry i t across the cell. Secondary to this facilitated diffusion is the redox state of the intestinal epithelium. Recently, however, experimental evidence hm been accumulated to cast doubt on the mucosal block theory as sufficient mechanism to account for observed phenomena (4). A theory which involves control of iron movement by a specific transport mechanism linked to metabolic energy was proposed by Crosby and his collaborators (6) and by Schachter and his group (6) and is shown in Figure 2. Once again, iron moves in the ferrous form, demanding an initial reduction of d i e t q iron from the ferric state. After the iron enters the mucosal cell, it is bound to endogenous low molecular weight ligands or is stored in the form of a macromolecule complex such as ferritin. Movement into the blood is mediated by a specific transport mechanism intimately linked to metabolic energy, indicated by the ATP in the diagram. Regulation and control is governed by the active transporting system of the serosal membrane of the intestine. Once in the blood, the ferrous iron is oxidized and bound to transferrin for subsequent utilization by the organism. Our own mechanism to account for iron transport is shown in Figure 3. We believe that primary control resides in the presence of endogenous or exogenous ligands or chelating agents which are able to bind either ferrous or ferric ions to form soluble low molecular weight complexes. The iron chelates move into the cells where the iron is either exchanged with other low molecular weight endogenous ligan& or is bound in a storage form to macromolecules, including ferritin. Iron is transported into the blood along with the original chelate with which it entered the cell or with another substance secreted by the intestinal cell itself. LUMEN
BLOOD
INTESTINE
A TP
Figure 2. The active transport mechanism for Intestinal regulation and control of iron uptake.
Once in the blood, the iron chelate may then move to depot cells where the iron is bound and transferred to the &globulin, transferrin; thence it goes to the reticulocytes, where it is utilized in heme synthesis, or is excreted via the kidneys and thus made unavailable to the organisms. Neither redox reactions or metabolic energy are directly involved. Experiments with Liver Slices
.005.01
.05
1
CITRATE IM1
Our initial experiments concerning iron metabolism were carried out using rat liver slices incubated in a physiological medium in which iron was present as 69Fe citrate (7). The solution chemistly of iron is such that the maximum concentration of unhydrolyzed ferric ion present a t pH 7.0 is of the order of 10-l7M . Furthermore, ferrous iron is rapidly oxidized during aerobic incubation so that a definitive measurement of its concentration is very diacult. For this reason we were forced unwittingly to use chelates in our first experiments. The total iron accumulated was independent of metabolic energy, as indicated by the lack of effect of a wide variety of respiratory inhibitors such as anaerobiosis, cyanide, azide, dinitrophenol, and others. Kinetics of the accumulation process were quite analogous to Freundlich adsorption isotherms and led us to propose that the maximum amount of iron that could be accumulated was directly regulated by the number of unbound iron-binding sites that were available within the cells (8). When these sites became saturated, the cells ceased to take up the metal. The energy of activation of uptake was -400 calories/mole/ degree and led us to suspect that reaction was limited by physical diffusion rather than by some metabolic process. It was possible that this permeability barrier was the limiting membrane of the liver cells. However, this proved not to be the case when the liver slices and cells were treated with a wide variety of physical and chemical agents known to destroy cell membrane integrity; the treatment failed to change the kinetic behavior of the uptake process (9). The rate-limiting step in accumulation was the transfer of the iron from the low molecular weight chelate to the iron binding site in the cell. Within the organism, the liver is bathed by plasma and not by synthetic buffer media. Furthermore we know that the iron is carried in the blood bound to the specific prglobulin transferrin. We therefore prepared serum from rabbit blood in which all of the iron on the transferrin was replaced with isotopic tracer, 69Fe. During this process we completely removed all low molecular weight endogenous chelating agents from the serum by dialysis. We now incubated rabbit liver slices in this plasma and measured the initial rate of
Figure 3. The equilibrium binding and chelotlon mechanism for intestinal reg~lotionand control of iron uptake.
Figure 4. The initial rats o f 69Fo uptoke from dialyzed rabbit serum function of exogenous dtroto concontration.
0%a
iron uptake as a function of concentration of exogenous citrate added to the dialyzed plasma, with the results seen in Figure 4 (10). Although there is transfer in the absence of added citrate, it is probably due to the presence of low molecular weight chelating agents elaborated by the liver cells themselves. The addition of exogenous citrate causes a threefold increase in the rate of uptake. Many other chelates exhibit similar effects, as does the concentrated dialysate from normal plasma. Intestinal Transport
We then turned our attention to the possibility that a similar mechanism to SEROS4L that of the liver slice was MUCOS4L regulating and controlling intestinal transport. We designed and built two devices to measure in uitro transmembrane iron movement. Figure 5 shows the large scale apparatus which permits continual measurement of isotope movement from mucosal to serosal, or vice-versa, with a large segment of mounted intact intest,ine as shown. Both compart- INTESTINE ments are vigorously aerated and circulated a t all times. The gas phase can be changed readily from aerobic to anaerobic conditions with ease and sim- Figure 5. The large scale apparaplicity. A wide variety of tus which permit, continuo1 msocOmDOundScan be added rurement of tronsints~tind iron to kucosal or serosal sides movemen' to determine their influence. It has long been known that about 30% of the intestinal transport of sodium is dependent upon metabolic energy in the form of ATP (11). This active sodium transport is one of the most extensively studied of all transport mechanisms. We therefore measured the transport of 22Naand "Fe chelated to ethylenediaminetetracetic acid (EDTA). The transport from the mucosal compartment to the serosal compartment was measured both aerobically and anerobically and a t the same time the biopotential (also a direct function of metabolic energy) was monitored. Results from this experiment are shown in Volume 42, Number 12, December 1965
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683
Table 1. Effect of Aerobiosis and Anaerobiosis on ZzNa and 59Fe-EDTATransport and Transmucosal Potential
"Ns*
=DFeEDTAb Biopotential (m
-
13.8 0.93
s)
-5.5mv
8.9 1.07 0.0
-4.9
+0.14 + 5 . 5 mv
Ioitial concentration of Na+ is 0.137 M. Unita of transport given ks pmole/hr om1 intestine. Initial concentration of FrEDTA is 3 X 10-3 M. Unita of transport given as m(.moles/hr. om'. I
Table 1. Whereas the flux of sodium is inhibited by the anticipated amount and the hiopotential is reduced to 0, iron-chelate transport is relatively unaffected, if not slightly enhanced. Similar results were obtained for a wide variety of chelates including citrate, fructose, and other biological ligands. Metabolic energy is not involved with the movement of iron chelates across the membrane (12). It was still possible that this passive movement might be regulated by a membrane hound carrier which facilitates the movement across the harrier. If this were so, one would expect to find kinetics of transport quite similar to those observed in enzyme-substrate interactions. We measured the rate of transport as a function of iron citrate over a wide concentration range; the results are presented in Figure 6. There is a direct linear relationship of rate of transport with concentration. Since Fick's law of diiusion is obeyed, the probability that a membrane-hound carrier is mediating transport seems highly unlikely.
Figure 6.
Rate of in ritro transport or a function of iron citrote
What factors then might regulate iron movement? We assayed a variety of biological and non-biological iron-chelates in the in vitro system and measured the rates of transmembrane iron flux (Fig. 7). Of particular interest is the rapid transport mediated by EDTA and nitrilotriacetate (NTA). We will see below that although their movement is comparable, their hiological utilizahility is predicated upon the ability of the iron molecule to move from the chelates into a utilizable form in the cell. Ascorhate, long considered to he essential in the metabolism of iron, proves to he less effective than citrate in the mobilization of the iron. The apparently low value for fructose is due to the formation of polynuclear complexes, as will he discussed. To this point in our story, we can conclude that iron transport across the intestinal membrane is passive and does not involve a membrane-hound carrier. Flux is highly dependent on low molecular weight chelate to soluhilize the iron. The rate of iron movement is a direct function of the chemical nature of the chelate. 684
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Journal o f Chemical Education
hour, Figure 7. Rates of tranrmembrone iron flux in vitro for a variety of biological and non-biological iron-chelates.
Biological Iron Chelates
While this phase of our work was in progress, our attention was attracted to some very elegant work by J. B. Neilands and his associates (IS) at the University of California, Berkeley. Their research was concerned with the uptake of iron by several species of microorganisms, including bacteria and fungi which were able to sequester and utilize iron from alkaline environments where other organisms were unable to survive. This research showed that the biochemical mechanisms operative in these iron-gleaning organisms was the secretion into the environment of large quantities of complex organic molecules with high affinities for iron. The first of these compounds he was able to isolate and identify is shown in Figure 8. It is a simple peptide of dihydroxyhenzoic acid and glycine, which forms a
Figure 8. The simple peptide of dihydroxybenroic acid and glysine isolated from "low iron" cultures of Bocciflur rubtifir.
bright red ferric complex. They then isolated several highly complicated molecules generically named ferrichromes, which were even more effective in their ability to chelate and soluhilize the metal. I n fact, some organisms could remove iron from the stainless steel tanks in which they were grown. Neiland's group was able to elucidate the structure of ferrichrome a and of ferrichrome b, complex cyclic compounds. They consist of cyclic polypeptides, including hydroxamate derivatives of ornithine and lysine as well as glycine and other organic acids. Ferrichrome a is shown in Figure 9. Recently the crystal structure of this molecule was ascertained and both electron spin resonance and Mossbauer spectral determinations have been carried out on it (14). It again should he emphasized that only a few organisms elaborate these complex chelates as a hiochemical adaptation to a harsh environment. The survival value of these compounds is inestimable.
Recently researchers a t Ciba Pharmaceutical Company (15) isolated a compound related to the fernchromes called ferrioxamine b, also shown in Figure 9. This compound was studied as a pharmacological agent to remove excess iron from humans in cases of iron toxicity such as encountered by children and adults swallowing excess amounts of iron medication or in patients with hemochromatosis or other iron storage pathologies. Unlike many other chelates of the EDTA family which had been tried in iron removal (16), the specificity of desferrioxamine for iron was so unique that i t did not remove other essential trace elements from the organisms. The therapeutic results have been promising and it appears that desferrioxamines will probably be an effective method for treating iron toxicity. Thus microorganisms and man seem to share a basic biochemical mechanism for iron metabolism. Studies using higher plants, particularly soybeans, by J. C. Brown and his collaborators (17) have also revealed the essentiality of chelation for iron transport. They have isolated and identified natural chelates, secreted by the roots, which permit soil iron to be solnbilized and utilized in the growth of the bean plant. Two strains of soybeans diering in but one genetic locus were obtained. One grows on alkaline soils while the other one is unable to do so without iron supplementation. Stem exudates of the competent strain showed that both malate and citrate are the chelating agents involved. Both of these acids are present in relative abundance in most plants as a metabolic intermediate in the oxidation of carbohydrates. However, here they serve a significant role as chelating agents for metal transport. The agronomists and horticulturists have often used various synthetic chelates to prevent iron and other trace metal deficiencies in higher plants (18). We have achieved an interesting symbiotic relationship of organic chemists to higher plants, in which if the plant is unable to synthesize its own chelating agents, one of the major chemical companies will do so for it.
biological origin that mght chelate iron, we discovered to our amazement that sugars and other polyhydroxy compounds form very stable and soluble complexes with both ferric and ferrous iron (19). The conditions for the formation require a high initial acid concentration, pH 2, to prevent hydrolysis of the metal, and molar excess of the sngar or polyol of the order of 10-20 times that of the metal present. When the pH is adjusted to 7.0, the iron remains in solution and forms an intense red-brown colored carbohydrate complex. We have begun to study the chemical nature of this complex and its stability. Using the techniques of electron spin resonance, nuclear magnetic resonance, and magnetic susceptibility, we found that in the pH region of 3.5 to 10 we obtained no spin signal characteristic of the ferric iron, no line broadening of the NMR spectrum of the alkyl hydrogens of the sugar, and a greatly reduced magnetic susceptibility (20). These facts pointed to the participation of polynuclear complexes, systems now enjoying a great deal of attention from the inorganic chemist. We found a variety of monosaccharides ranging from 3 to 7 carbons in length that were effective. The most active of the naturally occurring sugars were fructose and glucose. If we represent the sugar molecule by S, we propose in Figure 10 that two of the protons from hydroxyl groups of the sugar are displaced by binding the iron atom. Another hydroxyl is obtained by the hydrolysis of water which gives rise to the third proton. The coordination sphere is completed by three water molecules from the solution. This would be a typical monomeric configuration and wonld be expected to display an ESR signal, NMR line broadening and magnetic susceptibility. In the second row of Figure 10, we see two possibilities for a dimeric conformation. Such molecules wonld behave as our experiments have indicated. However, recently, by means of analytical
Sugar Complexes
Let us now return to the lumen of the intestine where, unfortunately, we have not been able to define the endogenous chelating agent elaborated by the cells. In the course of our studies of a variety of compounds of
.
.
o----t;.---o~c4,~ NH~(CH,I,-N'
\
F/
I' '',.
d
O\/
ICH2k
'CO-NH
? $CH215
Figure 9. The structure of ferrishrome o (top] and ferrioxomine b (left).
Figure 10. The *"gar-iron complexes. Two protons from the hydroxy groups of the sugar are dirplmsed b y binding the iron atom ( w g m = SI. The second row indicate. two possibilities for dimer conformatiom The third row indicates proposed restion of polymeric conformdon.
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ultracentrifugation and gel filtration on acrylamide resins, we have discovered that ferric fructose exists in a high molecular weight form as well. A proposed section of a polymeric molecule is shown in the bottom of Figure 10. Our best estimate a t this time indicates that molecular weights are in the region from 30,000 to 60,000. These large polymers must be in equilibrium with low molecular weight monomers and dimers since we can demonstrate that ferric fructose will pass slowly through dialysis membranes and attain equilibrium concentrations. Much of our basic effort a t this time is directed to understanding the chemical nature of these interesting complexes. Our discovery of the iron-sugar complexes, quite by chance, led us to an understanding of the molecular basis for a very interesting iron storage pathology. The natives of South Africa are often stricken by a disease known as "Bantu siderosis." It is characterized by many of the iron storage symptoms that are noted in other diseases such as hemochromatosis and transfusion siderosis. These natives subsist on corn gmel cooked in cast iron pots. Not only is their diet deficient in protein, which had previously been thought to lead to the iron storage, but also i t contains tremendous amounts of both carbohydrate as starch and iron as ferric hydroxide from the rusty iron pots in which they cook their food. When this diet reaches the acid environment of the stomach, the iron is hydrolyzed, as is much of the carbohydrate, and as the bolus moves from the acid environment of the gastric region into the intestine. it is neutralized. Large ouantities of iron are chelated' as the low mole&& weight soluble carbohydrate complexes. Instead of the iron precipitating and being no longer available for the organism, it is now available to the intestinal mucosa for transport into the blood stream and ultimate deposition. We thus have one of the first biochemical or "molecular" diseases which is characterized by an abnormality in a chelation phenomenon. Is sugar one of the normal chelates within the lumen of the human intestine? Of this we know little. Certainly many of the organic acids and amino acids and carbohydrates of the diet could play a very important role here ( d l ) . Elmer Brown and his associates (22) have shown that once the iron enters the intestinal cells, i t is bound to low molecular weight amino acid complexes. Particularly serine and glycine seem to be involved. Whether or not these are the true transport form of the metal in the intestinal cell is still not known. We are continuing our search for other biological chelates that may be important in the regulation and control of intestinal iron transport. We believe that there may be specific substances elaborated by the intestine, much as in the roots of plants and the cytoplasm of microorganisms, that are directly involved in the transport of iron.
The Role of Low Molecular Weight Chelotes in the Transport of lron to Transferrin and Other lron Binding Proteins
Once the iron has crossed the intestinal epithelium, it enters the blood where the possibility of one or more of several fates await it. We have developed a technique for measuring transport of trace metals in whole 686
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Journal of Chemical Educofion
Table 2.
Organ and Tissue Distribution of "Fe as a Function of Chelate
NTA Blood Bladder Liver Kidney Spleen
EDTA
0.4
0.8
Valuea as % total iron in all tissues counted.
animals (Id). Rats are anesthetized, segments of small intestine are ligated, iron as the "Fe chelate is injected directly into this tied-off loop and uptake is allowed to proceed for a given interval of time, usually two hours. At the end of this period, the section into which the radioactive chelate has been injected is removed, and the residual activity within the body of the animal is determined by whole animal counting. We are thus able to measure the effective amount of the metal transported as well as to dissect the various organs and determine specifically how much of the iron moved into each tissue compartment. Examination of Table 2 reveals the differences between two chelates whose rate of transport is essentially the same. Whereas EDTAiron moves across the membranes very rapidly, its ability to transfer the metal, either to tissues or speciiic proteins, is extremely poor. Within two hours the bulk of the iron is excreted in the urine, unavailable and useless to the organism. NTA-iron on the other hand is hardly excreted a t all, and the bulk of the metal is found in the blood, liver, and other tissues. We have begun to explore the mechanisms for this aspect of regulation and control, utilizing a model system of biological consequence. Transferrin specifically binds two atoms of iron per molecule of protein. Many other physical characteristics of the binding sites have been studied in detail (25). It suffices to say that each site is identical in every respect. The association constant of iron for the protein is approximately loa1. The electron spin resonance signals are identical, indicating the same ligand field surrounds both metal ions and the nature of the amino acid residues binding the iron are known to be three phenolic groups from tyrosines and two imidazole nitrogens from histidines. It is easy to remove the ferric ion from the protein by reduction at low pH. We are thus able to obtain a purified apo-transferrin and measure spectrophotometrically the parameters which regulate and control the movement of iron from a low molecular weight chelate to the protein by the specific absorption of the iron-protein a t 470 mp. Results of the experiment utilizing three chelates-NTA, citrate, and EDTAare shown in Figure 11 (24). Cursory examination would indicate that perhaps the rate of transfer is controlled by the dissociation constants of the various iron chelates. Thus EDTA is stronger than citwte, which is stronger than NTA; and one might formulate the two step reaction: Fe-Ch Fea+
G
+ Tr *
Fe'+
+ Ch
Fe-Tr
This proves not to be the case. Not only are the concentrations of Fe3+ so low that existing rate constants for binding reactions could not permit the reaction to be carried out a t the measured velocities. but there is no
mechanism, per se, is a passive one. No metabolic energy is required directly for its operation. Indirectly, however, energy from the cell must be utilized to synthesize the ligands which are involved as well as the hemes and other tetrapyrols and proteins. Iron transport is quitc unlike the active process heretofore observed for many cations, particularly Naf and KC. Thus the biological world, like the political world is filled with "activists" and "passivists." Both are iudispensible for regulation and control.
direct relationship between the rate of this reaction and the concentration of the iron calculated to exist in the three solutions. This led us t o propose another series of reactions: Ch-Fe
+ Tr
a
Ch-Fe-Tr
a
Ch
+ FeTr
The important aspect of this reaction mechanism is the formation of a ternary complex of the chelate, the iron, and transferrin. Our recent experiments have been able to demonstrate directly the presence of such a ternary complex both by spectral measurements as well as direct measurement of equilibrium constants for the first reaction. The rate limiting reaction appears to be the breakdown of the ternary complex to form ironprotein. NTA not only immediately binds to the protein to form the ternary complex, but also rapidly dissociates, leaving the iron-protein. Citrate on the other hand forms the complex rapidly, but is slowly liberated. EDTA forms the ternary complex with an association constant much lower than that of citrate, but does not "unload" and remains bound as the ternary complex, unable to proceed. Such a mechanism of regulation and control of tissue binding sites by specific interaction of metal chelates and proteins, nucleic acids, or other biological polymers offers an explanation for specific utilization of iron in various forms by the mammalian system.
Acknowledgments
Not enough thanks can be given to the fine group of collaborators associated with this research over the past several years. Their names appear as co-authors on various aspects of the work; their contributions have been enormous. It is also a pleasure to thank the various foundations and government agencies which have made this work possible. These include the Hartford Foundation, The U.S. Public Health Service, the U S . Atomic Energy Commission, The Smith, Kline and French Foundation, the Abbott Research Fund, and the California Cancer Society. The author also wishes to thank the U.S. Public Health Service for its Research Career Development Award which has significantly aided the progress of this research. Literature Cited (1) BEUTLER, E., FAIRBANKS, V. F., AND FAHEY, J. L., 'LClinicaI T)isordnrs of Iron Metabolism." Greene and Stratton, - --......~., .. (2) HAHN,P. F.,ET AL.,J ; ~ x p t l . ' ~ e d78, . , 169 (1943). (3) GRANICK, S., Bull. New York Aead. Med., 30, 81 (1954). (4) MOORE, C. V., in "The Harvey Lectures," 55, 67 (1961). 151 W. H., J . Clin. . . WAEBY,M. 8.. JONES,L. G., AND CROSBY, ~nvest.,43, 1433 (1964). (6) G. B.. SCHACHTER, D.. AND SCHENKER, H., Am. J . ,-, DO=-DLE. ~ h y s i d l .198; , 609 (1960): (7) SALTMAN, P., FISKIN,R. D., AND BELLINGER, S. B., J . Bi01. Chem., 220,741 (1956). P., FRISCA,H., FISKIN,R. D., AND ALEX,T., J. (8) SALTMAN, Biol. Chem., 221,741 (1956). P., Exptl. Cell Res., 18, 560 (1960). (9) BASS,R., AND SALTMAN, P.. ET AL.,Arch. Biochem. and Biophys., 88, 222 (101 . . CHARLEY, (1960).' 111 P. F.. AND SOLOMON. A. K.. J . G€%. Physiol., 41, ~ -) -CURRAN. , 143 ~ 9 5 7 ) '. (12) SALTMAN, P., AND HELBOCK, H. in "The Use of Radioisotopes in Animal Nutrition and Physiology," International Atomic Energy Agency, Vienna, 1965, p. 301. J. B., Bact. Rev., 21,101 (1957). (13) NEILANDS, J. D., AND TEMPLETON, D. H., 114) . . ZALEIN.A,. FORRESTER, ~ c i a c e i46, , 261 (1964). J., NOUV.rev. franc. h h a t . , 3, l(1963). (15) TRIPOD, (16) TRIPOD,J., in "Iron Metabolism," edited by GROSS, F., Springer-Verlag, Berlin, 1964, p. 503. (17) BROWN, J. C., AND TIFFIN,L. D., Plant Phwiol., 40, 395 (1965). S., AND MARTELL, A. E., ('Organic Sequestering (18) CAABREK, Agent,s,"John Wiley & Son, Inc., New York, 1959, p. 416. 1191 P.. ET AL., Biochim. et Biophys. Acta, 69, 313 . . CHARLEY. (1963).' ' (20) AASA,R., ET AL.,Biochim. et Biophys. Acta, 80, 430 (1964). H., (21) GROEN,J., VAN DEN BROEK,W. A,, AND VELDMAN, Biochim. et Biophys. Acta, 1, 315 (1947). (22) BROWN, E. B., AND ROTAER, M. L., J. La& and Clin. Med., 62,357 (1963). (23) AMA, R., ET AL.,Biochim. et Biophys. Acta, 75, 203 (1963). P., Snm, C., AND BATES,G. W., Fed. Proe., 23, (24) SALTMAN, 135 (1964).
Summary
I t is practically impossible in a limited review of this kind to give proper credit to all of the many investigators in the field of irou metabolism who have contributed meaningfully to this problem. This review has been unabashedly a rather personal one. It is hoped that in the future many of the differences of fact and artifact between the various proposed mechanisms of irou transport will be resolved. I n essence the regulation and control of iron metabolism through the intestine as well as across all biological membranes is determined by the ability of the iron to be chelated by low molecular weight ligands. The chemical nature of the ligand determines the rapidity with which the iron will move across the membrane. The ultimate site of deposition of the iron within the various tissues is regulated and controlled both by the availability of the specific binding sites on macromolecules within the cell or within the tissues and the ability of the low molecular weight iron chelate to interact with that bipolymer and ultimately transfer the iron to it. The transport
Figure 11. EDTA.
~
The uptake of iron by apotronrferrin from NTA, strate, and
+
+
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