Inorganic Chemistry in Biology and Medicine Arthur E. Martell, EDITOR Texas A&M University Based on a symposium sponsored by the Division of Inorganic Chemistry at the 178th Meeting of the American Chemical Society, Washington, D.C. , September 10-11, 1979. ACS SYMPOSIUM SERIES 14 0 AMERICAN CHEMICAL SOCIETY WASHINGTON, D.C. 1980 In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. Library of Congress CIP Data Inorganic chemistry in biology and medicine. (ACS symposium series; 140 ISSN 0097-6156) Includes bibliographies and index. 1. Metals in the body—Congresses. 2. Metals— Therapeutic use—Congresses. 3. Cancer—Chemother- apy—Congresses. 4. Chelation therapy—Congresses. 5. Chemistry, Inorganic—Congresses. I. Martell, Arthur Earl, 1916- II. American Chemical Society. Division of Inorganic Chemistry. III. Series. IV. Series: American Chemical Society. ACS symposium series; 140. 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Sartorelli Raymond B. Seymour Gunter Zweig In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. FOREWORD The ACS SYMPOSIU M SERIE S was founded in 197 4 to provide a medium for publishin format of the Series parallels that of the continuing ADVANCE S IN CHEMISTR Y SERIE S except that in order to save time the papers are not typeset but are reproduced as they are sub- mitted by the authors in camera-ready form. Papers are re- viewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation. In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. PREFACE At its inception , the origina l plan for this symposiu m was to emphasiz e the medica l aspect s of inorgani c chemistry , rather than to go over once more new development s in bioinorgani c chemistry , importan t as the subject is, since the latter topic has been treate d many times in recent symposi a reviews and monographs . The objective s of this symposiu m were to review and interpre t the remarkabl e advance s that have occurre d recentl y in medica l inorgani c chemistr y and to stimulat e interes t on the part of inorgani c chemist s to becom e involve d in the developin g researc h problem s in this area. The interaction function s of metal ions in physiologica l systems are very complex , and the precise nature of these interaction s and processe s are, for the most part, unknown . In additio n to the application s of metal ions and complexe s for medica l purposes , extensiv e fundamenta l studie s are needed to understan d the basis of these application s and thereb y make it possibl e to carry out systemati c improvement s in curren t method s as well as to develo p new approache s in this interestin g field. Of the approximatel y eighty metalli c elements , a considerabl e numbe r have been identifie d as essentia l to life; many others have been indicate d as possibl y essential , while a large numbe r of metals are of concer n because of toxic effects that result when they are introduce d into the body acci - dentally or throug h environmenta l influences . Major meta l ions such as Na + , K + , Mg 2+ , and Ca 2+ are importan t in maintainin g electrolyt e concentra - tion in body fluids or as skeleta l constituents . Many of the transitio n metal ions are essentia l in trace amount s for the activatio n of enzyme systems . In many cases, these essentia l metal ions become toxic or even carcinogeni c when presen t at sufficien t levels to overwhel m the natura l ligands and macromolecule s that functio n as carrier s for these ions, and thus more than saturate the normal physiologica l processe s for their control . Under such conditions , they may function , as do many unnatura l toxic metals , by reacting with other biomolecules , distortin g or blockin g their essentia l functions . In many cases, the difference s between the essentia l and toxic levels are surprisingl y narrow. This duality of behavio r between natura l and toxic levels constitute s the basis of threshol d concentration s for several carcinogeni c metals—below which these metals exist as essentia l and noncarcinogeni c compounds . It also provide s a strong refutatio n of the validit y of the linear extrapolatio n metho d stil l in active use for the interpretatio n of carcinogenicit y of compound s observe d at high concentra - tion levels in test animals . vii In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. The topics covere d in this symposiu m were selecte d so as to provid e example s of current and potentia l medica l application s of metal compounds . The emphasi s and amount of attentio n given were in many cases not in proportio n to the importanc e or activity levels of these applications , for a number of reasons . The use of platinu m complexe s for the treatmen t of cancer is perhap s under-represente d becaus e severa l symposia , some of which have been published , have been held on this subject in recent years. Similarly , iron nutrition , althoug h very important , has been omitted because it is well covere d by periodi c and continuin g conference s and conferenc e proceeding s devoted entirely to this field of research . New development s of ionophore s and on the use of chelatin g agents for the remova l of radioactiv e metals from the body were not given the attentio n that they deserve in this symposiu m becaus e these subjects were treated in separat e symposi a at the same America n Chemica l Because of the large numbe r and complexit y of the function s of metal ions in physiologica l systems , the application s of complexe s of both essentia l and unnatura l metal ions for medica l purpose s are expecte d to expand dramaticall y in the next decade . It is hoped that this book will help to attract more inorgani c chemist s to this field, to provid e the expertis e in coordinatio n chemistr y neede d for the achievemen t of significan t new development s in this potentiall y importan t area of medicine . The Editor wishe s to express his appreciatio n for the many helpful suggestion s receive d from professiona l colleague s durin g the formativ e stages of this symposium . Specia l thanks are due to L. G. Marzill i for assistanc e with subject matte r planning , and to J. H. Timmon s for valuabl e editoria l assistance . Texas A&M University College Station, Texas August 7, 1980 A. E. MARTELL viii In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 1 Molecular and Biological Properties of Ionophores BERTON C. PRESSMAN, GEORG E PAINTER, and MOHAMMA D FAHIM Department of Pharmacology, University of Miami, Miami, FL 33101 The ionophores ar which form lipid-solubl complexe transpor cations across low polarity barriers such as organic solvents and lipids (1). From a biological standpoint, the most important low polarity barrier is the lipid bilayer which lies within biological membranes; ionophores possess unique and potent biological proper- ties which derive from their ability to perturb transmembrane ion gradients and electrical potentials. Each ionophore has its own characteristic ion selectivity pattern arising from the interac- tion between the conformational options of the host ionophore and the effective atomic radius and charge density of the guest cation. The ability of ionophores to complex and transport cations has an ever growing list of applications in experimental biology and technology and may ultimately provide the basis for novel cardiovascular drugs. Ionophores are also intriguing intel- lectually as objects for study of chemical and physical complexa- tion processes at the molecular level and as challenges to the state of the art of chirally selective organic synthesis (2) . Several reviews are available for expanding the description of ionophores provided here (3,4,5). General Structural Features of Ionophores Several of the general structural features of ionophores are illustrated in Figure 1. All ionophores deploy an array of liganding oxygen atoms about a cavity in space into which the com- plexed cation fits. X-ray crystallography reveals that the prin- cipal bonding energy is provided by induced dipolar interaction between the complexed cation and those specific oxygens which are filled in. Valinomycin consists of alternating residues of hydroxyacids and aminoacids constituting a cyclic dodecadepsipeptide. In space the ring undulates defining a bracelet 4 Å. high and 10 Å in diam- eter. The liganding oxygens, the ester carbonyls, form a three 0-8412-0 5 8 8-4/ 80/47-140-003$05.00/ 0 © 1980 American Chemica l Society In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 4 INORGANI C CHEMISTR Y IN BIOLOG Y AND MEDICIN E VANCOMYCIN ENNIATIN B MACROLIDE ACTINS CYCLOHEXYL ETHER MONENSIN NIGERICIN Figure 1. Structures of representative ionophores. The oxygen atoms that x-ray crystallography indicates to be primarily involved in liganding to cations are filled in. In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 1. PRESSMA N ET AL. Properties of Ionophores 5 dimensiona l cage which accommodates K + (r = 1.33 X) much more snugl y tha n Na + (r = 0.95 ft) resultin g in a K + :Na + preferenc e of 10,000:1 (4). Enniati n B is a cycli c hexadepsipeptide ; the smalle r rin g result s in a relativel y plane r arra y of ligandin g oxygen atoms; th e more open and more flexibl e cage result s in a K + :Na + discrimi - natio n of onl y 3:1 (6). A new featur e appear s in the cycli c tetraesters , the macro- lid e nactins . In additio n to the este r carbonyls , fou r hetero - cycli c ethe r oxygens participat e in complexation ; the oxygens are arrange d at the apice s of a cubi c cage. Fiv e varian t nactin s are known dependin g whether 0-4 of the R groups are methyl s (nonactin ) or ethyl s (monactin , dinactin , trinactin , tetranactin)(7) . While the aforementione d ionophore s are Streptomyce s metabo- lites , the crown polyethers , the depicte d prototyp e of which is dicyclohexyl-18-crown-6 are syntheti c (8) Althoug h the y lac k th e intricat e conformation multipl e asymmetric carbo ertie s are analogous . Whil e the y are les s efficien t ion carriers , thei r lac k of labil e linkage s confer s increase d chemica l stability ; they fin d extensiv e use in organi c synthesi s for solubilizin g electrolytes , e.g. enolates , in nonpola r solvent s thereb y pro - vidin g reactiv e naked anion s (9 ) . Th e ionophore s thu s far describe d lac k ionizabl e groups and ar e collectivel y classifie d as neutra l ionophores ; thei r complexes acquir e the net charg e of whatever ion is complexed. We shal l now examine two representative s of the carboxyli c subclas s of iono - phores . Onl y the anioni c form of thes e ionophore s complex cations , hence the y form electricall y neutra l zwitterioni c complexes. Thi s distinctio n is fundamenta l for explainin g the profoun d difference s i n biologica l behavio r of the ionophor e subclasses , hence we pre - fe r carboxyli c ionophor e to the term polyethe r antibioti c used by Westle y (5) . The latte r term, furthermore , lead s to functiona l ambiguit y wit h the etherea l macrolid e nactin s and crown polyether s which are neutra l ionophores . Th e naturall y occurrin g carboxyli c ionophores , typifie d by monensin, lac k the structura l redundancy of the neutra l iono - phores . Monensin consist s of a formall y linea r arra y of hetero - cycli c ether-containin g rings , however the molecula r chiralit y arisin g from the ring s and asymmetric carbon s favor s the molecul e assuming a quasi-cycli c configuration . Additiona l stabilizatio n of the rin g is conferre d by head-to-tai l hydrogen bonding . In additio n to it s ligandin g ethe r oxygens, monensin has a pai r of ligandin g hydroxy l oxygens (10). Th e tai l portio n of nigerici n closel y resemble s monensin, however, an additiona l tetrahydropyrano l rin g thrust s the head carboxy l group int o the complexatio n sphere . Thus, in additio n to th e induce d dipol e ion bonds previousl y described , nigerici n com- plexe s featur e a tru e ioni c bond. Despit e major similaritie s in structure , nigerici n prefer s K + ove r Na + by a facto r of 100 whil e monensin prefer s Na + ove r K" 1 " by a facto r of 10 (11 ) . In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 6 INORGANIC CHEMISTRY IN BIOLOGY AND MEDICINE Dynamics of Ionophore-Mediate d Transpor t Neutra l Ionophores . The relationshi p between equilibriu m ionophor e affinitie s and dynamic biologica l transmembrane trans - por t is detaile d in Figur e 2. The transpor t cycl e catalyze d by neutra l ionophore s is give n on the left . Ionophore added to a biologica l membrane partition s predominatel y int o the membrane. A portio n of the ionophor e diffuse s to the membrane interfac e where i t encounter s a hydrate d cation . A loos e encounte r complex is formed followe d by replacemen t of the cationi c hydratio n spher e by engulfmen t of the catio n by the ionophore . The dehydrate d com- ple x is lipid-solubl e and hence can diffus e acros s the membrane. Th e catio n is the n rehydrated , released , and the uncomplexed iono - phore free d to retur n to it s initia l stat e withi n the membrane. Th e net reactio n catalyze d is the movement of an io n wit h it s charge acros s the membrane. Two independen t factor governin g net transpor t tential , i.e . AE^B , and the concentratio n gradient , [M+ ] ^/[M+ ] B • At equilibrium , the electrochemica l potentia l (a combined functio n of electrica l and concentratio n terms) of M*" on sid e A becomes equa l to the electrochemica l potentia l of on sid e B, i.e . PM A = PMB * IN TERMS of experimentall y measurable parameters , the relationshi p AE ^ = -59 mV log [M + ] A /[M + ] B applies . Thi s signifie s tha t if the electrica l term, AE^B , exceeds the concentratio n term, 59 mV log [M^/Mj], the io n wil l flo w down the potentia l gradien t and dissipat e it (electrophoreti c transpor t mode). If the concentratio n term exceeds the pre-exist - in g potentia l term, the movement of down it s concentratio n term wil l increas e AE ^j g (electrogeni c transport) . The relevan t sig - nificanc e of thi s transpor t mode is tha t neutra l ionophore s per - tur b not onl y the transmembrane ion gradient s of biologica l systems but als o thei r transmembrane electrica l potentials . Sinc e th e latte r are so importan t in biologica l control , it is not sur - prisin g tha t the neutra l ionophore s are exceedingl y toxi c towards intac t animals . Carboxyli c Ionophores . Carboxyli c ionophore-mediate d trans - por t is detaile d on the lef t of Figur e 2. The form assumed withi n th e membrane at the star t of the transpor t cycl e is an electri - call y neutra l zwitterion , M^-I"; anioni c fre e I" is presumably too pola r to be stabl e at tha t locus . When thi s specie s diffuse s to th e membrane interface , it is subjec t to solvation ; the catio n can be hydrate d and removed from the complex. The resultan t highl y pola r I" is oblige d to remain at the interfac e unti l a new charg e partner , represente d by N+'R^O, arrives . Once in position , N + In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980. [...]... i t i s In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 18 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE 2SII 20N ISM it!i\ sec 1000 MIREHSIR f 1IMI/K( I.I IKE 2**1/11 HAL USE 60 90 120 ISO MINUTES AFTER 00SE Figure 9 Pharmacokinetics of monensin in the dog In the upper trace, 100 fig/kg monensin was injected into a... subsequent increase i n i n t r a c e l l u 2 2 + 2 + + 2 + 2 + + In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 .37 7.7 12.1 13.1 + X-206 Salinomycin A-204 + 000025 6.1 Monensin 000009... l l , and the s o l v e n t becomes l e s s + In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 10 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE M Figure 4 CD spectra of the carboxylic acid free anion and K complex forms of salinomycin The free anionic form was generated by the addition of excess tri-nbutylamine and the... regarded as a conservative test In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 30 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE hemoglobin l e v e l of 6.19 g/100 ml; whereas rats fed n i c k e l at 5 and 50 yg/g of d i e t had hematocrits of 32.1% and 33.8% and hemoglobin l e v e l s of 8.31 and 8.92 g/100 ml, r e s p... In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 20 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE may accumulate over ten times t h i s l e v e l of monensin as a combinat i o n of parent compounds and m e t a b o l i t e s of unknown pharmacologic a l e f f e c t s (35) This data was obtained 12 hours a f t e r administ... r i n g the environmental continuum encountered by an ionophore when t r a n s v e r s i n g a b i o l o g i c a l membrane In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 2 2 D T 1.69 1.71... a r i s Bovine pancreas Spinach Rat liver Mouse melanoma Potato Jack bean Lemna p a u c i c o s t a t a Rumen b a c t e r i a l Soybean by N i e l s e n ( 3 3 ) In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE Table II Effects on rats of nickel, iron, and their interaction... obtained from a nonanesthetized dog that received the monensin orally (2 mg/kg) as a concentrate applied to a small quantity of feed The plasma levels obtained by administration of an oral dose approached those obtained by injection, indicating that the major portion of the oral dose passed through the plasma and into the tissues before being eliminated In Inorganic Chemistry in Biology and Medicine; ... requirement of c h i c k s , and Schnegg and Kirchgessner (6) reported a s i m i l a r requirement f o r r a t s I f animal data were extrapolated to man, the d i e t a r y n i c k e l requirement of humans In Inorganic Chemistry in Biology and Medicine; Martell, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980 32 INORGANIC CHEMISTRY IN BIOLOGY A N D MEDICINE would probably be i... % and hemoglobin l e v e l of 1 0 0 9 g / 1 0 0 ml, whereas r a t s fed n i c k e l at 5 and 5 0 yg/g of d i e t had hematocrits of 4 0 8 % and 4 2 0 % and hemoglobin l e v e l s of 1 1 7 7 and 1 2 0 9 g / 1 0 0 ml, r e s p e c t i v e l y In Experiment 3 , n i c k e l - d e p r i v e d r a t s had an average hematocrit of 2 6 8 % and # 2 l + 3 2 In Inorganic Chemistry in Biology and Medicine; . crystallography indicates to be primarily involved in liganding to cations are filled in. In Inorganic Chemistry in Biology and Medicine; Martell,. primar y increas e i n intracellula r Na + activit y t o a subsequen t increas e i n intracellu - In Inorganic Chemistry in Biology and Medicine; Martell,