throbber
Gordon W. Gribble'
`Dartmouth College
`Hanover, New Hampshire 03755
`
`Fluoroacetate Toxicity
`
`Under the stress of World War I1 chemists in England,
`Germany and their allied countries sought to develop
`chemicals (independently, of course!) which would inca-
`pacitate, maim, or kill the enemy. These remarkably suc-
`cessful researches led to the synthesis and large-scale pro-
`duction of several types of warfare agents: nerve gases,
`vesicant agents, tear gases, harassing compounds, and,
`perhaps the most frightening of all, water poisons.
`For the latter kind of chemical agent it can be easily
`envisaged that a secret agent could poison the water sup-
`. .
`" . .
`nlv of a large enemv ~onulace with hut a small amount of
`"
`a toxic chemical. The requirements for a water poison are
`strineent: it should he colorless. odorless. soluble. stable.
`and rhighly toxic, preferably with a delayed action to pre:
`vent early detection. I t therefore must have come as quite
`a surprise to chemists in England, Germany, and Poland
`when they discovered independently during the early stag-
`es of the war that a simple derivative of acetic acid fulfills
`all of the above criteria for an ideal water poison!
`This compound is methyl fluoroacetate (MFA) and it,
`along with fluoroacetic acid (FA) and 2-fluoroethanol,
`represents one of the most toxic classes of non-protein
`substances known.
`
`0
`I1
`FCH,COCH,
`(MFA)
`
`0
`II
`FCH,COH
`(FA)
`
`FCH,CH,OH
`
`As seen in Table 1 these compounds are more toxic, on
`a per weight basis, than several other well-known deadly
`poisons.
`Preparation and Properties
`MFA was first prepared in 1896 by the Belgian chemist
`Swarts (I) by treatment of methyl iodoacetate with silver
`fluoride, a rather expensive and inefficient procedure.
`
`ICH,CO,CH, + AgF - FCH,CO,CH, + AgI
`
`MFA, because of its great toxicity, came into consider-
`ation a t the start of World War I1 as a potential warfare
`agent ( 2 4 ) . MFA seemed especially suitable as a water
`poison because of its "ideal" physical and chemical prop-
`erties for this type of agent (see Table 2). In addition, its
`toxic action is delayed making early detection of MFA in
`water supplies difficult.
`By the end of the war several countries including Eng-
`land, the United States, Poland, and Germany had de-
`veloped efficient pilot-plant methods for the preparation
`of MFA.
`Saunders and his colleagues in England used a rotating
`autoclave at 220°C to produce MFA from methyl chloroa-
`cetate and potassium fluoride in 54% yield (5). A similar
`method was developed by a Polish group (6). Schrader in
`Germany found that ethyl diazoacetate and hydrofluoric
`acid furnished ethyl fluoroacetate (7). Several other syn-
`
`1 Recipient of a Public Health Service Research Career Devel-
`opment Award (1K04-GM23756-01) from the National Institute of
`General Medical Sciences.
`460 / Journalof Chemical Education
`
`Table 1. Toxicities of MFA and Other Toxic Compoundso
`
`MFA
`FCHzCOINa
`mustard gas
`parathion
`strychnine
`NaCN
`DFP (nerve agent)
`OTaken from several sources.
`The dose required to kill 50% of the animals by subcutaneous
`injection.
`
`6
`. . .
`. . .
`. . .
`0.5
`10
`4
`
`. . .
`0.2
`0.7
`10
`5
`. . .
`. . .
`
`Table 2. Selected Physical Properties of MFAa
`
`Property
`hoilina ~ o i n t
`meltingpoint
`water solubility
`odor
`a Reference ( 4 )
`
`Value
`104' (760 mm)
`-32'
`15%
`faintlv fruitv at 10 DDm
`
`theses of these fluoro esters were disclosed after the war
`(2).
`Unlike the other haloacetates MFA is remarkably resis-
`tant to displacement of fluoride by nucleophiles. For ex-
`ample, MFA when refluxed for 1 hr with 10% NaOH gives
`no fluoride. After 20 hr of reflux with 20% KOH only 50%
`of fluoride is liberated (51. Similarly, erhyl bromoacetace
`is at least 400,000 tlmes more reactive than MFA towards
`sulfite ion. This inherent stabilitv of the C-F bond in
`MFA must also account for the aisence of lachrymatory
`properties for MFA, unlike the cbloro-, bromo-, and
`iodoacetates which are acute tear-producing agents. These
`haloacetates act as powerful alkylating agents, like mustard
`gas and dimethyl sulfate, and react with cellular nucleo-
`philes such as the SH group of proteins and the nucleo-
`nhilic nitroeens of nucleic acid bases. In stark contrast to
`these haloacetates, MFA is not a biological alkylating agent
`and. in fact. the C-F bond in MFA remains intact throuah-
`out t h e course of poisoning! Thus, paradoxically, it is the
`great stability of MFA and FA which leads to their unique
`toxic action. What then is the explanation for the great
`toxicity of MFA and FA?
`Toxic Action
`Several investigators in 1948-49 observed that citric
`acid accumulated in the tissues of MFA-treated animals
`(8-10). It was then logically suggested that FA entered the
`citric acid cycle (Krehs) and somehow prevented the fur-
`ther utilization of citric acid (11, 12).
`Before exploring the exact nature of this interplay of FA
`with the citric acid cycle let us review the latter (13). The
`role served by the citric acid cycle is to oxidize fatty
`acids, carbohydrates, and amino acids.To do this, these
`foodstuff molecules must first he degraded to acelic acid
`(in the form of acetyl coenzyme A). Finally, acetic acid is
`oxidized according to the following equation.
`
`Adama Makhteshim Ltd. Exhibit 1038
`
`

`
`The importance of the citric acid cycle, shown in its en-
`tirety below, cannot be overstated since the energy of one
`acetic acid molecule is transformed and conserved in the
`form of 12 ATP molecules which in turn serve as essential
`energy carriers in living cells.
`Returning to the mode of action of FA, it was observed
`that FA does not inhibit any isolated individual citric acid
`cycle enzyme (11, 14). This would appear to demand the
`conclusion that FA is converted in oivo to a different,
`more toxic substance which enters the citric acid cycle.
`Peters bas provided convincing evidence that this toxin is
`fluorocitrate (FC) which is synthesized in uiuo from FA
`(11, 14).
`
`The FC subsequently inhibits the enzyme aconitase,
`which is concerned with the conversion of citric acid to
`aconitic acid and or isocitric acid. This inhibition leads tu
`a fatal huilduu of citric acid in the tissues, culminating in
`violent convuisions and death from cardiac failure opre-
`spiratory arrest (see Fig. 1).
`The evidence for Peters' proposal is quite strong: (1) FC
`can be isolated from FA poisoned animals; (2) FC inhibits
`highly purified aconitase whereas FA does not; and (3)
`acetate exhibits a protective effect, presumably by inter-
`fering with the conversion of FA into FC.
`
`carbohydrates '.
`
`pyruvic
`
`fatty
`acids
`
`C H ~ ~ A
`
`fluoroacetic
`
`In addition, structure-toxicity studies provide further
`evidence in support of the FC hypothesis.
`Structure-Activity Relationships
`One might reasonably predict that a compound capable
`of in uioo conversion to FA would be toxic. Indeed, it is
`clear from Table 3 that this is exactlv the case. Fluo-
`roethanol and fluoroacetaldehyde can he oxidized in i,ruo
`to FA while tluoroacetvl tluoride, tluoroacetamide, and
`fluoroacetic anhydride Ean be hydrolyzed to FA. All are
`about as toxic as MFA. On the other hand, compounds
`such as difluoroacetic acid, chloroacetyl fluoride, and
`methyl fluoroformate are nontoxic since they cannot be
`hydrolyzed to FA. Not surprisingly, the chlorine atom in
`1-chloro-2-fluoroethane is apparently not displaced by
`water in the organism and, as a result, is nontoxic.
`Since only those fatty acids containing an even number
`of carbon atoms are degraded to acetic acid in uiuo, it is
`found that only the corresponding o-fluorocarhoxylic acids
`are toxic. The odd-carbon o-fluorocarboxylic acids are
`non-toxic. As seen in Table 4 the results are striking in-
`deed.
`The observed alternation in toxicity is a beautiful veri-
`fication of the theory of 0-oxidation of fatty acids, sum-
`marized in Figure 2 (15).
`In accord with the @oxidation theory as applied above
`to o-fluorocarboxylic acids, the following derivatives are
`all non-toxic, even though they possess an even-numbered
`carbon chain. I t is readily seen that in each case one or
`more of the steps in the 0-oxidation theory is impossible.
`
`CH,
`I
`FCH,CCH,CO,H
`
`In spite of the general understanding of FA toxicity, the
`details of the FC-aconitase inhibition are unknown al-
`though schemes have been advanced (16).
`Natural Occurrence
`At least three natural sources of FA and derivatives
`
`Table 3. Toxicity of Selected Fluoroacetyl
`and Related Compoundsa
`
`Toxicb
`
`NontoxicC
`
`citric p\
`accumulation
`
`CO~H
`I
`
`a References (2-4)
`b Toxicity comparable with that of MFA
`No MFA-like toxicity.
`
`Table 4. Toxicitv ofw-Fluorocarboxvlatesa
`
`Figure 1. Citric acid cycle
`
`References (2-41.
`
`Volume 50. Number 7. July 1973 / 461
`
`Adama Makhteshim Ltd. Exhibit 1038
`
`

`
`Treatment
`As mentioned earlier, large doses of acetate or an acetate
`source such as glycerol monoacetate or ethanol help to
`prevent the lethal synthesis of FC from FA. However, this
`is, in effect, preventive treatment and is effective only if
`administered immediately after poisoning and thus prior
`to the FC synthesis and citrate accumulation. In view of
`the fact that citrate is reasonably effective in lowering the
`Pb(II) concentration in lead poisoned animals (21). it
`would he interesting to see if Ph(I1) treatment can lower
`the citrate buildup and prevent the fatal onslaught of con-
`vulsions which result from high citrate concentrations in
`the tissues. Citrate forms a strong complex with Ph(I1)
`which apparently is readily excreted. Since citric acid ac-
`cumulation probably disturbs the normal calcium ion bal-
`ance in the organism another treatment which is therefore
`suggested is administration of calcium ion. This may help
`to restore the normal calcium balance by removing excess
`citrate through complexation.
`Since sodium fluoroacetate is used commercially as a
`rodenticide (trade name: "1080") proper caution should
`be exercised in its handling as several cases of human poi-
`soning have been recorded (22-24). The lethal dose for a
`150 lb man is estimated to be about 400 mg (25). Particular
`attention should he paid to the fact that the fluoroacetate
`and fluorocitrate residing in the carcass remains toxic for
`a long period, unlike most other organic poisons.
`Literature Cited
`(11 swarta, F., BUII A
`,In*, , . .. . , .
`(2) Ssrtori,M. F., Che
`131 Ssundcrs, B. C.. "
`C., 196I.Vol.2,~~. 196-204.
`(4) Ssunders, B. C., "Some Aspeels of the Chemirry and Torie Action of Organie
`Compounds Containing Phmpharus and Fluorine," Cambridge UnivemiW Press,
`1957, pp. 114-70.
`(5) Ssundor8.B. C.. andStacey,G. J.. J Cham. Soc., 1773119481.
`(6) Gryszkiewicz-Trochimowski, E., Sporzynrki, A . and Wnuk. J., Rsu. Tr.u Chrm..
`66.413 (1947).
`(7) Sehrader, G., "The Development of New Imetieiden." Final Report No. 714, Brit-
`ish Intelligence Ohjrctives Sub-Committee, London, 1947, p. 19.
`(81 Buffa, P., and Peters, R. A,. J . Phyaiol.. 110,488 11949).
`191 Kslnifsky, G., andBerron, E. S. G.,Arrh. Biarham.. 19,75(19481.
`1101 Potter.V.R..andBush. H.. CaneerRes.. 10.353119501.
`
`31. 675 ii8961; BUII. so<. chim.. 15, 1134
`RW. B ~ I ~ . ,
`
`~
`
`E
`
`.
`
`.
`
`1121 Martius, C.,Liebi#sAnn.
`(131 Mshler. H. R., and Col
`Row, NewY0.k.
`(141 Peters, R. A., Ploc.
`,
`(13). pp. mi-604.
`(151
`(16) SPLY~I, J . F., andDickmen, S.R., J. B i d Chem., 220,193il9561.
`(17) Maraia. J. S. C., OnderstrpoorfJ Vet. Sei. An.. 18.203 (19431: 20.67 (1944).
`(I81 hnner, W.. Blil Msd. J., 1. 1314 (1904); Peters, R. A,, and Hall, R. J..
`187.573(1960).
`(191 MeEwan.T.. QuaensiandJAgr Sci., 21.1(19641.
`(201 Chenoweth,M. B., sndGilman.A.,il Phormocol.. 87,90119461.
`(211 Fried, J. F.. Rasenthal, M. W.. and Schubert, J.. .%c.
`Soc. Erp B i d ar
`92,331 119561.
`(221 Gajdu~ek, D. C.. andLuther, G . A m J . Dil. Child., 79,310(l9501.
`123) Harrisson. J. W.E..,etol.. J,Amer Med. Asror., 149,1520(L952).
`(241 Broekmann, J . L., MeDoweIl, A. V., and Leedl, W. G.. J. Amlr Med. Ass
`. . .. , . . . . , .
`I I ? P I I P S S ,
`Flick. L.J.,andBoehd F. W., Vet. Med., 41,196119461.
`
`.
`
` .
`
`1251
`
`Figure 2.8-Oxidation of fatty acids.
`
`have been discovered to date. The South African plant
`gifilaar (dichpetalum cymosum) contains potassium flu-
`oroacetate ( I n , the Sierra Leone shrub ratshane (Dicha-
`petalum toxicarium) seems to contain a w-fluorooctade-
`cenoic acid (la), and the Australian plant Gastrolobium
`grandiflorum has present in its leaves fluoroacetic acid
`(19). Less than an ounce of gimlaar leaves is enough to
`kill a sheep, and it is claimed that one half a leaf is fatal
`to an ox (4).
`Another interesting aspect of FA toxicity is its remark-
`ahle species specificity. For example, MFA is highly toxic
`to the Texas pocket gopher (LDloo < 0.05 mg/kg) hut not
`to the South African clawed toad (LDso > 500 mg/kg)
`(20).
`
`462 / Journal o f Chemical Education
`
`Adama Makhteshim Ltd. Exhibit 1038

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