Mechanisms of haloalkane and haloalkene biotransformation

Hafopcnated hydroe&onv constitute a farge group of cfremisafs with diverse applications in medicine. industry. agricut rure snd commerce. An important advance m medicine was t.he introduction of shCwoformas a general artestfre~ic by James SimfKon in t&17; the intelest in flaiogen alcd compnmds as rtnestheticscontinues unah;lted. and wveral new :Igents have f-eertmtrrxfucedrecently. Tfte biotransformation of ftafogenated chemic& fur+ fdng been of interest lo f&tlllWofogiUS. ~itf,lJU@ &ffCl SfiOWed, O-I 1883. tftat urinary chfortde excretion increased after chloroform admit&ration. Jo%mofecufer weight hafogenatedafkanes wfe

tfmu&t

for many

years to be afma

The objective of this review is todescribe briefly the pathways and mechanisms of frafoalkane and hatoafkene bi~ransfo~~ firm. Dehalogenation often occurs 7s a consequence of hafoirlkane and hafoafkene metabolism; except in the case of inorganic tfuotide formation. the toxicity of hafogenated cftemie& is not attributable to the halide refe.ased. axhl&ul or 0xygell&illR resctioas A common pathway for the biotransformation of haioafkanes involves C-H bond oxidation: R-CJ-L-X + [OJ-+R-CH(OH)X -*R-C{ =O)H R-Cfi-X1 + [Of-tR-C(OH)Xs -+R-Ct=O)X.

(f)

metabofiealIy inert. Su~quent studies (2) Growed that hafogenated hydrocarbons. wftere [Oj indicates a cytochrome Pwh ds vof:ltife anesthetrs. ale not JSQdependent po’lysubstrate monob~~rnic~~y men and are readify bio oxygenasocatafysed oxidation. Tfre rest&~~~fl~~~~ the involvement of tfre ingge~-~ohyd~s are unstable and yield microsomaf drug met&ofizing system was the correspoding afdehyde (I ) or acyf describedrubsequeatf;;~. halide (2) as produces.which may be conDuring the m 15 years. tfre biotrans verted to tfuz respective afcoftol or acid. formation of hafogenatedhydrocarbonshas Many examples of this reaction pathway berm stirdied exten&eiy, and reviews-j are known. Tfte metabolism of and a tnoe have been published. it dibaJometfnutesto carbon monoxide pro. Lsnow weff-es&fisfted tftat most ftafogen ceeds along a similar reaction p&way: ated compounds require biotransformation C&x; + ~Oj-cHtOH~w, to pmduce tfteir toxic effects. This bioactf (31 CH~OH}~~HC(=O~X + H’ + X- (4) vatron fnay fofffsw Iwo generaf pa&ways. HC(==O)X-+CO + H‘ + X-. (5) Prrit. frafogemted compounds may yield stabk. but toxic. metabofiles. The b&turn+ Similarly. tfu biotransformation of formation of dihalometfxutes to carbon chloroform to phosgene may occur by an rno~xi~~ and of l~~xy~~ to analogous reaction: fflwrrde +erve as exampfes. Second. CHCk f (Oj-ctOH)Cf, (61 hr~fogenti compoundsmaj yield reactive CtOH$%-+C(=O)Ck + H- + Cl ‘.(7) e&tropfrifr me~abnfites which alkyfate seffufar macromofecufes. The metabohsm Tfie pftosgene produced may interact with of ~h~~~ to -gene and tfreepox* ceffufar ma~mmofecufes and is egg to iica of hafegenat& ahume~are exampfes of fre the reactiTc intermediate responsible for &is bioactivation pathway. cell damage.

Much evidence sup~xrttstftesc reaction sequences as a common mechanism for haloalkaoe metabolism. The reactions arc the cytochromc Pcatafysetl by 45~de~~ent ~fysubstmte monooxygenase system: this is supportedby the microsornal location OF the enzymes, the requirement for both NADPH and dioxygen. tfte responsivenessto enzyme inducers, such as phenoba~it~l, and inhibitors. such as SKF 52.5-A. and inhibition by carbon monoxide. Reaction mechanism studies on the biotransformation of dihaiome~a~es to carbon monoxide show that the oxygen atom appearing in the carbon monoxide is derived fmm dioxygen. and the reaction shows a large deutcrium isotope effect (V~~W/V~~.U~N= 7.7; Ref. 7). f3euterium isotope effects have also been observed in the ~ta~lism of chloroform, bromofotm, methoxyflurane, halothaneand enffurane”~4. Hafogenated alkenes are well known to undergo e~xidation reactions and the resulting epoxides are frequently highfy reactive and have been implicated in the toxicity of this class of compounds”. The general reaction is x2c=cXz

4 (O]->xzc-9

xlr.

(Pf

The cytocfuome P-45tLdependent polysubstrate mono-oxgenase system is also involved in hafogenated afkene me~~lism; for example, the reaction is catafysed by microsomaf enzymes recjuiring NADPH and dioxygen, is increased by treatmentof animals with enzyme indilcers. and is inhibited by carbon monoxide and other mon~oxygen~~ inhibitors. The epoxides thus fotmed may react with tissue constituents and trigger the production of cell damage or may undergo novel re arrangements accomfrmned by halide migratior?. The biotransformation of tricftforoethylene is an excellent example of this reaction. The intermediate epoxide, I. I ,5rearranges to yield ~hfo~ox~, chloraf hydrate which may be oxidized to hicfrforoacetic acid or reduced to trichforoethanol. While oxhfative reactions usually involve C-H bond oxidation, direct oxidation of the bafogen atom could also occur. In general, hypetvafent alkyf halides are either unknown or exist only as transient intermediates. Hypervalent aryl icufidesare

stable compounds, and eytochrome P-450 has been reported to catafyse tfte transfer of oxygen fmm iodc~sobenzeneto iodobek

7irl?S - .Seprc&t~r zene”.

Thr

I YX2

form;l!ton

2-dichloroethanc

of

as an

I ,2_dichlorocerhane

I-chlorosc~

intermediate

metabolism

in

has been

ih

c;ltslysed

the

bl

S-tranhfcrascz.

a

lamily

glutathione of

cy~o~ohc

i

GSH

RX-GSR

i X

.

(131

Reductive reactions Thz metabolism of carbon Ietrachloride

where GSH

is glulaUuone, RX is an alkyl

hnlide. GSR

is the plutathione conjugate.

lo chL rofonn and of halolhane to 2 - chloro

and X

” I, ‘.I tritluoroethane and 2Wdlchloro- I, I-difluoroethylene are

example.

examples

of

the

cytochrome

P-4SUdependent reductive metabolism of halogenated chemicals. The metabolism of carbon tetrachloride to chloroform

is cata-

lyseld by microsomal enzymes. is dependent on the presence of cytochrome

P-450

in

is the liberated inorganic halide. For iodomclhane

is metabohLcd

to

S-methylglutathione. and the major urinary metabolite of 1.3-d&?mopropane ih N- rcctyl-S(3-bromopro;_

I)cysteinc;

the

coilversion of glutathionc conjug;lles to mercapturic acids is oh.servcd frcquentl>. and many halogenated compounds arc excreted as mercapmrtc acids. Mechanistic

reconstituted enzyme systems. is increased by treatment of animals with enzyme induc-

studies show that the :eaction proceeds with

ers. and is inhibited by carbon monoxide,

consistent with a nucleophilic (F&2) attack

tihus establishing the involvement of cyttr

of glutathione on the carbon atom bearing the halogen.

chrome P-450 in the reaction. The

reduction

of carbon

tetrachloride

irppears to follow the pathway? CC14 + e ~-b-Ccl3

+ CI

-Ccl. + lipid-+CHCln + lipid radical.

(9) (10)

inversion of configuration’“;

this tinding is

When ,gmz-dihaloalkanes. such as dihalomethanes or chloramphenicol. are involved. the imermediatr S(cr-haloalkyl)glutathione conjugates are unstable and yi.:ld aldehydes as products.

cell, which a~ thought to be involved in

The metabolism of dihalomcthancs to formaldehyde is an example of this biotransformation. When vie-dihaloalkunes are involved, the Sintermediate

carbon tetrachloride indulced liver damage. The trichloromethyl rac,ical may undergo further reduction to the trichloromethyl carbanion, which may :yield dichllbrocar-

(G-haloalkyl)gluaathione intermediates may undergo rearrangement to yield episuifonium ions. Such episulfonium ions are highly reactive and have heen shown to be

bene after u-eliminatior~l of chlotice;

mutagenic”.

The intermediate trichll,romethyl radical may initiate peroxidatife changes in the

drolysis of dichlorocartene

hy-

yields .:arhon

Conclusions

monoxide:

Considerable -CCL + e---CCL -CCla-+:CC12 :CCL + HzO-K’O

+

(II) Cl

+ 2HCl.

(12) f 13)

strides have been mJdr m

elucidating the mechanisms by \+hich hak~genated chemicals undergo blotran+ formation or bioactivarion. if a toxic metabolite is produced. but much remaina

Similar pathways for hslothane reduction have been described, and persuasi\,e evi-

10 bc done. The intriguing suggestion that

dence for the reduction of halothane to a carbene has been presenned.

halogen atoms may undergo direct enzynr atic oxidation needs IO be inve$rigated

The glutathionedependent reducllon of l.2-dihaloethanes to ethylene and of

further. Additional biological studies are needed to understand the role of lipid pcrolc-

a-haloketones to methyl ketonl:s by cytosolic enzymes has heen descritxd, but

hepatotoxicity.

Gnce recent ytudics al>cb

detailed reaction mec!?.~nism studiei have

suggest a role

for phosgene fonnatioll.

not been repotted.

Similarly, the role of episulfonium :on fijrmation in the toxiciry of I ,2-dihaitwthancb

idation

ddrion ofthc dcta~lcd nlezhanf\m\ h.dogcnated chemical\

cryzymer:

suggested, but direct evidcnre is lachinp.

known

grc.~e\t challcnpc lor the i‘uturc i\ the CIUL,I-

in

carbon

tetrachloride-induced

needs further work and must h: contrastrd

Conjogation reactions

with the biolo@cal

The coqiugation of monohaloalkanes wllth glutathione is a common reactIon and

dehydes, mutagens that are also mctabolites of 1.Lhhalocthanes. Perhaps the

effects of haloacetal-

qc.

pro&cc

h> u hkch ~,ell dam-

While the alh>latlon ccf ~~clluI.u mazn~

molecule\

ha\

producrion

of cell

been assoc~atcd mith the damage. the oreraIl prcL

ccss IS very pllorl) undcrxttxd. cation

of

recent

The apph-

ad%;:nces m cell

and

molecular h~olopy 10 p:-ohlcrn~ m IIIUCOIogy will undoubtedly pr’+ve fruitful.