Environmental Engineering Reference
In-Depth Information
Selenoproteins. have. antioxidant. or. other. redox. functions,. which. is. why. Se.
deiciency. often. enhances. the. probability. of. developing. cancers. or. viral.
infections;. diseases. associated. with. Se. deiciency. include. Keshan. disease.
and. male. infertility. in. humans,. and. white. muscle. disease. in. livestock.
(Whanger.1989,.Ellis.et.al..2004,.Diwadkar-Navsariwala.et.al..2006,.White.and.
Broadley. 2009).. There. is. a. relatively. narrow. window. between. the. amount.
of.Se.required.as.a.nutrient.and.the.amount.that.is.toxic,.and.hence,.Se.dei-
ciency.and.toxicity.are.both.common.problems.worldwide.(Terry.et.al..2000)..
As.an.illustration,.daily.intake.of.50.μg.Se.is.recommended.for.humans,.but.
long-term.intake.of.10.times.higher.levels.may.lead.to.chronic.Se.poisoning..
The.one-time.ingestion.of.plant.material.containing.1000.mg/kg.dry.weight.
(dry.wt.).or.more.Se.can.even.lead.to.acute.Se.poisoning.and.death.(Draize.
and.Beath.1935,.Rosenfeld.and.Beath.1964,.Wilber.1980)..Such.high.Se.levels.
(1,000-10,000.mg/kg.dry.wt.).occur.in.so-called.hyperaccumulator.plant.spe-
cies.that.are.endemic.on.seleniferous.soils.in.the.Western.United.States.and.
parts.of.China.where.Se.is.naturally.present..Human.and.livestock.Se.poison-
ing,.both.chronic.and.acute,.are.serious.problems.in.these.seleniferous.areas.
(Ohlendorf.et.al..1986,.Harris.1991,.Kabata-Pendias.1998,.Terry.et.al..2000).
Higher.plants.readily.take.up.selenate.or.selenite.from.their.environment.
and.incorporate.it.into.organic.compounds.using.S.transporters.and.S.assim-
ilation.enzymes..In.short,.inorganic.selenate.is.taken.up.and.reduced.to.sel-
enite,.then.selenide,.and.combined.with.O-acetylserine.(OAS).to.form.SeCys..
This.seleno-amino.acid.can.be.nonspeciically.incorporated.into.proteins.in.
the.place.of.Cys,.leading.to.toxicity..SeCys.can.also.be.converted.to.seleno-
methionine.(SeMet),.which.also.can.be.misincorporated.into.proteins..SeMet.
can.also.be.converted.to.volatile.dimethylselenide.(DMSe).(Lewis.et.al..1966,.
Hansen. et. al.. 1998).. Furthermore,. SeCys. can. be. broken. down. to. elemental.
Se. and. alanine. (Pilon. et. al.. 2003).. Elemental. Se. is. insoluble. and. relatively.
innocuous..SeCys.can.also.be.methylated.to.form.methyl-SeCys,.which.can.
be. safely. accumulated. since. it. is. not. incorporated. into. proteins. (Neuhierl.
et.al..1999)..Methyl-SeCys.can.also.act.as.a.precursor.for.the.production.of.
another.form.of.volatile.Se,.dimethyldiselenide.(DMDSe).(Terry.et.al..2000,.
Sors.et.al..2005)..This.is.the.main.volatile.form.of.Se.emitted.by.hyperaccu-
mulator.species.
The. capacity. of. plants. to. accumulate,. metabolize,. and. volatilize. Se. is.
all. useful. for. Se. phytoremediation.. Soil. or. water. rich. in. Se,. either. natu-
rally. or. due. to. human. activities. (e.g.,. mining,. oil. reining,. agriculture),.
may. be. cleaned. up. using. plants,. which. may. either. release. the. Se. into.
the. atmosphere. in. relatively. nontoxic. volatile. form. and/or. accumulate.
it. in. their. harvestable. tissues.. If. Se. is. the. only. pollutant. present,. the. use.
of. Se-accumulating. plants. for. phytoremediation. may. produce. a. value-
added.crop.that.may.be.sold,.offsetting.the.phytoremediation.costs..Some.
selenocompounds. have. particularly. potent. anticarcinogenic. properties.
(Unni.et.al..2005),.e.g.,.methyl-SeCys..Examples.of.plant.species.that.accu-
mulate. this. compound. are. the. crop. species. broccoli. and. garlic,. and. the.
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