▁in ▁brain ▁mit och ond ria , ▁ γ - gl ut am yl c yst e ine ▁can ▁serve ▁as ▁the ▁re duct ant ▁instead . ▁The ▁protein ▁encoded ▁by ▁this ▁gene ▁protect s ▁from ▁CD 9 5 - indu ced ▁apo pt osis ▁in ▁cult ured ▁breast ▁cancer ▁cells ▁and ▁in hib its ▁ 5 - li po xygen ase ▁in ▁blood ▁cells , ▁and ▁its ▁over expression ▁del ays ▁end oth el ial ▁cell ▁death ▁and ▁increases ▁resistance ▁to ▁to xic ▁challeng es , ▁especially ▁ox id ative ▁stress . ▁This ▁protein ▁is ▁one ▁of ▁only ▁a ▁few ▁prote ins ▁known ▁in ▁higher ▁verte br ates ▁to ▁contain ▁s elen oc yst e ine , ▁which ▁occurs ▁at ▁the ▁active ▁site ▁of ▁gl ut ath ione ▁per ox id ase ▁and ▁is ▁c oded ▁by ▁the ▁n ons ense ▁( stop ) ▁cod on ▁T GA . ▁ ▁GP X 1 ▁forms ▁a ▁highly ▁re active ▁s elen en ic ▁acid ▁intermediate , ▁providing ▁insight ▁into ▁the ▁way ▁that ▁the ▁protein ▁environment ▁stabil izes ▁these ▁inter medi ates ▁and ▁p aving ▁the ▁way ▁for ▁new ▁th era pe ut ics . ▁S elen en ic ▁acid ▁is ▁protected ▁by ▁the ▁protein ▁environment ▁from ▁re active ▁groups ▁within ▁the ▁protein . ▁The ▁mechanism ▁of ▁action ▁is ▁based ▁on ▁s elen en ic ▁acid ▁react ing ▁with ▁the ▁amid ▁or ▁am ine ▁bond ▁of ▁another ▁protein , ▁forming ▁a ▁sen yl ad m ide ▁bond , ▁suggesting ▁a ▁role ▁for ▁this ▁bond