▁important ▁role ▁in ▁the ▁development ▁of ▁primitive ▁life ▁on ▁Earth . ▁ ▁P rec ursor ▁to ▁the ▁ant io x id ant ▁gl ut ath ione ▁Due ▁to ▁the ▁ability ▁of ▁th i ols ▁to ▁under go ▁re do x ▁re actions , ▁c yst e ine ▁has ▁ant io x id ant ▁properties . ▁Its ▁ant io x id ant ▁properties ▁are ▁typically ▁expressed ▁in ▁the ▁tri pe pt ide ▁gl ut ath ione , ▁which ▁occurs ▁in ▁humans ▁and ▁other ▁organ isms . ▁The ▁system ic ▁avail ability ▁of ▁or al ▁gl ut ath ione ▁( G SH ) ▁is ▁neg lig ible ; ▁so ▁it ▁must ▁be ▁b ios yn thes ized ▁from ▁its ▁constitu ent ▁am ino ▁ac ids , ▁c yst e ine , ▁g ly c ine , ▁and ▁gl ut am ic ▁acid . ▁While ▁gl ut am ic ▁acid ▁is ▁usually ▁sufficient ▁because ▁am ino ▁acid ▁nit ro gen ▁is ▁rec yc led ▁through ▁gl ut am ate ▁as ▁an ▁inter medi ary , ▁di et ary ▁c yst e ine ▁and ▁g ly c ine ▁supp lement ation ▁can ▁improve ▁synth esis ▁of ▁gl ut ath ione . ▁ ▁P rec ursor ▁to ▁iron - s ulf ur ▁clusters ▁C yst e ine ▁is ▁an ▁important ▁source ▁of ▁sul f ide ▁in ▁human ▁met abol ism . ▁The ▁sul f ide ▁in ▁iron - s ulf ur ▁clusters ▁and ▁in ▁nit ro gen ase ▁is ▁extracted ▁from ▁c yst e ine , ▁which ▁is