Glutathionylspermidine is an intermediate formed in the biosynthesis of trypanothione, an essential metabolite in defence against chemical and oxi-dative stress in the Kinetoplastida. The kinetic mechanism for glutathionyl-spermidine synthetase (EC ) from Crithidia fasciculata (CfGspS) obeys a rapid equilibrium random ter-ter model with kinetic constants KGSH= 609lm, KSpd= 157lmand KATP= 215lm.