Ajoene is an inhibitor and subversive substrate of human glutathione reductase and Trypanosoma cruzi trypanothione reductase: Crystallographic, kinetic, and spectroscopic studies

Ajoene ((E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide), a garlic-derived natural compound, is a covalent; inhibitor as well as a substrate of human glutathione reductase (GR) and Trypanosoma cruzi trypanothione reductase (T R). The 2.1-Angstrom resolution crystal structure of GR inhibited by (E)-aj oene revealed a mixed disulfide between the active site Cys58 and the CH2=C H-CH2-SO-CH2-CH=CH-S moiety of ajoene. The modified enzyme has a markedly i ncreased oxidase activity when compared to free GR, GR reduces (Z)-ajoene w ith a k(cat)/K-m of 6.8 x 10(3) M-1 s(-1) yielding 4,5,9-trithiadodeca-1,6, 11-triene (deoxyajoene) and 4,8,9,13-tetrathiahexadeca-1,6,10,15-tetraene a s stable reaction products. The reaction leads also to the formation of sin gle-electron reduced products and concomitantly superoxide anion radicals a s shown by coupling the reaction to the reduction of cytochrome c. The inte ractions between the flavoenzymes and ajoene are expected to increase the o xidative stress of the respective cell. The antiparasitic and cytostatic ac tions of ajoene may at least in part be due to the multiple effects on key enzymes of the antioxidant thiol metabolism.