The influence of glutathione and cysteine levels on the cytotoxicity of helenanolide type sesquiterpene lactones against KB cells

The biological activities of sesquiterpene lactones have been attributed to their reactivity with the cysteine residues of functional proteins forming covalent bonds via Michael type addition. In the present study we investig ated the influence of different L-cysteine (cys) and glutathione (GSH) conc entrations on the cytotoxicity of the sesquiterpene lactones (STLs) helenal in, 11 alpha ,13-dihydrohelenalin acetate and chamissonolide against KB cel ls. Due to the significantly higher reactivity of the alpha -methylene-gamm a -lactone (ML) towards cys as compared with the cyclopentenone (CP) site a t physiological pH, addition of 20, 50 and 100 molar equivalents of cys dec reased the cytotoxicity of helenalin and chamissonolide, whereas the cytoto xicity of 11 alpha ,13-dihydrohelenalin acetate remained unaffected. In con trast, the influence of GSH addition on the cytotoxicity of 11 alpha ,13-di hydrohelenalin acetate depends on the concentration of GSH added. Concentra tion-effect curves obtained for chamissonolide and GSH resembled the declin e in cytotoxicity after cys addition. Helenalin showed a biphasic shape of the concentration-effect curve for the 100:1 GSH/helenalin ratio resembling at higher doses the chamissonolide and in lower doses the 11 alpha ,13-dih ydrohelenalin acetate curve at 50-fold excess. These results can be explain ed by the different reactivity and equilibrium conditions for thiol additio n of the two reactive centers of bifunctional STLs in cellular test systems and verified a clear correlation between the different reactivity of their electrophilic centers and the observed biological effects in in-vitro cell systems. (C) 2001 Elsevier Science Ltd. All rights reserved.