An electrophilic
quinone methide (2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone, BHT-QM) functions in the toxicity of
butylated hydroxytoluene (
BHT) in both rodent liver and lung.
BHT-QM has also been demonstrated to mediate
tumor promotion in mouse skin by another metabolite of
BHT,
2,6-di-tert-butyl-4-hydroperoxy-4-methyl-2,5-cyclohexadienone (
BHTOOH). In the present study, the role of
BHT-QM in the cytotoxicity of
BHTOOH was investigated. The toxicity of
BHTOOH was potentiated by
glutathione depletion and inhibited by
thiol compounds, indicating that
BHTOOH is activated to a
thiol-reactive, toxic intermediate. This activation process was suggested to be
iron-dependent by the ability of an Fe(III)-specific
chelator to inhibit
BHTOOH toxicity. Comparative study of analogs of
BHTOOH in which the 4-methyl group was substituted with CD3, ethyl, isopropyl or tert-butyl supported the hypothesis that
BHT-QM mediates this toxicological response. The decreased rate of reactivity of
quinone methides that occurs as the 4-alkyl group is enlarged was accompanied by a corresponding reduction in toxic potency. The structural requirements for
quinone methide toxicity were also explored with a series of
BHTOOH analogs substituted at the 2- and 6-positions of the molecule. Reducing the lipophilicity of the 2,6-tert-butyl groups is known to increase
quinone methide reactivity with
glutathione but does not diminish the rate of
quinone methide formation from the
hydroperoxide. Interestingly, alteration of only one of the tert-butyl groups did not change the toxic potency, whereas removal or replacement of both tert-butyl groups dramatically reduced the toxicity in control cells but not
glutathione-depleted cells.(ABSTRACT TRUNCATED AT 250 WORDS)