Acute pulmonary toxicity and
tumor promotion by the
food additive 2,6-di-tert-butyl-4-methylphenol (
BHT) in mice are well documented. These effects have been attributed to either of two
quinone methides, 2,6-di-tert-butyl-4-methylenecyclohexa-2,5-dienone (
BHT-QM) formed through direct oxidation of
BHT by pulmonary
cytochrome P450 or a
quinone methide formed by hydroxylation of a tert-butyl group of
BHT (to form
BHTOH) followed by oxidation of this metabolite to
BHTOH-QM.
BHTOH-QM is a more reactive electrophile compared to
BHT-QM due to intramolecular interactions of the side-chain
hydroxyl with the carbonyl
oxygen. To further examine this bioactivation pathway, an analogue of
BHTOH was prepared, 2-tert-butyl-6-(1'-hydroxy-1'-methyl)ethyl-4-methylphenol (BPPOH), that is structurally very similar to
BHTOH but forms a
quinone methide (BPPOH-QM) capable of more efficient intramolecular hydrogen bonding and, therefore, higher electrophilicity than
BHTOH-QM. BPPOH-QM was synthesized and its reactivity with water,
methanol, and
glutathione determined to be >10-fold higher than that of
BHTOH-QM. The conversions of BPPOH and
BHTOH to
quinone methides in lung microsomes from male BALB/cByJ mice were quantitatively similar, but in vivo the former was pneumotoxic at one-half of the dose required for the latter and one-eighth of the dose required for
BHT, as determined by increased lung
weight:body weight ratios following a single i.p. injection. Similar differences were found in the doses of
BHT,
BHTOH, or BPPOH required for
tumor promotion after a single initiating dose of
3-methylcholanthrene followed by three weekly
injections of the
phenol. The downregulaton of
calpain II, previously shown to accompany lung
tumor promotion by
BHT and
BHTOH, also occurred with BPPOH. The correlation between
biologic activities of these
phenols and the reactivities of their corresponding
quinone methides provides additional support for the role of
BHTOH-QM as the principal metabolite responsible for the effects of
BHT on mouse lung.