Using Physical Organic Chemistry Knowledge to Predict Unusual Metabolites of Synthetic Phenolic Antioxidants by Cytochrome P450

Biotransformation, especially by
human CYP450 enzymes, plays a
crucial role in regulating the toxicity of organic compounds in organisms,
but is poorly understood for most emerging pollutants, as their numerous
“unusual” biotransformation reactions cannot retrieve
examples from the textbooks. Therefore, in order to predict the unknown
metabolites with altering toxicological profiles, there is a realistic
need to develop efficient methods to reveal the “unusual”
metabolic mechanism of emerging pollutants. Combining experimental
work with computational predictions has been widely accepted as an
effective approach in studying complex metabolic reactions; however,
the full quantum chemical computations may not be easily accessible
for most environmentalists. Alternatively, this work practiced using
the concepts from physical organic chemistry for studying the interrelationships
between structure and reactivity of organic molecules, to reveal the
“unusual” metabolic mechanism of synthetic phenolic
antioxidants catalyzed by CYP450, for which the simple pencil-and-paper
and property-computation methods based on physical organic chemistry
were performed. The phenol-coupling product of butylated hydroxyanisole
(BHA) (based on spin aromatic delocalization) and ipso-addition quinol metabolite of butylated hydroxytoluene (BHT) (based
on hyperconjugative effect) were predicted as two “unusual”
metabolites, which were further confirmed by our in vitro analysis. We hope this easily handled approach will promote environmentalists
to attach importance to physical organic chemistry, with an eye to
being able to use the knowledge gained to efficiently predict the
fates of substantial unknown synthesized organic compounds in the
future.