Trapping of transiently formed quinone methide during enzymatic conversion of N-acetyldopamine to N-acetylnorepinephrine

We have demonstrated that quinone methide formation is an important aspect of insect physiology and proposed that enzymatically generated quinone methides react nonenzymatically with water or other nucleophiles to form Michael-1,6-addition products [(1988) Adv. Insect Physiol. 21, 179-231; (1989) J. Cell. Biochem. suppl. 13C, 58]. Using a purified o-quinone isomerase from the larval cuticle of Sacrophaga bullata and mushroom tyrosinase, we now demonstrate that transiently formed N-acetyldopamine quinone methide from N-acetyldopamine can be trapped by methanol to produce beta-methoxy N-acetyldopamine. The methanol adduct thus formed was found to be a racemic mixture and can be resolved into the optical isomers on cyclodextrin chiral column. These results confirm our contention that enzymatically generated quinone methides are nonenzymatically and nonstereoselectively transformed to Michael-1,6-adducts by reaction with water or other nucleophiles.