Rapid Halogen Substitution and Dibenzodioxin Formation during Tyrosinase-Catalyzed Oxidation of 4-Halocatechols

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Radiofluorinated N-Octanoyl Dopamine ([18F]F-NOD) as a Tool To Study Tissue Distribution and Elimination of NOD in Vitro and in Vivo

To mitigate pretransplantation injury in organs of potential donors, N-octanoyl dopamine (NOD) treatment might be considered as it does not affect hemodynamic parameters in braindead (BD) donors. To better assess optimal NOD concentrations for donor treatment, we report on the fast and facile radiofluorination of the NOD-derivative [¹⁸F]F-NOD [¹⁸F]5 for in vivo assessment of NOD's elimination kinetics by means of PET imaging. [¹⁸F]5 was synthesized in reproducibly high radiochemical yields and purity (>98%) as well as high specific activities (>20 GBq/μmol). Stability tests showed no decomposition of [¹⁸F]5 over a period of 120 min in rat plasma. In vitro, low cell association was found for [¹⁸F]5, indicating no active transport mechanism into cells. In vivo, [¹⁸F]5 exhibited a fast blood clearance and a predominant hepatobiliary elimination. As these data suggest that also NOD might be cleared fast, further pharmacokinetic evaluation is warranted.