Reactivities of Quinone Methides versus o-Quinones in Catecholamine Metabolism and Eumelanin Biosynthesis

Abstract

Melanin is an important biopolymeric pigment produced in a vast majority of organisms. Tyrosine and its hydroxylated product, dopa, form the starting material for melanin biosynthesis. Earlier studies by Raper and Mason resulted in the identification of dopachrome and dihydroxyindoles as important intermediates and paved way for the establishment of well-known Raper–Mason pathway for the biogenesis of brown to black eumelanins. Tyrosinase catalyzes the oxidation of tyrosine as well as dopa to dopaquinone. Dopaquinone thus formed, undergoes intramolecular cyclization to form leucochrome, which is further oxidized to dopachrome. Dopachrome is either converted into 5,6-dihydroxyindole by decarboxylative aromatization or isomerized into 5,6-dihydroxyindole-2-carboxylic acid. Oxidative polymerization of these two dihydroxyindoles eventually produces eumelanin pigments via melanochrome. While the role of quinones in the biosynthetic pathway is very well acknowledged, that of isomeric quinone methides, however, remained marginalized. This review article summarizes the key role of quinone methides during the oxidative transformation of a vast array of catecholamine derivatives and brings out the importance of these transient reactive species during the melanogenic process. In addition, possible reactions of quinone methides at various stages of melanogenesis are discussed.

Synthesis and physiological implications of melanic pigments (Review)

“…Necessary for the synthesis of the catecholamines, the tyrosine amino-acid and its hydroxylated DOPA product, represent the starting point of the biosynthesis of the melanic pigments. Raper and Mason, between 1930 and 1940, were the first to establish the biosynthesis path of eumelanin (43). Melanin is formed following the cascading reactions, enzymatically catalyzed.…”

Insect cuticular melanins are distinctly different from those of mammalian epidermal melanins

“…Insects do not seem to possess DCT that will generate DHICA (Sugumaran, Duggaraju, Generozova, & Ito, 1999). But use DCDT and produce DHI as the sole product of dopachrome conversion (Sugumaran, 2002(Sugumaran, , 2016Sugumaran & Barek, 2016). Therefore, insects make more of DHI melanin and less of DHICA melanin.…”

Quick screening of true tyrosinase inhibitors from natural products using tyrosinase‐immobilized magnetic nanoparticles and a magnetic microplate

“…The enzyme catalyzes two distinct reactions in melanin biosynthesis: the hydroxylation of l ‐tyrosinase to l ‐3,4‐dihydroxyphenylalanine ( l ‐DOPA) and its subsequent oxidation to dopaquinone. Quinone is a highly reactive compound and undergoes a series of nonenzymatic and enzymatic reactions to form melanin . Although melanin is principally responsible for skin color and plays a protective role against skin photocarcinogenesis, abnormal melanin pigmentation is associated with dermatological conditions, such as freckles, melasma, and senile lentigines .…”

“…Quinone is a highly reactive compound and undergoes a series of nonenzymatic and enzymatic reactions to form melanin. [3,4] Although melanin is principally responsible for skin color and plays a protective role against skin photocarcinogenesis, abnormal melanin pigmentation is associated with dermatological conditions, such as freckles, melasma, and senile lentigines. [5] Tyrosinase is also responsible for the undesired enzymatic browning of fruits that occurs during senescence or following damage incurred during postharvest handling and processing.…”