Differential mechanism of oxidation of N‐acetyldopamine and N‐acetylnorepinephrine by cuticular phenoloxidase from Sarcophaga bullata

Abstract

AbstractThe mechanism of oxidation of two related sclerotizing precursors—N‐acetyldopamine and N‐acetylnorepinephrine—by the cuticular phenoloxidase from Sarcophaga bullata was studied and compared with mushroom tyrosinase‐mediated oxidation. While the fungal enzyme readily generated the quinone products from both of these catecholamine derivatives, sarcophagid enzyme converted N‐acetyldopamine to a quinone methide derivative, which was subsequently bound to the cuticle with the regeneration of o‐dihydroxy phenolic function as outlined in an earlier publication [Sugumaran: Arch Insect Biochem Physiol, 8, 73 (1988)]. However, it converted N‐acetylnorepinephrine to its quinone and not to the quinone methide derivative. Proteolytic digests of N‐acetyldopamine‐treated cuticle liberated peptides that had covalently bound catechols, while N‐acetylnorepinephrine‐treated cuticle did not release such peptides. Acid hydrolysis of N‐acetyldopamine‐treated cuticle, but not N‐acetylnorepinephrine‐treated cuticle liberated 2‐hydroxy‐3′,4′‐dihydroxyacetophenone and arterenone. These results further confirm the unique conversion of N‐acetyldopamine to its corresponding quinone methide derivative and N‐acetylnorepinephrine to its quinone derivative by the cuticular phen‐oloxidase. Significance of this differential mechanism of oxidation for sclerotization of insect cuticle is discussed.