Characterization of a new enzyme system that desaturates the side chain of N‐acetyldopamine

A novel enzyme system that desaturates the side chain of the catecholamine derivative, N-acetyldopamine (NADA), was isolated and characterized from the larval cuticle of Sarcophaga bullata. The NADA desaturase system which converts NADA to 1,2-dehydro-NADA, surprisingly, does not resemble dehydrogenases such as succinate dehydrogenase. It uniquely performs the desaturation reaction by oxidizing NADA to its corresponding quinone and subsequently converting the resultant quinone to 1,2-dehydro-NADA via NADA quinone methide. Accordingly, desaturase enzyme preparation contained both o-diphenoloxidase activity and NADA quinone:NADA quinone methide isomerase activity. In addition, inhibition studies as well as trapping experiments also confirmed the obligatory formation of NADA quinone as the transient intermediate of the NADA desaturation. It is the first report of a cell-free system causing the side chain desaturation of any catecholamine derivative.