On the oxidation of 3,4‐dihydroxyphenethyl alcohol and 3,4‐dihydroxyphenyl glycol by cuticular enzyme(s) from Sarcophaga bullata

Abstract

AbstractThe catabolic fate of 3,4‐dihydroxyphenethyl alcohol (DHPA) and 3,4‐dihydroxyphenylethyl glycol (DHPG) in insect cuticle was determined for the first time using cuticular enzyme(s) from Sarcophaga bullata and compared with mushroom tyrosinase‐medicated oxidation. Mushroom tyrosinase converted both DHPA and DHPG to their corresponding quinone derivatives, while cuticular enzyme(s) partly converted DHPA to DHPG. Cuticular enzyme(s)‐mediated oxidation of DHPA also accompanied the covalent binding of DHPA to the cuticle. Cuticle‐DHPA adducts, upon pronase digestion, released peptides that had bound catechols. 3,4‐Dihydroxyphenyl‐acetaldehyde, the expected product of side chain desaturation of DHPA, was not formed at all. The presence of N‐acetylcysteine, a quinone trap, in the reaction mixture containing DHPA and cuticle resulted in the generation of DHPA‐quinone‐N‐acetylcysteine adduct and total inhibition of DHPG formation. The insect enzyme(s) converted DHPG to its quinone at high substrate concentration and to 2‐hydroxy‐3′,4′‐dihydroxyacetophenone at low concentration. They converted exogenously added DHPA‐quinone to DHPG, but acted sluggishly on DHPG‐quinone. These results are consistent with the enzymatic transformations of phenoloxidase‐generated quinones to quinone methides and subsequent nonenzymatic transformation of the latter to the observed products. Thus, quinone methide formation in insect cuticle seems to be caused by the combined action of two enzymes, phenoloxidase and quinone tautomerase, rather than the action of quinone methide‐generating phenoloxidase (Sugumaran: Arch Insect Biochem Physiol 8, 73–88, 1988). It is proposed that DHPA and DHPG in combination can be used effectively to examine the participation of (1) quinone, (2) quinone methide, and (3) dehydro derivative intermediates in the metabolism of 4‐alkylcatechols for cuticular sclerotization.