Investigation of an Ortho-quinone isomerase from larval cuticle of the American silkmoth,Hyalophora cecropia

Insect cuticles catalyze the formation of N-acetylnorepinephrine (NANE) and N-β-alanylnorepinephrine (NBANE) from N-acetyldopamine (NADA) and N-β-alanyldopamine (NBAD), respectively. An enzyme, involved in the reaction, has now been isolated from fifth stage larval cuticle of Hyalophora cecropia and partially characterized. The enzyme alone has hardly any activity towards NADA, but together with diphenoloxidases [catechol oxidases (EC 1.10.3.1) or laccases (EC 1.10.3.2)] it will produce NANE as the main product from NADA, indicating that NADA-quinone is the actual substrate for the enzyme. The enzyme is presumably an ortho-quinone para-quinone methide isomerase, and formation of NANE is due to non-enzymatic addition of water to the quinone methide. The enzyme combination mushroom tyrosinase-cuticular isomerase has pH optimum at 5.5, and the optimal substrate concentration is about 10 mM NADA.Together with the endogenous cuticular diphenoloxidases the isomerase can account for the formation of NANE observed when pieces of intact cuticle are incubated with NADA, and for the presence of NANE and NBANE in sclerotized cuticle.The possible roles of the enzyme in sclerotization and defense reactions in insects are briefly discussed.