Characterization of quinone tautomerase activity in the hemolymph of Sarcophaga bullata larvae

The hemolymph of Sarcophaga bullata larvae was activated with either zymosan or proteolytic enzymes such as chymotrypsin or subtilisin and assayed for phenoloxidase activity by two different assays. While oxygen uptake studies readily attested to the wide specificty of activated phenoloxidase, visible spectral studies failed to confirm the accumulation of quinone products in the case of 4‐alkyl substituted catechols such as N‐acetyldopamine and N‐β‐alanyldopamine. Sepharose 6B column chromatography of the activated hemolymph resolved phenoloxidase activity into two fractions, designated as A and B. Peak A possessed typical o ‐diphenoloxidase ( o ‐diphenol, oxygen oxidoreductase EC 1.10.3.1) activity, while peak B oxidized physiologically important catecholamine derivatives such as N‐acetyldopamine, N‐acetylnorepinephrine, and N‐β‐alanyldopamine into N‐acetylnorepinephrine, N‐acetylarterenone, and N‐β‐alanylnorepinephrine, respectively, and converted 3,4‐dihydroxyphenylacetic acid, 3,4‐dihydroxymandelic acid, and 3,4‐dihydroxyphenylglycol into 3,4‐dihydroxymandelic acid, 3,4‐dihydroxybenzaldehyde, and 2‐hydroxy‐3′,4′‐dihydroxyacetophenone, respectively. These transformations are consistent with the conversion of phenoloxidase‐generated quinones to quinone methides and subsequent non‐enzymatic transformations of quinone methides. Accordingly, Peak B contained both o ‐diphenoloxidase activity and quinone tautomerase activity. Sepharose 6B column chromatography of unactivated hemolymph resulted in the separation of quinone tautomerase from prophenoloxidase. The tautomerase rapidly converted both chemically made and mushroom tyrosinase‐generated quinones to quinone methides. Thus the failure to observe the accumulation of quinones with N‐acyl derivatives of dopamine and related compounds in the whole hemolymph is due to the rapid conversion of these long lived toxic quinones to short lived quinone methides. The latter, being unstable, undergo rapid non‐enzymatic transformations to form side‐chain‐oxygenated products that are non‐toxic. The possible roles of quinone isomerase and its reaction products—quinone methides—as essential components of sclerotization of cuticle and defense reaction of Sarcophaga bullata are discussed.