Trapping of transiently formed quinone methide during enzymatic conversion of N-acetyldopamine to N-acetylnorepinephrine

We have demonstrated that quinone methide formation is an important aspect of insect physiology and proposed that enzymatically generated quinone methides react nonenzymatically with water or other nucleophiles to form Michael-1,6-addition products [(1988) Adv. Insect Physiol. 21, 179-231; (1989) J. Cell. Biochem. suppl. 13C, 58]. Using a purified o-quinone isomerase from the larval cuticle of Sacrophaga bullata and mushroom tyrosinase, we now demonstrate that transiently formed N-acetyldopamine quinone methide from N-acetyldopamine can be trapped by methanol to produce beta-methoxy N-acetyldopamine. The methanol adduct thus formed was found to be a racemic mixture and can be resolved into the optical isomers on cyclodextrin chiral column. These results confirm our contention that enzymatically generated quinone methides are nonenzymatically and nonstereoselectively transformed to Michael-1,6-adducts by reaction with water or other nucleophiles.     

On the mechanism of side chain oxidation of N-beta-alanyldopamine by cuticular enzymes from Sarcophaga bullata

The metabolism of N-beta-alanyldopamine (NBAD) by Sarcophaga bullata was investigated. Incubation of NBAD with larval cuticular preparations resulted in the covalent bindings of NBAD to the cuticle and generation of N-beta-alanyl-norepinephrine (NBANE) as the soluble product. When the reaction was carried out in presence of a powerful quinone trap viz., N-acetylcysteine, NBANE formation was totally abolished; but a new compound characterized as NBAD-quinone-N-acetylcysteine adduct was generated. These results indicate that NBAD quinone is an obligatory intermediate for the biosynthesis of NBANE in sarcophagid cuticle. Accordingly, phenylthiourea--a well-known phenoloxidase inhibitor--completely inhibited the NBANE production even at 5 microM level. A soluble enzyme isolated from cuticle converted exogenously supplied NBAD quinone to NBANE. Chemical considerations indicated that the enzyme is an isomerase and is converting NBAD quinone to its quinone methide which was rapidly and nonenzymatically hydrated to form NBANE. Consistent with this hypothesis is the finding that NBAD quinone methide can be trapped as beta-methoxy NBAD by performing the enzymatic reaction in 10% methanol. Moreover, when the reaction was carried out in presence of kynurenine, two diastereoisomeric structures of papiliochrome II-(Nar-[alpha-3-aminopropionyl amino methyl-3,4-dihydroxybenzyl]-L-kynurenine) could be isolated as by-products, indicating that the further reactions of NBAD quinone methide with exogenously added nucleophiles are nonenzymatic and nonstereoselective. Based on these results, it is concluded that NBAD is metabolized via NBAD quinone and NBAD quinone methide by the action of phenoloxidase and quinone isomerase respectively. The resultant NBAD quinone methide, being highly reactive, undergoes nonenzymatic and nonstereoselective Michael-1,6-addition reaction with either water (to form NBANE) or other nucleophiles in cuticle to account for the proposed quinone methide sclerotization.     

Quinone methides—and not dehydrodopamine derivatives—as reactive intermediates of β-sclerotization in the puparia of flesh fly Sarcophaga bullata

Cuticular phenoloxidase(s) from Sarcophaga bullata larvae oxidized a variety of o-diphenolic compounds. While catechol, 3,4-dihydroxybenzoic acid, dopa, dopamine, and norepinephrine were converted to their corresponding quinone derivatives, other catechols such as 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenethyl alcohol, 3,4-dihydroxyphenyl glycol, 3,4-dihy-droxymandelic acid, and N-acetyldopamine were oxidized to their side-chain oxygenated products. In addition, the enzyme-catalyzed oxidation of the latter group of compounds accompanied the formation of colorless catecholcuticle adducts consistent with the operation of β-sclerotization. Radioactive trapping experiments failed to support the participation of 1,2-dehydro-N-acetyldopamine as a freely formed intermediate during phenoloxidase-mediated oxidation of N-acetyldopamine. When specifically tritiated substrates were provided, cuticular enzyme selectively removed tritium from [7-³H]N-acetyldopamine and not from either [8-³H] or [ring-³H]N-acetyldopamine during the initial phase of oxidation. The above results are consistent with the generation and subsequent reactions of quinone methides as the initial products of enzyme-catalyzed N-acetyldopamine oxidation and confirm our hypothesis that quinone methides and not 1,2-dehydro-N-acetyldopamine are the reactive intermediate of β-sclerotization of sarcophagid cuticle. Quinone methide sclerotization resolves a number of conflicting observations made by previous workers in this field.     

Quinone methide sclerotization: A revised mechanism for β-sclerotization of insect cuticle

Molecular mechanisms responsible for the stiffening and tanning of insect cuticle are reviewed. Two mechanisms, viz., quinone tanning and β-sclerotization, both involving catecholamine derivatives as sclerotizing precursors, are known to strengthen the cuticle. Quinone tanning mechanism invokes the generation and reactions of o-benzoquinones as the sclerotizing agents, whereas β-sclerotization dictates the activation of catecholamine side chains prior to their incorporation into cuticle. The reactive intermediate for the latter process was proposed by other workers to be 1,2-dehydro-N-acetyldopamine and its quinone. The role of these two compounds in β-sclerotization is critically evaluated. Based on our observation that incubation of cuticular enzyme from Sarcophaga bullata with 4-alkylcatechols results in the production of soluble side chain oxygenated compounds and the formation of catechol-cuticle adducts, an alternate mechanism for β-sclerotization is proposed. This mechanism calls for the generation of quinone methides, tautomers of 4-alkyl-quinones, as the initial products of oxidation of catecholamine derivatives in cuticle. Quinone methides formed spontaneously react with available nucleophiles in cuticle, resulting in the generation of catechol-cuticle adducts and side chain hydroxylated products. Further oxidation of adducts and coupling to other structural units ensure crosslinking of cuticular components. The proposed quinone methide sclerotization accounts for all the chemical observations made on the β-sclerotized cuticle.     

On the mechanism of formation of N-acetyldopamine quinone methide in insect cuticle

The mechanism of formation of quinone methide from the sclerotizing precursor N-acetyldopamine (NADA) was studied using three different cuticular enzyme systems viz. Sarcophaga bullata larval cuticle, Manduca sexta pharate pupae, and Periplaneta americana presclerotized adult cuticle. All three cuticular samples readily oxidized NADA. During the enzyme-catalyzed oxidation, the majority of NADA oxidized became bound covalently to the cuticle through the side chain with the retention of o-diphenolic function, while a minor amount was recovered as N-acetylnorepinephrine (NANE). Cuticle treated with NADA readily released 2-hydroxy-3′,4′-dihydroxyacetophenone on mild acid hydrolysis confirming the operation of quinone methide sclerotization. Attempts to demonstrate the direct formation of NADA-quinone methide by trapping experiments with N-acetylcysteine surprisingly yielded NADA-quinone-N-acetylcysteine adduct rather than the expected NADA-quinone methide-N-acetylcysteine adduct. These results are indicative of NADA oxidation to NADA-quinone and its subsequent isomerization to NADA-quinone methide. Accordingly, all three cuticular samples exhibited the presence of an isomerase, which catalyzed the conversion of NADA-quinone to NADA-quinone methide as evidenced by the formation of NANE—the water adduct of quinone methide. Thus, in association with phenoloxidase, newly discovered quinone methide isomerase seems to generate quinone methides and provide them for quinone methide sclerotization.     

Reexamination of the mechanisms of oxidative transformation of the insect cuticular sclerotizing precursor, 1,2-dehydro-N-acetyldopamine

1,2-dehydro-N-acetyldopamine (dehydro NADA) is an important catecholamine derivative formed during the sclerotization of insect cuticle. Earlier we have reported that tyrosinase-catalyzed oxidation of dehydro NADA produces a reactive quinone methide imine amide that forms adducts and cross-links through its side chain, thereby accounting for sclerotization reactions. Recently, laccase has also been identified as a key enzyme associated with sclerotization. Hence, we re-examined oxidation of dehydro NADA by tyrosinase and laccase using high performance liquid chromatography – tandem mass spectrometry. Tyrosinase-catalyzed oxidation of dehydro NADA not only generated dimers as reported earlier, but also generated significant amounts of oligomers. The course of laccase-catalyzed oxidation of dehydro NADA significantly differed from the tyrosinase reaction kinetically and mechanistically. Laccase failed to produce any detectable quinone or quinone methide as the primary two-electron oxidation product. Since laccases are known to generate primarily semiquinones as the initial products, lack of accumulation of two-electron oxidation products indicated that laccase reaction is primarily occurring via free radical coupling mechanism. Consistent with this proposal, laccase-catalyzed oxidation of dehydro NADA, resulted in the production of largely dimeric products and failed to produce any significant amount of oligomeric materials. These studies call for radical coupling as yet another major mechanism for sclerotization of insect cuticle.     

Mechanism of activation of 1,2-dehydro-N-acetyldopamine for cuticular sclerotization

The mechanism of oxidation of 1,2-dehydro-N-acetyldopamine (dehydro NADA) was examined to resolve the controversy between our group and Andersen's group regarding the reactive species involved in β-sclerotization. While Andersen has indicated that dehydro NADA quinone is the β-sclerotizing agent [Andersen, 1989], we have proposed quinone methides as the reactive species for this process [Sugumaran, 1987; Sugumaran, 1988]. Since dehydro NADA quinone has not been isolated or identified till to date, we studied the enzymatic oxidation of dehydro NADA in the presence of quinone traps to characterize this intermediate. Accordingly, both N-acetylcysteine and o-phenylenediamine readily trapped the transiently formed dehydro NADA quinone as quinone adducts. Interestingly, when the enzymatic oxidation was performed in the presence of o-aminophenol or different catechols, adduct formation between the dehydro NADA side chain and the additives had occurred. The structure of the adducts is in conformity with the generation and reactions of dehydro NADA quinone methide (or its radical). This, coupled with the fact that 4-hydroxyl or amino-substituted quinones instantly transformed into p-quinonoid structure, indicates that dehydro NADA quinone is only a transient intermediate and that it is the dehydro NADA quinone methide that is the thermodynamically stable product. However, since this compound is chemically more reactive due to the presence of both quinone methide and acylimine structure on it, the two side chain carbon atoms are “activated.” Based on these considerations, it is suggested that the quinone methide derived from dehydro NADA is the reactive species responsible for cross-link formation between dehydro NADA and cuticular components during β-sclerotization.     

Nonenzymatic transformations of enzymatically generated N-acetyldopamine quinone and isomeric dihydrocaffeiyl methyl amide quinone

We have recently demonstrated that the side chain hydroxylation of N-acetyldopamine and related compounds observed in several insects is caused by a two-enzyme system catalyzing the initial oxidation of catecholamine derivatives and subsequent isomerization of the resultant quinones to isomeric quinone methides, which undergo rapid nonenzymatic hydration to yield the observed products [Saul, S.J. and Sugumaran, M. (1989) FEBS Lett. 249, 155-158]. During our studies on o-quinone/p-quinone methide tautomerase, we observed that quinone methides are also produced nonenzymatically slowly, under physiological conditions. The quinone methide derived from N-acetyldopamine was hydrated to yield N-acetylnorepinephrine as the stable product as originally shown by Senoh and Witkop [(1959) J. Am. Chem. Soc. 81, 6222-6231], while the isomeric quinone methide from dihydrocaffeiyl methylamide exhibited a new reaction to form caffeiyl amide as the stable product. The identity of this product was established by UV and IR spectral studies and by chemical synthesis. We could not find any evidence of intramolecular cyclization of N-acetyldopamine quinone to iminochrome-type compound(s). The importance of quinone methides in these reactions is discussed.     

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

The 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.     

Further studies on the mechanism of oxidation of N-acetyldopamine by the cuticular enzymes from Sarcophaga bullata and other insects

In accordance with our earlier results, quinone methide formation was confirmed to be the major pathway for the oxidation of N-acetyldopamine (NADA) by cuticle-bound enzymes from Sarcophaga bullata larvae. In addition, with the use of a newly developed HPLC separation condition and cuticle prepared by gentle procedures, it could be demonstrated that 1, 2-dehydro-NADA and its dimeric oxidation products are also generated in the reaction mixture containing a high concentration of NADA albeit at a much lower amount than the NADA quinone methide water adduct, viz., N-acetylnorepinephrine (NANE). By using different buffers, it was also possible to establish the accumulation of NADA quinone in reaction mixtures containing NADA and cuticle. That the 1,2-dehydro-NADA formation is due to the action of a NADA desaturase system was established by pH and temperature studies and by differential inhibition of NANE production. Of the various cuticle examined, adult cuticle of Locusta migratoria, presclerotized cuticle of Periplaneta americana, and white puparial cases of Drosophila melanogaster exhibited more NADA desaturase activity than NANE generating activity, while the reverse was observed with the larval cuticle of Tenebrio molitor and pharate pupal cuticle of Manduca sexta. These studies indicate that both NADA quinone methide and 1, 2-dehydro NADA are formed during enzymatic activation of NADA in insect cuticle. Based on these results, a unified mechanism for β-sclerotization involving quinone methides as the reactive species is presented.     

Oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde: electrochemical and HPLC analysis of the reaction mechanism

Cyclic voltammetric and chronoamperometric data are consistent with a process in which 3,4-dihydroxymandelic acid (DOMA) is oxidized initially in a two-electron step to its corresponding o-benzoquinone. This species is unstable and undergoes the rate-determining loss of CO2 (k = 1.6 s-1 at pH 6 and 25 degrees C) to give an unobserved p-benzoquinone methide intermediate that rapidly isomerizes to 3,4-dihydroxybenzaldehyde (DOBAL), DOBAL is also electroactive at the applied potential and is oxidized in a two-electron step to 4-formyl-1,2-benzoquinone. Subsequent reactions of 4-formyl-1,2-benzoquinone include the oxidation of unreacted DOMA and the hydration of its aldehyde functional group. Oxidation of DOMA directly to its p-benzoquinone methide apparently does not occur. Derivatives of mandelic acid (e.g., 4-hydroxymandelic acid) that are expected to give only their corresponding p-benzoquinone methides upon oxidation afford redox behavior that differs distinctly from that for DOMA.     

Catechin and hydroxybenzhydrols as models for the environmental photochemistry of tannins and lignins.

The photochemistry of several model plant-derived compounds has been studied in aqueous solution. In particular, the reactions of catechin as a model tannin and methoxy-substituted hydroxybenzhydrols as model lignin functionalities were investigated. Tannins and lignins constitute a significant portion of the humic substances in aquatic systems, which are themselves the main component of dissolved organic matter thought to be responsible for the absorption and attenuation of light in these environments. Catechin (1) was found to undergo a reversible photoisomerization reaction to give epicatechin (2). Such a reaction is an explicit example of a photon absorbing process that enables catechin (1) and its derivatives to act as natural sunscreens by attenuating light energy through non-destructive reactions. The methoxy-substituted hydroxybenzhydrols were found to undergo photosolvolysis reactions via efficient generation of quinone methide intermediates. The intermediate quinone methides were observed to be longer lived, and thus more stable, than previously studied hydroxybenzhydrol derivatives. The meta-hydroxybenzhydrol isomer (5) was found to undergo additional chemistry leading to the production of a ring-closed fluorene from the quinone methide intermediate.     

4-alkyl-o-quinone/2-hydroxy-p-quinone methide isomerase from the larval hemolymph of Sarcophaga bullata. I. Purification and characterization of enzyme-catalyzed reaction

An enzyme which catalyzes the conversion of certain 4-alkyl-o-benzoquinones to 2-hydroxy-p-quinone methides has been purified to apparent homogeneity from the hemolymph of Sarcophaga bullata by employing conventional protein purification techniques. The purified enzyme migrated with an approximate molecular weight of 98,000 on gel filtration chromatography. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it migrated as a single band with a molecular weight of 46,000, indicating that it is made up of two identical subunits. It exhibited a pH optimum of 6.0 and readily converted chemically synthesized as well as enzymatically generated quinones derived from N-acetyldopamine, N-beta-alanyldopamine, and 3,4-dihydroxyphenethyl alcohol to highly unstable 2-hydroxy-p-quinone methides. The quinone methides thus formed were rapidly and nonenzymatically hydrated to form side chain hydroxylated o-diphenols as the stable product. In support of this proposition, when the enzyme reaction with N-acetyldopamine quinone was conducted in the presence of 10% methanol, racemic beta-methoxy-N-acetyldopamine was recovered as an additional product. The quinones of N-acetylnorepinephrine, N-beta-alanylnorepinephrine, and 3,4-dihydroxyphenylglycol were also attacked by the isomerase, resulting in the formation of N-acetylarterenone, N-beta-alanylarterenone and 2-hydroxy-3',4'-dihydroxyacetophenone, respectively as the stable products. The isomerase converted the dihydrocaffeiyl methyl amide quinone to its quinone methide analog which rapidly tautomerized to yield caffeiyl methyl amide. The importance of quinone isomerase in insect immunity and sclerotization of insect cuticle is discussed.     

o-quinone/quinone methide isomerase: a novel enzyme preventing the destruction of self-matter by phenoloxidase-generated quinones during immune response in insects

Melanization and encapsulation of invading foreign organisms observed during the immune response in insects is known to be due to the action of activated phenoloxidase. Phenoloxidase-generated quinones are deposited either directly or after self-polymerization on foreign objects accounting for the observed reactions. Since the reactions of quinones are nonenzymatic, they do not discriminate self from nonself and hence will also destroy self-matter. In this report we present evidence for the presence of a novel quinone/quinone methide isomerase in the hemolymph of Sarcophaga bullata which destroys long-lived quinones and hence acts to protect the self-matter. Quinone methides, formed by the action of this enzyme on physiologically important quinones, being unstable undergo rapid hydration to form nontoxic metabolites.     

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

The 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.     

Studies on the enzymes involved in puparial cuticle sclerotization in Drosophila melanogaster.

The properties of cuticular enzymes involved in sclerotization of Drosophila melanogaster puparium were examined. The cuticle-bound phenoloxidase from the white puparium exhibited a pH optimum of 6.5 in phosphate buffer and oxidized a variety of catecholic substrates such as 4-methylcatechol, N-beta-alanyldopamine, dopa, dopamine, N-acetyldopamine, catechol, norepinephrine, 3,4-dihydroxyphenylglycol, 3,4-dihydroxybenzoic acid, and 3,4-dihydroxyphenylacetic acid. Phenoloxidase inhibitors such as potassium cyanide and sodium fluoride inhibited the enzyme activity drastically, but phenylthiourea showed marginal inhibition only. This result, coupled with the fact that syringaldazine served as the substrate for the insoluble enzyme, confirmed that cuticular phenoloxidase is of the "laccase" type. In addition, we also examined the mode of synthesis of the sclerotizing precursor, 1,2-dehydro-N-acetyldopamine. Our results indicate that this catecholamine derivative is biosynthesized from N-acetyldopamine through the intermediate formation of N-acetyldopamine quinone and N-acetyldopamine quinone methide as established for Sarcophaga bullata [Saul, S. and Sugumaran, M., F.E.B.S. Letters 251, 69-73 (1989)]. Accordingly, successful solubilization and fractionation of cuticular enzymes involved in the introduction of a double bond in the side chain of N-acetyldopamine indicated that they included o-diphenoloxidase, 4-alkyl-o-quinone:p-quinone methide isomerase, and N-acetyldopamine quinone methide:dehydro N-acetyldopamine isomerase and not any side chain desaturase.     

Active-site-directed reductive alkylation of xanthine oxidase by imidazo[4,5-g]quinazoline-4,9-diones functionalized with a leaving group

A new class of purine antimetabolites, directed toward xanthine oxidase, was designed by employing some of the features found in the bioreductive alkylator mitomycin C. The design involved functionalizing the purine-like imidazo[4,5-g]quinazoline ring system as a quinone (4,9-dione) bearing a 2 alpha leaving group. Due to the presence of the electron-deficient quinone ring, the leaving group cannot participate in alkylation reactions. Reduction to the hydroquinone (4,9-dihydroxy) derivative, however, permits elimination of the leaving group to afford an alkylating quinone methide. In spite of the electronic differences, both quinone and hydroquinone derivatives of the imidazo[4,5-g]quinazoline system are able to enter the purine-utilizing active site of the enzyme. Thus, the hypoxanthine-like quinone derivative [2-(bromomethyl)-3-methylimidazo[4,5-g]quinazoline-4,8, 9(3H, 7H)-trione] and its hydroquinone derivative can act as reducing substrates for the enzyme, resulting in conversion to the xanthane-like 6-oxo derivatives. Hydrolysis studies described herein indicate that the hypoxanthine-like hydroquinone derivative eliminates HBr to afford an extended quinone methide species. The observed alkylation of the enzyme by this derivative may thus pertain to quinone methide generation and nucleophile trapping during enzymatic oxidation at the 6-position. Enzymatic studies indicate that the hypoxanthine-like quinone is an oxidizing suicide substrate for the enzyme. Thus, the reduced enzyme transfers electrons to this quinone, and the resulting hydroquinone inactivates the enzyme. As with mitomycin C, reduction and quinone methide formation are necessary for alkylation by the title quinone. This system is therefore an example of a purine active-site-directed reductive alkylator.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oxidation chemistry of 1,2-dehydro-N-acetyldopamines: direct evidence for the formation of 1,2-dehydro-N-acetyldopamine quinone

Two-electron oxidation of catecholamines either by phenol oxidase or by chemical oxidants such as sodium periodate produces their corresponding o-quinones as observable products. But, in the case of 1,2-dehydro-N-acetyldopamine, an important insect cuticular sclerotizing precursor, phenol oxidase catalyzed oxidation has been reported to generate a quinone methide analog as a transient, but first observable product. ?Sugumaran, M., Semensi, V., Kalyanaraman, B., Bruce, J. M., and Land, E. J. (1992) J. Biol. Chem. 267, 10355-10361. The corresponding quinone has escaped detection until now. However, in this paper, for the first time, we present direct evidence for the formation of dehydro-N-acetyldopamine quinone and show that it can readily be produced from the tautomeric quinone methide imine amide during the chemical oxidation of dehydro-N-acetyldopamine under acidic conditions. This situation is in sharp contrast to other known alkyl-substituted catechol oxidations, where quinone is the first observable product and quinone methide is the subsequently generated product. Dehydro-N-acetyldopamine quinone thus formed is also highly unstable. Semiempirical molecular orbital calculation also indicates that quinone methide imine amide is more stable than the quinone. Chemical considerations indicate that the quinone methide tautomer, and not the dehydro-N-acetyldopamine quinone, is responsible for crosslinking the structural proteins and chitin polymer in the insect cuticle. Therefore, the quinone methide tautomer, and not the quinone, is the key reactive intermediate aiding the hardening of insect cuticle.     

alpha,beta-Dehydro-3,4-dihydroxyphenylalanine derivatives: potential schlerotization intermediates in natural composite materials.

Proteins containing the post-translationally modified amino acid L-3,4-dihydroxyphenylalanine (DOPA) undergo autosclerotization as a means of assuring cohesive resilience in many structural matrices found in nature. To explore the chemical mechanism of sclerotization, we examined the oxidation products of relatively simple analogs of a peptidyl DOPA residue, such as N-acetylDOPA ethyl ester and N-acetyldopamide, together with those of several oligopeptides. Oxidation, induced by either of two catecholoxidases or by sodium periodate, resulted in the Lewis base catalyzed formation of derivatives of the unusual amino acid 3,4-dihydroxy-alpha,beta-dehydroDOPA (delta DOPA). The N-acetyl delta DOPA ethyl ester representative of this group of derivatives was characterized by NMR and uv spectroscopy. A variety of peptides developed analogous uv spectra upon oxidation. A similar reaction was observed upon oxidation of 3,4-dihydroxyphenylpropanoic (dihydrocaffeic) acid, but not after oxidation of N-acetyldopamine. Evidence is presented that this conversion is the result of a rearrangement of the DOPA quinone moiety to its delta DOPA tautomer, and that this tautomerization can be a dominant fate for peptidyl DOPA quinone, provided a Lewis base catalyst is available and competing reactions are minimized. Formation of delta DOPA in natural or synthetic polymers would increase the variety of crosslinks available to sclerotizing matrices. delta DOPA has been found in naturally occurring oligopeptides isolated by other workers from several marine species.     

Anthracycline antibiotic reduction by spinach ferredoxin-NADP+ reductase and ferredoxin

Spinach NADPH:ferredoxin oxidoreductase (EC 1.6.7.1) catalyzes the NADPH-dependent reduction of the anthracyclines daunomycin, aclacinomycin A, and nogalamycin and their respective 7-deoxyanthracyclinones. Under anaerobic conditions, the endogenous rate of O2 reduction by NADPH catalyzed by ferredoxin reductase (0.12 s-1 at pH 7.4) is augmented by the anthracyclines and 7-deoxyanthracyclinones. The catalytic constants are approximately equivalent for this augmentation for all substrates (approximate V of 2 s-1 and KM of 75 microM). Both O2- and H2O2 are made. Under anaerobic conditions, anthracycline reduction catalyzed by ferredoxin reductase results in the elimination of the C-7 substituent to provide a quinone methide intermediate. Following tautomerization by C-7 protonation, 7-deoxyanthracyclinones are obtained. Under appropriate conditions these may be further reduced to the 7-deoxyanthracyclinone hydroquinones. For daunomycin, the quinone methide is formed rapidly after reduction and is easily monitored at 600 nm. It may react with electrophiles other than H+, as demonstrated by its competitive trapping by p-carboxybenzaldehyde. It may also react with nucleophiles, as demonstrated by its competitive trapping by N-acetylcysteine. For aclacinomycin, quinone methide formation is also rapid although no distinct transient near 600 nm occurs. In addition to protonation, it reacts with itself providing the 7,7'-dimer. With ethyl xanthate as a thiolate nucleophile, adducts derived from addition to C-7 are obtained. For nogalamycin, quinone methide formation is not rapid. Nogalamycin is reduced to its hydroquinone, which slowly converts in a first-order process [k = (1.2 +/- 0.2) X 10(-3) s-1, pH 8.0, 30 degrees C] to the quinone methide, which is then quenched by protonation. Spinach ferredoxin in its reduced form is chemically competent for anthracycline reduction. Its effect on both the aerobic and anaerobic reactions catalyzed by ferredoxin reductase is to increase severalfold the overall velocity for anthracycline reduction. In conclusion, the aerobic reaction pathways for the anthracyclines as mediated by ferredoxin reductase are remarkably similar, while the anaerobic reactions are remarkably different. If these anthracyclines exert their antitumor activity by a common anaerobic pathway, it is most likely that the pathway is determined by the properties of the anthracycline as complexed to its in vivo target. The behavior of ferredoxin further suggests that not only low-potential flavin centers but also iron-sulfur centers should be regarded as important loci for anthracycline reductive activation.