Widespread ancient whole-genome duplications in Malpighiales coincide with Eocene global climatic upheaval

Whole-genome duplications (WGDs) are widespread and prevalent in vascular plants and frequently coincide with major episodes of global and climatic upheaval, including the mass extinction at the Cretaceous-Tertiary boundary (c. 65 Ma) and during more recent periods of global aridification in the Miocene (c. 10-5 Ma). Here, we explore WGDs in the diverse flowering plant clade Malpighiales. Using transcriptomes and complete genomes from 42 species, we applied a multipronged phylogenomic pipeline to identify, locate, and determine the age of WGDs in Malpighiales using three means of inference: distributions of synonymous substitutions per synonymous site (Ks ) among paralogs, phylogenomic (gene tree) reconciliation, and a likelihood-based gene-count method. We conservatively identify 22 ancient WGDs, widely distributed across Malpighiales subclades. Importantly, these events are clustered around the Eocene-Paleocene transition (c. 54 Ma), during which time the planet was warmer and wetter than any period in the Cenozoic. These results establish that the Eocene Climatic Optimum likely represents a previously unrecognized period of prolific WGDs in plants, and lends further support to the hypothesis that polyploidization promotes adaptation and enhances plant survival during episodes of global change, especially for tropical organisms like Malpighiales, which have tight thermal tolerances.

Massive mitochondrial gene transfer in a parasitic flowering plant clade

Recent studies have suggested that plant genomes have undergone potentially rampant horizontal gene transfer (HGT), especially in the mitochondrial genome. Parasitic plants have provided the strongest evidence of HGT, which appears to be facilitated by the intimate physical association between the parasites and their hosts. A recent phylogenomic study demonstrated that in the holoparasite Rafflesia cantleyi (Rafflesiaceae), whose close relatives possess the world's largest flowers, about 2.1% of nuclear gene transcripts were likely acquired from its obligate host. Here, we used next-generation sequencing to obtain the 38 protein-coding and ribosomal RNA genes common to the mitochondrial genomes of angiosperms from R. cantleyi and five additional species, including two of its closest relatives and two host species. Strikingly, our phylogenetic analyses conservatively indicate that 24%–41% of these gene sequences show evidence of HGT in Rafflesiaceae, depending on the species. Most of these transgenic sequences possess intact reading frames and are actively transcribed, indicating that they are potentially functional. Additionally, some of these transgenes maintain synteny with their donor and recipient lineages, suggesting that native genes have likely been displaced via homologous recombination. Our study is the first to comprehensively assess the magnitude of HGT in plants involving a genome (i.e., mitochondria) and a species interaction (i.e., parasitism) where it has been hypothesized to be potentially rampant. Our results establish for the first time that, although the magnitude of HGT involving nuclear genes is appreciable in these parasitic plants, HGT involving mitochondrial genes is substantially higher. This may represent a more general pattern for other parasitic plant clades and perhaps more broadly for angiosperms.

Recent studies have suggested that plant genomes have undergone potentially rampant horizontal gene transfer (HGT), especially in the mitochondrial genome. Here, using phylogenomic approaches, we demonstrate that as much as ∼40% of the mitochondrial genes in the parasitic plant species Rafflesiaceae are acquired from their hosts via HGT. These transgenes are likely functional in their recipient species and in some cases appear to have displaced native copies in the same genomic location. These results establish for the first time that, although the magnitude of HGT involving nuclear genes is appreciable in parasitic plants, HGT involving mitochondrial genes is substantially higher.

Horizontal transfer of expressed genes in a parasitic flowering plant

BACKGROUND

Recent studies have shown that plant genomes have potentially undergone rampant horizontal gene transfer (HGT). In plant parasitic systems HGT appears to be facilitated by the intimate physical association between the parasite and its host. HGT in these systems has been invoked when a DNA sequence obtained from a parasite is placed phylogenetically very near to its host rather than with its closest relatives. Studies of HGT in parasitic plants have relied largely on the fortuitous discovery of gene phylogenies that indicate HGT, and no broad systematic search for HGT has been undertaken in parasitic systems where it is most expected to occur.

RESULTS

We analyzed the transcriptomes of the holoparasite Rafflesia cantleyi Solms-Laubach and its obligate host Tetrastigma rafflesiae Miq. using phylogenomic approaches. Our analyses show that several dozen actively transcribed genes, most of which appear to be encoded in the nuclear genome, are likely of host origin. We also find that hundreds of vertically inherited genes (VGT) in this parasitic plant exhibit codon usage properties that are more similar to its host than to its closest relatives.

CONCLUSIONS

Our results establish for the first time a substantive number of HGTs in a plant host-parasite system. The elevated rate of unidirectional host-to- parasite gene transfer raises the possibility that HGTs may provide a fitness benefit to Rafflesia for maintaining these genes. Finally, a similar convergence in codon usage of VGTs has been shown in microbes with high HGT rates, which may help to explain the increase of HGTs in these parasitic plants.

Crosslink precursors for the dipteran puparium

During sclerotization of puparial proteins, tyrosine, lysine, and histidine were converted to highly basic aromatic metabolites. Peptides generated from the sclerotized cuticle with N-bromosuccinimide included the basic derivatives among the hydrolysis products. The absorbance maxima of the aromatic metabolites were 25 nm lower than those of the conventional tyrosyl peptides, with phenolic character poorly expressed or absent. Post-translational modification of the structural proteins preceded visual expression of tanning because aromatic conjugates also were present prior to pupariation. These results are consistent with a crosslinking mechanism favoring covalent bonding between protein chains.

Studies on some Pharmacognostic profiles of Pithecell'obium dulce Benth. Leaves (Leguminosae)

The macroscopical characters of the leaves, leaf constants, physico-chemical constants, extractive values, colour, consistency, pH, extractive values with different solvents, micro chemical test, fluorescence characters of liquid extracts and leaf powder after treatment with different chemical reagents under visible and UV light at 254mn, measurement of cell and tissues were studied to fix some pharmacognostical parameters for leaves of Pithecellobium, dulce Benth which will enable the future investigators for identification of the plant. Preliminary phytochemical study on different extracts of the leaves were also performed.

Design, synthesis, and evaluation of novelly substituted benzimidazole compounds as angiotensin II receptor antagonists

5-Nitrobenzimidazole derivatives with varying substituents at 2-position have been designed, synthesized, and evaluated for angiotensin II antagonistic activity. A drug-receptor interaction model has been proposed.

Purification, characterization and molecular cloning of prophenoloxidases from Sarcophaga bullata

Prophenoloxidase (PPO) is a key enzyme associated with both melanin biosynthesis and sclerotization in insects. This enzyme is involved in three physiologically important processes viz., cuticular hardening, defense reactions and wound healing in insects. It was isolated from the larval hemolymph of Sarcophaga bullata and purified by employing ammonium sulfate precipitation, Phenyl Sepharose chromatography, DEAE-Sepharose chromatography, and Sephacryl S-200 column chromatography. The purified enzyme exhibited two closely moving bands on 7.5% SDS-PAGE under denaturing conditions. From the estimates of molecular weight on Sephacryl S-100, TSK-3000 HPLC column and SDS-PAGE, which ranged from 90,000 to 100,000, it was inferred that the enzyme is made up of a single polypeptide chain. Activation of PPO (K(a)=40 microM) was achieved by the cationic detergent, cetyl pyridinium chloride below its critical micellar concentration (0.8 mM) indicating that the detergent molecules are binding specifically to the PPO and causing the activation. Neither anionic, nor nonionic (or zwitterionic) detergents activated the PPO. The active enzyme exhibited wide substrate specificity and marked thermal unstability. Using primers designed to conserved amino acid sequences from known PPOs, we PCR amplified and cloned two PPO genes from the sarcophagid larvae. The clones encoded polypeptides of 685 and 691 amino acids. They contained two distinct copper binding regions and lacked the signal peptide sequence. They showed a high degree of homology to dipteran PPOs. Both contained putative thiol ester site, two proteolytic activation sites and a conserved C-terminal region common to all known PPOs.

A new mechanism for the control of phenoloxidase activity: inhibition and complex formation with quinone isomerase

Insect phenoloxidases participate in three physiologically important processes, viz., cuticular hardening (sclerotization), defense reactions (immune reaction), and wound healing. Arrest or even delay of any of these processes compromises the survival of insects. Since the products of phenoloxidase action, viz., quinones, are cytotoxic, uncontrolled phenoloxidase action is deleterious to the insects. Therefore, the activity of this important enzyme has to be finely controlled. A novel inhibition of insect phenoloxidases, which serves as a new regulatory mechanism for control of its activity, is described. The activity of phenoloxidases isolated from both Sarcophaga bullata and Manduca sexta is drastically inhibited by quinone isomerase (isolated from Calliphora), an enzyme that utilizes the phenoloxidase-generated 4-alkylquinones. In turn, phenoloxidase reciprocated the inhibition of isomerase. By forming a complex and controlling each other's activity, these two enzymes seem to regulate the levels of endogenously quinones. In support of this contention, an endogenous complex consisting of phenoloxidase, quinone isomerase, and quinone methide isomerase was characterized from the insect, Calliphora. This sclerotinogenic complex was isolated and purified by borate extraction of the larval cuticle, ammonium sulfate precipitation, and Sepharose 6B column chromatography. The complex exhibited a molecular mass of about 620-680 kDa, as judged by size-exclusion chromatography on Sepharose 6B and HPLC and did not even enter 3% polyacrylamide gel during electrophoresis. The phenoloxidase activity of the complex exhibited a wide substrate specificity. Incubation of the complex with N-acetyldopamine rapidly generated N-acetylnorepinephrine, dehydro-N-acetyldopamine, and its dimers. In addition, transient accumulation of N-acetyldopamine quinone was also observed. These results confirm the presence of phenoloxidase, quinone isomerase, and quinone methide isomerase in the complex. Attempts to dissociate the complex with even trace amounts of SDS ended in the total loss of quinone isomerase activity. The complex does not seems to be made up of stoichiometric amounts of individual enzymes as the ratio of phenoloxidase to quinone isomerase varied from preparation to preparation. It is proposed that the complex formation between sequential enzymes of sclerotinogenic pathway is advantageous for the organism to effectively channel various reactive intermediates during cuticular hardening.

Insect melanogenesis. III. Metabolon formation in the melanogenic pathway-regulation of phenoloxidase activityy by endogenous dopachrome isomerase (decarboxylating) from Manduca sexta

Tyrosinase initiates melanogenesis in a variety of organisms. The nature of melanin formed is modified subsequently by dopachrome isomerase and other melanogenic proteins. Earlier, we reported the partial purification of dopachrome isomerase (decarboxylating) from the hemolymph of Manduca sexta and demonstrated the generation of a new quinone methide intermediate during melanogenesis (Sugumaran, M., and Semensi, V. (1991) J. Biol. Chem. 266, 6073-6078). In this paper, we report the purification of this enzyme to homogeneity and a novel inhibition mechanism for regulation of phenoloxidase activity. The activity of phenoloxidase isolated from M. sexta was markedly inhibited by purified dopachrome isomerase. In turn, phenoloxidase also reciprocated by inhibiting the isomerase activity. Preformed dopaminechrome did not serve as the substrate for the isomerase; but dopaminechrome that generated in situ by phenoloxidase was readily converted to melanin pigment by the phenoloxidase/isomerase mixture. Furthermore, the isomerase, which has a molecular weight of about 40,000 in native state, exhibited retardation during affinity electrophoresis on sodium dodeyl sulfate (SDS)-polyacrylamide gel electrophoresis gel copolymerized with tyrosinase and migrated with a molecular weight of 50,000, indicating complex formation with phenoloxidase. Electrophoresis of pupal cuticular extract on polyacrylamide gel, followed by activity staining revealed the presence of a protein band carrying both phenoloxidase and isomerase activity. Accordingly, a high-molecular-weight melanogenic complex was isolated from the pharate cuticle of M. sexta. The complex catalyzed the generation of melanochrome from dopa, while the free phenoloxidase produced only dopachrome from the same substrate. When the complex was treated with trace amounts of SDS, which inhibited the activity of dopachrome isomerase present in the complex, then only the conversion of dopa to dopachrome was observed. These studies confirm the formation of a melanogenic complex between phenoloxidase and dopachrome isomerase. By forming a complex and regulating each other's activity, these two enzymes seem to control the levels of endogenous quinones.

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.

Characterization of a new phenoloxidase inhibitor from the cuticle of Manduca sexta

Melanin, the phenolic biopolymer that serves as a skin- and hair pigment-protecting agent against harmful solar radiation and a free radical trap, is biosynthesized in animals mainly by the action of tyrosinase also known as phenoloxidase. Regulation of tyrosinase and hence melanogenesis is vital for all animals. In this report, we present the isolation and characterization of a new, heat-labile glycoprotein inhibitor of phenoloxidase from the larvae of Manduca sexta. The inhibitor was isolated from the live larval cuticle by buffer extraction and purified to homogeneity employing ammonium sulfate precipitation, dialysis, and concanavalin A-Sepharose chromatography. It migrated with a molecular weight of 380,000 on SDS-PAGE gels and inhibited the activity of insect and plant as well as fungal phenoloxidases. Inhibitor formed a tight complex with phenoloxidases, which resisted dissociation even by 1% Triton X-100 or SDS. Selective inhibition of phenoloxidase, while acting on certain but not all different substrates, was observed. The physiological importance of this newly discovered high-molecular-weight phenoloxidase inhibitor is discussed.

Formation of a new quinone methide intermediate during the oxidative transformation of 3,4-dihydroxyphenylacetic acids: implication for eumelanin biosynthesis

Oxidation of dopa and dopamine requires a net removal six electrons to produce indolequinones, the monomeric precursors of eumelanin pigment. On the other hand, their 6-fluoroderivatives suffer only four-electron oxidation to yield the same products (M. E. Rice, B. Mogaddam, C. R. Creveling, and K. L. Kirk, Anal. Chem. 59, 1534-1536, 1987). Taking advantage of this novel fluorochemistry, we reexamined the oxidative mechanism of 3,4-dihydroxyphenylacetic acid and 6-fluoro-3,4-dihydroxyphenylacetic acid to throw more light on the nature of reactive intermediates formed during the reaction. Enzymatic or chemical oxidation of 3,4-dihydroxyphenylacetic acid generated the transient o-quinone which exhibited rapid intramolecular cyclization and side chain modification to produce 2, 5,6-trihydrobenzofuran and 3,4-dihydroxymandelic acid, respectively. However, when 6-fluoro-3,4-dihydroxyphenylacetic acid was oxidized either by tyrosinase or by sodium periodate, the resultant quinone uniquely exhibited only cyclization coupled with loss of fluoride ion. This clean reaction allowed us to establish the structures of the transient reactive intermediates. Two interconvertable isomeric forms of the product were isolated and characterized from the reaction mixture. If the oxidation was carried out in water, a yellow quinolactone accumulated in the reaction mixture. This compound was instantaneously converted to a purple quinone methide upon addition of a trace amount of sodium phosphate. Passage through a C(18) HPLC column caused the reverse transformation. The structures of these products were established by semiempirical molecular orbital calculations and NMR spectrometry. Comparison of the oxidation mechanisms of melanin precursors, dopa and dopamine, with that of 3,4-dihydroxyphenylacetic acids reveals that a similar quinone methide intermediate is likely to be formed during eumelanin biosynthesis.

Insect melanogenesis. II. Inability of Manduca phenoloxidase to act on 5,6-dihydroxyindole-2-carboxylic acid

Eumelanins in animals are biosynthesized by the combined action of tyrosinase, 3,4-dihydroxyphenylalanine (DOPA)chrome isomerase, and other factors. Two kinds of eumelanins were characterized from mammalian systems; these are 5,6-dihydroxyindole (DHI)-melanin and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin. In insects, melanin biosynthesis is initiated by phenoloxidase and supported by DOPAchrome isomerase (decarboxylating). Based on the facts that DOPA is a poor substrate for insect phenoloxidases and DHI is the sole product of insect DOPAchrome isomerase reaction, it is proposed that insects lack DHICA-melanin. Accordingly, the phenoloxidase isolated from the hemolymph of Manduca sexta failed to oxidize DHICA. Control experiments reveal that mushroom tyrosinase, as well as laccase, which is a contaminant in the commercial preparations of mushroom tyrosinase, are capable of oxidizing DHICA. Neither the whole hemolymph nor the cuticular extracts of M. sexta possessed any detectable oxidase activity towards this substrate. Thus, insects do not seem to produce DHICA-eumelanin. A useful staining procedure to localize DHICA oxidase activity on gels is also presented.

Laccase--and not tyrosinase--is the enzyme responsible for quinone methide production from 2,6-dimethoxy-4-allyl phenol

Tyrosinase, which is known to possess both monophenol monooxygenase activity (EC 1.14.18.1, tyrosine, 3,4-dihydroxyphenylalanine:oxygen oxidoreductase) and o-diphenoloxidase activity (EC 1.10.3.1, o-diphenol:oxygen oxidoreductase), has been shown to exhibit other related activities. Recently, a new reaction, viz., oxidative conversion of 2,6-dimethoxyallyl phenol to its quinone methide, catalyzed by commercial preparations of mushroom tyrosinase was reported (E. S. Krol, and J. L. Bolton, 1997, Chem. Biol. Interact. 104, 11-27). Since the reaction involves an unusual 1,6-oxidation rather than the conventional 1,4-oxidation, we reexamined this reaction more carefully. The o-diphenoloxidase activity and the dimethoxyallyl phenol oxidase activity of mushroom tyrosinase preparations exhibited different mobilities on size-exclusion chromatography on a Sephacryl S-200 column. A similar behavior was also witnessed on preparative isoelectric focusing in a rotofor cell. Different preparations of mushroom tyrosinase possessed varying ratios of these two activities, further confirming that they are due to two different enzymes. Native polyacrylamide gel electrophoresis followed by activity staining of the gel revealed different mobilities for these two activities. The protein band exhibiting dimethoxyallyl phenol oxidase activity could also be stained by syringaldazine, a well-known substrate for laccase (EC 1.10.3.2, p-diphenol:oxygen oxidoreductase). Two insect phenoloxidases, which are known for their wide substrate specificity, failed to oxidize dimethoxyallyl phenol to any detectable extent, thereby confirming that typical o-diphenoloxidases lack the ability to oxidize dimethoxyallyl phenol. On the other hand, laccase, which is known to convert syringaldazine to its quinone methide derivative, readily produced the quinone methide from dimethoxyallyl phenol. It is therefore concluded that laccase, which is present as a contaminant in the commercial preparations of mushroom tyrosinase--and not tyrosinase (o-diphenoloxidase)--is the enzyme responsible for catalyzing the new conversion of dimethoxyallyl phenol to its corresponding quinone methide.

Model sclerotization studies. 4. Generation of N-acetylmethionyl catechol adducts during tyrosinase-catalyzed oxidation of catechols in the presence of N-acetylmethionine

Incubation of catechol with mushroom tyrosinase in the presence of N-acetylmethionine resulted in the generation of an adduct. This product was identified to be N-acetylmethionyl catechol, on the basis of spectral characteristics and well-characterized chemical reaction of o-benzoquinone with N-acetylmethionine. Enzyme-catalyzed oxidation of catechol and the subsequent nonenzymatic addition of the resultant quinone to N-acetylmethionine accounted for the observed reaction. That the reaction is not confined to catechol alone, but is of general occurrence, can be demonstrated by the facile generation of similar adducts in incubation mixtures containing N-acetylmethionine, tyrosinase, and different N-acetylmethionines, such as 4-methylcatechol and N-acetyldopamine. Attempts to duplicate the reaction with insect cuticular phenoloxidases were not successful, as the excess N-acetylmethionine used in the reaction inhibited their activity. Nevertheless, occurrence of this nonenzymatic reactivity. Nevertheless, occurrence of this nonenzymatic reaction between N-acetylmethionine and mushroom tyrosinase-generated quinones indicates that a similar reaction between enzymatically generated quinones in the cuticle with protein-bound methionine moiety is likely to occur during in vivo quinone tanning as well.

Tyrosinase-catalyzed oxidation of 3,4-dihydroxyphenylglycine

The oxidation chemistry of dopa in relation to melanin biosynthesis has been extensively studied. However, the oxidation of its lower homolog viz., 3,4-dihydroxyphenylglycine has not been described. Using 3,4-dimethoxybenzaldehyde as the starting material, the chemical synthesis of 3,4-dihydroxyphenylglycine was accomplished by Strecker's synthesis and demethylation reactions. Tyrosinase readily oxidized this unusual amino acid to the expected quinone. The glycyl-o-benzoquinone thus formed was highly unstable like its higher analog, dopaquinone. However, unlike its counterpart, it failed to exhibit intramolecular cyclization reaction. Rather, glycyl-o-benzoquinone exhibited facile transformation(s) to ultimately generate 3,4-dihydroxybenzaldehyde as an isolatable product. A probable mechanism involving intermediary formation of unstable quinone methide and carbinolamine intermediates is proposed to account for the novel transformation of glycyl-o-benzoquinone to 3,4-dihydroxybenzaldehyde.

Characterization of a defense complex consisting of interleukin 1 and phenol oxidase from the hemolymph of the tobacco hornworm, Manduca sexta

Hemolymph of fifth instar Manduca sexta larvae collected under non-sterile conditions exhibited the presence of a novel high molecular weight protein complex, which was absent from the hemolymph collected aseptically. The high molecular weight complex consisted of, at least prophenol oxidase, phenol oxidase, and an interleukin 1-like molecule, thereby demonstrating the generation of this complex as a consequence of a host defense response. While the native phenol oxidase and the interleukin 1-like molecule possessed molecular weights of about 80,000 and 17,000, respectively, the complex had a molecular weight of about 400,000. Apart from prophenol oxidase, phenol oxidase, and interleukin 1, dopachrome isomerase and other, as of yet unidentified, proteins may be part of the complex as judged by the presence of additional bands observed during SDS-polyacrylamide gel electrophoresis. The significance of the assembly of this defense complex for insect host defense strategies is discussed.

On the presence of prophenoloxidase in the hemolymph of the horseshoe crab, Limulus

Melanization, and hence the participation of phenoloxidase, in defense mechanism of arthropods is well established. However, in the living fossil, horseshoe crab, it has been claimed that the prophenoloxidase system widely found in the hemolymph of most arthropods is absent. On the contrary, we present evidence for the presence of a prophenoloxidase system in the hemolymph of Limulus and a method to study its activation. Activation of prophenoloxidase was achieved by treatment with either the anionic detergent, SDS, or the cationic detergent, cetylpyridinium chloride. The detergents seemed to bind to the proenzyme below their critical micellar concentration and induce conformational changes that cause the activation of prophenoloxidase. In addition, a number of fatty acids and phospholipids also activated the prophenoloxidase. Proteases such as trypsin activated the enzyme only marginally. The approximate molecular weight of the proenzyme was found to be 70,000. Substrate specificity studies, product analysis and inhibition experiments revealed that the Limulus enzyme is a typical o-diphenoloxidase. The possible reasons for the failure to detect the phenoloxidase activity by earlier workers are discussed.

Oxidation of 3,4-dihydroxybenzylamine affords 3,4-dihydroxybenzaldehyde via the quinone methide intermediate

Dopamine and related compounds are known to be toxic to melanoma cells. Some of their toxicity may be related, in part, to the oxidation products generated from them upon their interaction with melanogenic enzymes. In this paper, we present our studies on the oxidation chemistry of 3,4-dihydroxybenzylamine, the lower homolog of dopamine. Mushroom tyrosinase catalyzed oxidation of 3,4-dihydroxybenzylamine rapidly generated the corresponding quinone. However, aminomethyl-o-benzoquinone thus formed did not accumulate in the reaction mixture, but readily transformed to another product that exhibited absorbance maxima at 280 and 310 nm. This compound was identified to be 3,4-dihydroxybenzaldehyde based on its HPLC elution profile, cochromatography with authentic sample and UV spectral properties. Possible mechanism for the formation 3,4-dihydroxybenzaldehyde from 3,4-dihydroxybenzylamine and the nature of cytotoxic quninonoid intermediates formed are discussed.

Detection of dopachrome isomerase on gels and membranes

Dopachrome isomerase, which mediates the conversion of dopachrome to dihydroxyindoles, is an enzyme affiliated with the melanogenic pathway found in most vertebrates and invertebrates. We have recently reported an activity-staining procedure for detecting this enzyme after electrophoresis on tyrosinease-embedded polyacrylamide gels using dopa [K. Nellaiappan, G. Nicklas, and M. Sugumaran, (1994) Anal. Biochem. 220, 122-128]. The usefulness of this method is limited to the availability of good quality of tyrosinase. To overcome the limitations of nonavailability as well as batch to batch variation in the specific activity of commercially available tyrosinase which will affect the staining, we have developed a new stain technique using chemically made dopachrome for the detection of dopachrome isomerase. The new method employs the use of dopachrome generated in situ by the oxidation of dopa with periodate. Dopachrome isomerase initially appears as a bluish-purple (melanochrome) band against an orange-red background. Eventual transformation of melanochrome to dark insoluble melanin polymer produces a black band corresponding to dopachrome isomerase activity. Continuous washing of the gel clears the background. Usefulness of this technique to stain dopachrome isomerase (a) in different organisms and (b) on polyacrylamide gels, agarose gels, and nitrocellulose membranes is demonstrated. In addition, conditions to stain the enzyme on nitrocellulose membranes using mushroom tyrosinase are described. Comparison of the effectiveness of tyrosinase versus periodate staining procedure reveals that the former method is superior to the latter in detecting dopachrome isomerase. The periodate method overcomes the problems associated with the quality of commercially available tyrosinase and its nonavailability in certain regions.

Proenzyme of Manduca sexta phenol oxidase: purification, activation, substrate specificity of the active enzyme, and molecular cloning

Phenol oxidase (PO) was isolated as a proenzyme (pro-phenol oxidase, pro-PO) from the hemolymph of Manduca sexta larvae and purified to homogeneity. Pro-PO exhibits a M(r) of 130,000 on gel filtration and two bands with an apparent M(r) of approximately 100,000 on SDS/PAGE, as well as size-exclusion HPLC. Activation of pro-PO was achieved either by specific proteolysis by a cuticular protease or by the detergent cetylpyridinium chloride at a concentration below the critical micellar concentration. A cDNA clone for M. sexta pro-PO was obtained from a larval hemocyte cDNA library. The clone encodes a polypeptide of approximately 80,000 Da that contains two copper-binding sites and shows high sequence similarity to POs, hemocyanins, and storage proteins of arthropods. The M. Sexta pro-PO, together with other arthropod pro-POs, contains a short stretch of amino acids with sequence similarity to the thiol ester region of alpha-macroglobulins and complement proteins C3 and C4.

Direct evidence for quinone-quinone methide tautomerism during tyrosinase catalyzed oxidation of 4-allylcatechol

The hapatotoxicity of safrole and related compounds has been attributed to the electrophilic compounds generated from the oxidation of these chemicals. In this paper, for the first time using quasi pre-steady state conditions, we present direct evidence for the rapid production of electrophilic allyl o-quinone and its tautomerization to more stable and reactive vinyl p-quinone methide during both the enzyme catalyzed and chemical mediated oxidation of 4-allylcatechol, an in vivo product of safrole metabolism.

Complex formation between mushroom tyrosinase and Manduca dopachrome isomerase

Melanin biosynthesis in animals is initiated by the ubiquitously present tyrosinase and is aided by dopachrome isomerase. We have characterized a novel dopachrome isomerase (decarboxylating) from the hemolymph of Manduca sexta that generates a new quinone methide intermediate during melanogenesis (Sugumaran, M. and Semensi, V. (1991) J. Biol. Chem. 266, 6073-6078). This enzyme has the ability to form a complex with mushroom tyrosinase as judged by a number of physicochemical studies. The isomerase exhibited a marked inhibitory effect on tyrosinase and tyrosinase reciprocated by inhibiting the isomerase. While the isomerase showed no activity toward preformed dopaminechrome, it readily influenced the stability of dopaminechrome generated in situ by tyrosinase. Moreover, mushroom tyrosinase, which lacked specific binding to Concanavalin A Sepharose column, after complexing with the isomerase exhibited binding to this column. The complex formation also affected the pI value as well as mobility on a size exclusion column of these enzymes. Enzymes executing sequential metabolic transformation are known to form complexes called metabolons. Based on these above studies, it is concluded that both the enzymes involved in insect melanogenic pathway--phenoloxidase and dopachrome isomerase--are able to form a metabolon complex.

A caution about the azide inhibition of enzymes associated with electrophilic metabolites

Azide is often used as an inhibitor to detect active site metal ions present in enzymes such as tyrosinases and laccases. Azide is not only a good chelator for metal ions, but also a powerful nucleophile. Consequently, some of the observed inhibition of tyrosinase by azide can be explained by the reaction of enzymatically generated quinones with azide to form azido catechol. In the light of this finding, routine use of inhibition studies with azide to detect the metal ions present at the active site of enzymes generating and/or consuming electrophilic metabolites is discouraged.

Model sclerotization studies. 3. Cuticular enzyme catalyzed oxidation of peptidyl model tyrosine and dopa derivatives

Incubation of N-acetyltyrosine methyl ester with cuticular enzymes, isolated from the wandering stages of Calliphora sp larvae, resulted in the generation of N-acetyldopa methyl ester when the reaction was carried out in the presence of ascorbate which prevented further oxidation of the o-diphenolic product. Enzymatic oxidation of N-acetyldopa methyl ester ultimately generated dehydro N-acetyldopa methyl ester. The identity of enzymatically produced N-acetyldopa methyl ester and dehydro N-acetyldopa methyl ester has been confirmed by comparison of the ultraviolet and infrared spectral and chromatographic properties with those of authentic samples as well as by nuclear magnetic resonance studies. Since N-acetyldopaquinone methyl ester was also converted to dehydro N-acetyldopa methyl ester and tyrosinase was responsible for the oxidation of N-acetyldopa methyl ester, a scheme for the cuticular phenoloxidase catalyzed conversion of N-acetyltyrosine methyl ester to dehydro N-acetyldopa methyl ester involving the intermediary formation of the quinone and the quinone methide is proposed to account for the observed results. The conversion of N-acetyldopa methyl ester to dehydro derivative remarkably resembles the conversion of the sclerotizing precursor, N-acetyldopamine, to dehydro-N-acetyldopamine observed in the insect cuticle. Based on these comparative studies, it is proposed that peptidyl dopa derivatives could also serve as the sclerotizing precursors for the sclerotization of the insect cuticle.

1,2-dehydro-N-beta-alanyldopamine as a new intermediate in insect cuticular sclerotization

Sclerotization of insect cuticle is an extremely important biochemical process for the successful survival of most insects. N-Acetyldopamine and N-beta-alanyldopamine are two widely used sclerotizing precursors. N-Acetyldopamine is converted by phenoloxidase, quinone isomerase, and quinone methide isomerase to generate the reactive intermediates quinone, quinone methide, and quinone methide imine amide for use in quinone tanning, quinone methide sclerotization, and alpha,beta-sclerotization, respectively. N-beta-Alanyldopamine has been claimed to be used only by phenoloxidase and quinone isomerase for quinone tanning and quinone methide sclerotization thereby attributing biochemical diversity to the coloration and strength of different cuticles. However, we demonstrate here that cuticular enzymes isolated from the larvae of Calliphora possess the capacity to generate 1,2-dehydro-N-beta-alanyldopamine from N-beta-alanyldopamine. Chemical synthesis of 1,2-dehydro-N-beta-alanyldopamine and its further oxidation are reported for the first time. Comparative biochemical studies confirm that both the sclerotizing precursors, N-acetyldopamine and N-beta-alanyldopamine, are used by all three different mechanisms of the unified theory of sclerotization.

Detection of dopachrome isomerase activity on gels

Dopachrome isomerase is a recently discovered enzyme associated with the melanogenesis process occurring in most vertebrates and invertebrates. It catalyzes the conversion of dopachrome to 5,6-dihydroxyindole(s). Based on the fact that 5,6-dihydroxyindoles are rapidly oxidized to melanochrome pigment by tyrosinases, we have developed a rapid, sensitive, and specific staining procedure for detection of dopachrome isomerase activity after gel electrophoresis. The method employs the use of commercially available mushroom tyrosinase entrapped in polyacrylamide gels for electrophoretic separation of dopachrome isomerase. Staining is achieved by the use of dopa solution. The dopachrome formed by the action of mushroom tyrosinase entrapped in the gel is converted to 5,6-dihydroxyindole(s) by dopachrome isomerase initially. The latter compound is subsequently oxidized by tyrosinase to purple-colored melanochrome. Therefore, dopachrome isomerase appears as a bluish-purple band against a pale orange-red background within 10 min. With the use of the new detection technique, the presence of hitherto unknown isozymes of dopachrome isomerase could be readily detected in polyacrylamide gels. This procedure is more sensitive than silver staining for detection of dopachrome isomerase and as little as 15 ng of purified protein could be easily detected on gels.

The ontogeny of dopachrome isomerase isozyme patterns in the tobacco hornworm Manduca sexta

Melanogenesis in animals is initiated by tyrosinase and augmented by recently discovered dopachrome isomerase. Mushroom tyrosinase embedded 5% polyacrylamide gels are used to specifically stain for dopachrome isomerase activity after electrophoretic separation. Using this new procedure, the ontogenic changes in dopachrome isomerase are demonstrated for the first time in any organism. While there is no isozyme variation within the same stage of development, there are profound variations in isozyme patterns during the developmental stages from egg to adult, indicating differential expression and/or processing of dopachrome isomerase genes to match the metabolic needs at these stages.

Biological and toxicological consequences of quinone methide formation

Quinone methides are a class of reactive, electrophilic compounds which are capable of alkylating cellular macromolecules. They are formed during xenobiotic biotransformation reactions and are hypothesized to mediate the toxicity of a large number of quinone antitumor drugs as well as several alkylphenols. In addition, oxidation of specific endogenous alkylphenols (e.g. coniferyl alcohol) and alkylcatechols (e.g. N-acetyldopamine, dopa) to quinone methides plays an important role in the synthesis of several complex plant and animal polymers, including lignin, cuticle and melanin. The role of quinone methides in these various processes is reviewed.

Evidence for the formation of a quinone methide during the oxidation of the insect cuticular sclerotizing precursor 1,2-dehydro-N-acetyldopamine

1,2-Dehydro-N-acetyldopamine (dehydro-NADA) is an important catecholamine derivative involved in the cross-linking of insect cuticular components during sclerotization. Since sclerotization is a vital process for the survival of insects, and is closely related to melanogenesis, it is of interest to unravel the chemical mechanisms participating in this process. The present paper reports on the mechanism by which dehydro-NADA is oxidatively activated to form reactive intermediate(s) as revealed by pulse radiolysis, electron spin resonance spectroscopy, high performance liquid chromatography, and ultraviolet-visible spectroscopic analysis. Pulse radiolytic one-electron oxidation of dehydro-NADA by N3. (k = 5.3 x 10(9) M-1 s-1) or Br2.- (k = 7.5 x 10(8) M-1 s-1) at pH6 resulted in the rapid generation of the corresponding semiquinone radical, lambda max 400 nm, epsilon = 20,700 M-1 cm-1. This semiquinone decayed to form a second transient intermediate, lambda max 485 nm, epsilon = 8000 M-1 cm-1, via a second order disproportionation process, k = 6.2 x 10(8) M-1 s-1. At pH 6 in the presence of azide, the first order decay of this second intermediate occurred over milliseconds; the rate decreases at higher pH. At pH 6 in the presence of bromide, the intermediate decayed much more slowly over seconds, k = 0.15 s-1. Under such conditions, the dependence of the first order decay constant upon parent dehydro-NADA concentration led to a second order rate constant of 8.5 x 10(2) M-1 s-1 for reaction of the intermediate with the parent, probably to form benzodioxan "dimers." (The term dimer is used for convenience; the products are strictly bisdehydrodimers of dehydro-NADA (see "Discussion" and Fig. 11)) Rate constants of 5.9 x 10(5), 4.5 x 10(5), 2.8 x 10(4) and 3.5 x 10(4) M-1 s-1 were also obtained for decay of the second intermediate in the presence of cysteine, cysteamine, o-phenylenediamine, and p-aminophenol, respectively. By comparison with the UV-visible spectroscopic properties of the two-electron oxidized species derived from dehydro-NADA and from 1,2-dehydro-N-acetyldopa methyl ester, it is concluded that the transient intermediate exhibiting absorbance at 485 nm is the quinone methide tautomer of the o-quinone of dehydro-NADA. Sclerotization of insect cuticle is discussed in the light of these findings.

Mechanistic studies on tyrosinase-catalysed oxidative decarboxylation of 3,4-dihydroxymandelic acid

Mushroom tyrosinase, which is known to convert a variety of o-diphenols into o-benzoquinones, has been shown to catalyse an unusual oxidative decarboxylation of 3,4-dihydroxymandelic acid to 3,4-dihydroxybenzaldehyde [Sugumaran (1986) Biochemistry 25, 4489-4492]. The mechanism of this reaction was re-investigated. Although visible-region spectral studies of the reaction mixture containing 3,4-dihydroxymandelic acid and tyrosinase failed to generate the spectrum of a quinone product during the steady state of the reaction, both trapping experiments and non-steady-state kinetic experiments provided evidence for the transient formation of unstable 3,4-mandeloquinone in the reaction mixture. The visible-region spectrum of mandeloquinone resembled related quinones and exhibited an absorbance maximum at 394 nm. Since attempts to trap the second intermediate, namely alpha,2-dihydroxy-p-quinone methide, were in vain, mechanistic studies were undertaken to provide evidence for its participation. The decarboxylative quinone methide formation from 3,4-mandeloquinone dictates the retention of a proton on the alpha-carbon atom. Hence, if we replace this proton with deuterium, the resultant 3,4-dihydroxybenzaldehyde should retain the deuterium present in the original substrate. To test this hypothesis, we chemoenzymically synthesized alpha-deuterated 3,4-dihydroxymandelic acid and examined its enzymic oxidation. Our studies reveal that the resultant 3,4-dihydroxybenzaldehyde retained nearly 90% of the deuterium, strongly indicating the transient formation of quinone methide. On the basis of these findings it is concluded that the enzymic oxidation of 3,4-dihydroxymandelic acid generates the conventional quinone product, which, owing to its unstability, is rapidly decarboxylated to generate transient alpha,2-dihydroxy-p-quinone methide. The coupled dienone-phenol re-arrangement and keto-enol tautomerism of this quinone methide produce the observed 3,4-dihydroxybenzaldehyde.

Molecular mechanisms for mammalian melanogenesis. Comparison with insect cuticular sclerotization

Melanogenesis is an important biochemical process for the production of skin pigments which protect many animals from the damage of solar radiation. The abnormalities in melanogenesis are associated with albinism, vitiligo, as well as malignant melanoma in humans. In the lower forms of animals viz., insects, the exoskeleton is hardened to protect their soft bodies by a process called sclerotization, which is often accompanied by melanization. Recent advances in the biochemistry of sclerotization and melanization reveal remarkable similarity between these two processes. The seven stages of sclerotization are: (a) enzymatic oxidation of N-acyldopamine, (b) Michael-1,4-addition reactions of N-acyldopamine quinone, (c) tautomerization of quinone to quinone methide, (d) Michael-1,6-addition of quinone methides, (e) tautomerization of N-acyldopamine quinone methide to 1,2-dehydro-N-acyldopamine, (f) enzymatic oxidation of 1,2-dehydro-N-acyldopamine, and (g) the reactions of resultant quinonoid compounds. Amazingly, striking similarities in the reaction sequences are found in the melanization process starting from dopa. These comparisons predict a central role for quinone methides as reactive intermediates during melanization. Accordingly, recent studies provide increasing evidence in favor of this proposition.

The mechanism of tyrosinase-catalysed oxidative decarboxylation of alpha-(3,4-dihydroxyphenyl)-lactic acid

Mushroom tyrosinase, which is known to catalyse the conversion of o-diphenols into o-benzoquinones, has been shown to catalyse the oxidative decarboxylation of 3,4-dihydroxymandelic acid [Sugumaran (1986) Biochemistry 25, 4489-4492]. To account for this unusual reaction, a quinone methide intermediate has been proposed. Since all attempts to trap this intermediate ended in vain, mechanistic studies were designed to support the formation of this transient product. Replacement of the alpha-proton in 3,4-dihydroxymandelic acid with a methyl group generates alpha-(3,4-dihydroxyphenyl)-lactic acid, the enzymic oxidation of which should produce 3,4-dihydroxyacetophenone as the end product if the oxidative decarboxylation proceeds through the quinone methide intermediate. Accordingly, chemically synthesized alpha-(3,4-dihydroxyphenyl)-lactic acid on enzymic oxidation produced 3,4-dihydroxyacetophenone as the major isolatable product. Non-steady-state kinetic analysis of the enzyme reaction attested to the transient formation of the conventional quinone product. Thus the enzymic oxidation of alpha-(3,4-dihydroxyphenyl)-lactic acid seems to generate the conventional quinone, which, owing to its instability, is rapidly decarboxylated to yield the transient quinone methide. The coupled dieneonephenol re-arrangement and ketol-enol tautomerism transforms the quinone methide into 3,4-dihydroxyacetophenone.

Lysolecithin--a potent activator of prophenoloxidase from the hemolymph of the lobster, Homarus americanas

The phenoloxidase system, which is involved in encapsulation and melanization of foreign objects in crustacean, is found to be present in an inactive proenzyme form in the hemocytes of the lobster, Homarus americanas. Activation of the enzyme could be achieved either by treatment with an anionic detergent such as sodium dodecyl sulfate, or by a cationic detergent such as cetylpyridinium chloride, but not by either nonionic detergent or zwitterionic detergent. In addition, a number of fatty acids also activated the proenzyme. However, phospholipids, especially lysolecithin proved to be the most potent activator of prophenoloxidase. Therefore, it is proposed that apart from the well established proteolytic mode of activation, prophenoloxidase can also be activated by this alternative mode involving lipids.

Quinone methide as a new intermediate in eumelanin biosynthesis

The conversion of dopachrome to dihydroxyindole(s), a key reaction in eumelanin biosynthetic pathway, has been shown to be under the control of dopachrome conversion factor. Dopachrome conversion factor isolated from the hemolymph of Manduca sexta larvae, which is devoid of any tyrosinase activity, exhibits a narrow substrate specificity and readily bleaches the iminochromes derived from the oxidation of L-dopa, L-dopa methyl ester, and alpha-methyl-L-dopa, but failed to attack the corresponding D-isomers. The product formed in the case of L-dopachrome was identified to be 5,6-dihydroxyindole. Therefore, aromatization of dopachrome seems to accompany its decarboxylation as well. However, the enzyme also converts L-dopachrome methyl ester to an indole derivative indicating that it can deprotonate the alpha-hydrogen when the carboxyl group is blocked. These results are accounted for by the transient formation and further transformation of a reactive quinone methide intermediate during the dopachrome conversion factor-catalyzed reaction. The fact that the enzyme-catalyzed conversion of alpha-methyl dopachrome methyl ester (where both decarboxylation and deprotonation are blocked) resulted in the generation of a stable quinone methide in the reaction mixture confirms this contention and supports our recent proposal that quinone methide and not indolenine is the key transient intermediate in the conversion of dopachrome to dihydroxyindole observed during melanogenesis.

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.

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.

N-acetyldopamine quinone methide/1,2-dehydro-N-acetyl dopamine tautomerase. A new enzyme involved in sclerotization of insect cuticle

The enzyme system causing the side chain desaturation of the sclerotizing precursor, N-acetyldopamine (NADA), was solubilized from the larval cuticle of Sarcophaga bullata and resolved into three components. The first enzyme, phenoloxidase, catalyzed conversion of NADA to NADA quinone and provided it for the second enzyme (NADA quinone isomerase), which makes the highly unstable NADA quinone methide. Quinone methide was hydrated rapidly and nonenzymatically to form N-acetylnorepinephrine. In addition, it also served as the substrate for the last enzyme, quinone methide tautomerase, which converted it to 1,2-dehydro-NADA. Reconstitution of NADA side chain desaturase activity was achieved by mixing the last enzyme fraction with NADA quinone isomerase, obtained from the hemolymph of the same organism, and mushroom tyrosinase. Therefore, NADA side chain desaturation observed in insects is caused by the combined action of three enzymes rather than the action of a single specific NADA desaturase, as previously thought.

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.

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.

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.

A novel quinone: quinone methide isomerase generates quinone methides in insect cuticle

Insoluble cuticle-bound enzyme(s) of Manduca sexta pharate pupae, which is known to convert N-acetyldopamine to N-acetylnorepinephrine through the intermediate formation of quinone methide, also converted exogenously supplied N-acetyldopamine quinone to N-acetylnorepinephrine. The presence of a quinone trap such as N-acetylcysteine in the reaction mixture containing N-acetyldopamine and cuticle prevented the formation of N-acetylnorepinephrine but readily yielded N-acetylcysteine-N-acetyldopamine quinone adduct as a dead-end product. These results indicate the oxidation of N-acetyldopamine to its quinone and its enzyme-catalyzed isomerization to quinone methide before yielding N-acetylnorepinephrine as the stable product. The role of this newly discovered isomerase in sclerotization of insect cuticle is discussed.