Identification of Products and Intermediates During L-Dopa Oxidation to Dopachrome Using High Pressure Liquid Chromatography with Electrochemical Detection

The Role of the Dopamine Melanin Pathway in the Ontogeny of Elytral Melanization in Harmonia axyridis

Polymorphic melanism in insects is a conspicuous phenotype which is derived from specific genotypes, and might be central to speciation processes via assortative sexual selection. At the molecular level, melanism in insects is attributed to the melanin pathway. DOPA decarboxylase (DDC) protein encoded by the DDC gene plays a central role in dopamine-melanin synthesis, the main component of melanin in insects. Although the mechanism of melanism has been elucidated in holometabolous insects, other physiological processes coupled with melanin synthesis are unknown. Herein, we identified DDC from the Asian multi-colored ladybird (Harmonia axyridis), an ideal holometabolous insect for studies of melanization due to highly variable color on their elytra. Analyses revealed that HaDDC (the DDC gene of H. axyridis) was constitutively expressed throughout all developmental stages. We performed RNAi technique to examine the melanin synthesis pathway of elytra in H. axyridis. The transcript levels of HaDDC were significantly suppressed after the injection of double-strand RNA of HaDDC (dsHaDDC) at 300 ng/individual in third instar larvae. Silencing HaDDC in third instar larvae did not result in mortality nor significantly affect pupation and eclosion. We further demonstrated that all adults of H. axyridis (forms succinea, spectabilis, and conspicua) with HaDDC silenced in third larvae showed abnormal phenotype which emerged as decreased elytra melanin. However, melanin was still observed in other parts of the adults such as head or pronotum. These results demonstrate for the first time that dopamine-derived melanin is the main contributor in elytra melanization in H. axyridis. Additionally, we provide evidence for DDC in regulating fecundity by showing that silencing of HaDDC in third instar larvae significantly reduced female egg-laying and egg hatching. As such, DDC is likely pleiotropic in respect of its role in melanin production and fecundity processes. These findings bring novel insights into melanin production in holometabolous insects, and contribute to the framework on which further studies may be conducted on the mechanism of pigment production and patterning in various types of insect coloration.

Fungal biotransformation of synthetic levodopa to stable dopamine in l-ascorbate-mediated aerobic-thermophilic biochemical process

In the present study, fungal biotransformation of synthetic levodopa to stable dopamine in an l-ascorbate-mediated thermophilic-aerobic biochemical reaction was investigated. A mutant strain of Aspergillus oryzae EMS-6 was used for the preparation of mycelial biomass. The mutant was previously developed through EMS-induced mutagenesis and repressed against l-cysteine HCl. Growth parameters such as rate of cultivation (48 h), initial pH (6) and incubation temperature (30 °C) supported 18.84 g/l biomass with 23 g/l glucose consumption. Thermophilic behaviour of culture was observed at 25-40 °C. Kinetic variables notably µ = 0.385 /h and Q_s, exhibited consistent growth pattern. Biochemical reactions were performed aerobically using mycelial biomass as the source of enzyme 'tyrosinase' in a digital hotplate equipped with magnetic stirrers. The reaction conditions included 5 mg/ml biomass and 2.5 mg/ml levodopa as basal substrate in a thermophilic reaction of 25 min duration acidified with l-ascorbic acid. TLC and HPLC analysis of reaction mixture confirmed the presence of levodopa and dopamine using a CN-9dth (R) column. Activation enthalpy and entropy of dopa decarboxylase (DDC) and its thermal inactivation showed an improved biotransformation of levodopa to dopamine at the optimal temperature (30 °C) as compared to other temperatures being employed. Overall, 3.68 mg/ml dopamine (4.55 mg/ml proteins) synthesis from 2.38 mg/ml levodopa was accomplished. The enhancement in metabolic activity of the mutant strain is ~ 2.75-fold improved when compared to the unoptimized reaction conditions, which is highly significant (HS) indicating an eco-commercially viable (LSD ~ 0.412) bioprocess.

Biosynthesis of betalains: yellow and violet plant pigments

Betalains are the yellow and violet pigments that substitute anthocyanins in plants belonging to the order Caryophyllales. These pigments have attracted much attention because of their bioactivities, which range from an antioxidant capacity to the chemoprevention of cancer. However, the biosynthetic pathway of betalains remains under discussion; the main steps have been characterized in recent years, but multiple side reactions are possible. The key enzymes involved have only recently been described, providing clues about the regulation of betalain biosynthesis. In this review, we provide a comprehensive view of the biosynthetic scheme of betalains and discuss the different reactions that have been demonstrated experimentally or proposed in the literature.

The use of gene silencing to study the role of dopa decarboxylase in mosquito melanization reactions

Mosquito melanization involves hydroxylation of tyrosine to dopa, which then is oxidized to dopaquinone by phenoloxidase, or decarboxylated to dopamine by dopa decarboxlase (DDC). An Armigeres subalbatus cDNA encoding DDC was cloned and real-time PCR analysis revealed increased transcripts in blood-fed and microfilariae (mf)-inoculated mosquitoes. A double subgenomic Sindbis virus was used to silence DDC and assess its role in melanization of mf. DDC transcription and activity were significantly decreased in silenced mosquitoes, as was the degree of mf melanization 48 h postinoculation; however, melanization increased after 72 and 96 h, demonstrating that DDC influences the rate of melanization. DDC-silenced mosquitoes also exhibit high mortality, over-feeding and abnormal movement, consistent with an involvement of DDC in neurotransmission.

Surface-enhanced Raman spectroscopy of DOPA-containing peptides related to adhesive protein of marine mussel, Mytilus edulis

The common blue marine mussel adheres to underwater surfaces using an adhesive protein (Mefp-1) extruded from its foot. This highly hydroxylated protein contains a number of unusual amino acids, including 3,4-dihydroxyphenylalanine (DOPA), which is thought to contribute to the crosslinking of the extruded threads and adhesion to the substratum. Mefp-1 adheres to a wide variety of surfaces and is ultimately biodegradable. In this study we use surface-enhanced Raman spectroscopy (SERS) to characterize the adsorption of DOPA-containing peptides on colloidal gold. The peptides are simplified fragments of the Mefp-1 consensus decapeptide repeat, Ala-Lys-Pro-Ser-Tyr-DHP-Hyp-Thr-DOPA-Lys. Our results show that the peptides TDeltaKA, PTDeltaKA, and PPTDeltaKA (where Delta represents DOPA) coordinate to the gold surface through the catechol oxygens of the DOPA residue and through primary amine groups. The diproline sequence introduces conformational constraints that influence the conformations of the adsorbed peptides. These findings lay the groundwork for developing synthetic adhesives for underwater and medical applications.

Some aspects of tyrosine secondary metabolism

The origin and fate of some tyrosine secondary metabolites within specialized eukaryotic cells are discussed in the light of our knowledge of the plasma environment to which they are exposed throughout their lifetime. Attention is focused on ar-dihydroxy and -trihydroxy derivatives and the corresponding quinoidal counterparts, as well as on the enzymic activities involved in the formation and degradation of these potentially toxic molecules. Some physiopathological and pharmacological implications of the above-mentioned topics are considered, taking into account the well known toxicity of reactive intermediates in molecular oxygen reduction, as well as the reactivity of both semiquinonic and quinonic products of catecholamine oxidation.

The effects of hydroxyl radical attack on dopa, dopamine, 6-hydroxydopa, and 6-hydroxydopamine

High pressure liquid chromatography with electrochemical detection (HPLC-ED) was employed in conjugation with a sensitive and specific salicylate hydroxylation assay to evaluate the immediate effects of hydroxyl radical (.OH) attack on four catechol intermediates of eumelanin, dopamine (3,4-dihydroxyphenylethylamine), its precursor dopa (3,4-dihydroxyphenylalanine), and their respective neurotoxic trihydroxyphenyl derivatives, 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine,6-OHDA) and 6-hydroxydopa(2,4,5-trihydroxyphenylalanine, TOPA). Semiquinone and quinone species were identified as the initial products of the oxidation of these four catechol substrates. The enhanced oxidations of the catechols when exposed to .OH attack was accompanied by marked decreases in the level of each semiquinone species. Quinone levels were elevated in reactions involving .OH attack on dopamine and 6-OHDA, but absent in reactions involving radical attack on dopa or TOPA, suggesting that dopaquinone (DOQ) and TOPA p-quinone (TOPA p-Q) are oxidized more rapidly by .OH than are the quinones of dopamine and 6-OHDA. The formation of 6-OHDA p-quinone (6-OHDA p-Q) in incubations involving DA and .OH suggest that the .OH-mediated hydroxylation of DA may be a mechanism for generating this potentially cytotoxic trihydroxyphenyl. The results of this study demonstrate for the first time that semiquinone and quinone intermediates of eumelanin are the initial products derived from the .OH-mediated oxidations of dopa, DA, TOPA, and 6-OHDA. These observations suggest that if .OH is generated beyond the capabilities of cytoprotective mechanisms, the radical can rapidly oxidize catechol precursors, augment melanogenesis, and generate additional cytotoxic quinoid intermediates of eumelanin.

The effects of glutathione and ascorbic acid on the oxidations of 6-hydroxydopa and 6-hydroxydopamine

The interactions of ascorbic acid (AA) and reduced glutathione (GSH) in the oxidations of the catecholaminergic neurotoxins 6-hydroxydopa (TOPA) and 6-hydroxydopamine (6-OHDA) were investigated by both high performance liquid chromatography with electrochemical detection (HPLC-ED) and spectrometric methods. These comparative studies showed TOPA and 6-OHDA to be extremely unstable, with 100% of the trihydroxyphenyls oxidized within 0.5 min at physiological pH in potassium phosphate buffer. Neither AA nor GSH was found capable of significantly impeding the oxidations of these trihydroxyphenyls, or of regenerating these substances by reducing back their oxidation products, even though such a redox exchange mechanism was demonstrated for AA and the dihydroxyphenyl dopamine. Although ineffective in keeping TOPA and 6-OHDA as reduced molecules, GSH may nevertheless influence the neurotoxicity of trihydroxyphenyls by interacting with their oxidation products forming glutathionyl conjugates, thereby switching the reaction pathway away from potentially toxic eumelanin precursors and toward the production of pheomelanin. Electrochemical analyses established the formation of two oxidation products derived from each trihydroxyphenyl, one detected at -100 mV and the other at +700 mV. AA had no effect on either oxidation product, whereas GSH significantly decreased the levels of both oxidation products. The component detected at +700 mV is the cyclized, reduced leukochrome. The identity of the component detected at -100 mV was not established, but it is considered to be either the p-quinone or the cyclized, oxidized aminochrome.