cysteinyldopa and dopaquinone

Melanin is a natural pigment produced within organelles, melanosomes, located in melanocytes. Biological functions of melanosomes are often attributed to the unique chemical properties of the melanins they contain; however, the molecular structure of melanins, the mechanism by which the pigment is produced, and how the pigment is organized within the melanosome remains to be fully understood. In this review, we examine the current understanding of the initial chemical steps in the melanogenesis. Most natural melanins are mixtures of eumelanin and pheomelanin, and so after presenting the current understanding of the individual pigments, we focus on the mixed melanin systems, with a critical eye towards understanding how studies on individual melanin do and do not provide insight in the molecular aspects of their structures. We conclude the review with a discussion of important issues that must be addressed in future research efforts to more fully understand the relationship between molecular and functional properties of this important class of natural pigments.

Topics: Benzoquinones; Benzothiazoles; Cysteinyldopa; Dihydroxyphenylalanine; Humans; Iris; Melanins; Melanocytes; Melanosomes; Molecular Structure

The contributions of pulse radiolysis towards characterisation of unstable ortho-quinones relevant to melanogenesis are reviewed. The quinones discussed include dopaquinone, the precursor of both eumelanogenesis and phaeomelanogenesis, and 5-S-cysteinyldopaquinone, an early component of the phaeomelanogenic pathway. Redox exchange between dopaquinone and 5-S-cysteinyldopa is shown to be a determinant of the balance between eumelanogenesis and phaeomelanogenesis. Ortho-quinones resulting from the oxidation of tertiary N,N-dialkylcatecholamines cyclise to redox-inactive betaines which fail to autoactivate tyrosinase. This is consistent with the dopa detected during melanogenesis catalysed by tyrosinase being formed indirectly by a combination of dopaquinone intramolecular reductive addition to form leucodopachrome (cyclodopa), followed by redox exchange between remaining dopaquinone and leucodopachrome. Rapid tautomerism of the ortho-quinone of 4-cyanomethylcatechol to a redox-inactive quinomethane likewise inhibits tyrosinase autoactivation. The incorporation of trihydric phenol moieties in melanin is modelled by the reactions of several ortho-quinones with phloroglucinol, which itself is not directly oxidised by tyrosinase due to the meta-positioning of the hydroxyl groups. The importance of a susceptibility towards nucleophilic attack as well as a propensity to undergo redox-exchange, in the chemistry of melanogenic ortho-quinones, is emphasised.

Topics: Benzoquinones; Catechols; Cysteine; Cysteinyldopa; Dihydroxyphenylalanine; Enzyme Activation; Humans; Melanins; Molecular Structure; Monophenol Monooxygenase; Oxidation-Reduction; Phloroglucinol; Pulse Radiolysis; Quinones

Melanogenesis, i.e., synthesis of melanin and melanosomes, is a "cascade" of event which is channelled by internal and external regulatory factors. The recognition and selection of this information and subsequent differentiation of melanogenesis (melanin type and melanosomal development) would be regulated significantly by melanosomal membrane. The melanogenesis type could be switched relatively easily by UV light, hormone, and availability of tyrosinase substrate. The role of sulphydryl compounds as a regulatory factor in melanogenesis type (in particular for pheomelanogenesis) may not be tied to its absolute presence or absence, but rather, to the effective concentration within the melanocyte at a given time. It is, therefore, probable that the morphogenesis of melanosomes may not follow immediately in response to melanogenesis-type changes, hence the melanocyte revealing more often mosaic forms of melanosomes in nature after exposure to non-genetic factors. The switch of melanogenesis would be significantly controlled by structural and functional availability of vesiculoglobular bodies which are encoded or associated with HMSA-5 (69 kDa) glycoprotein. This HMSA-5 protein shares a significant homology with gp75 "b-locus" protein. However, because of our hypothesis that vesiculoglobular bodies carry post- (and pre-) tyrosinase regulatory factors involving in both pheo- and eumelanogenesis, the term "b-protein" which focuses only on eumelanogenesis may not be applied to HMSA-5.

Topics: Animals; Antioxidants; Benzoquinones; Cell Compartmentation; Cysteinyldopa; Dihydroxyphenylalanine; Gene Expression Regulation; Hair Color; Humans; Melanins; Melanocyte-Stimulating Hormones; Melanocytes; Mice; Mice, Inbred C57BL; Mice, Mutant Strains; Models, Biological; Monophenol Monooxygenase; Organelles; Pituitary Hormones; Skin Pigmentation; Ultraviolet Rays

Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of l-cysteine, DQ immediately binds to the -SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. l-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of l-cysteine thiolate (Cys-S

Topics: Benzoquinones; Binding Sites; Cysteine; Cysteinyldopa; Density Functional Theory; Dihydroxyphenylalanine; Melanins; Models, Molecular; Protons

Human glutathione transferase M2-2 prevents the formation of neurotoxic aminochrome and dopachrome by catalyzing the conjugation of dopamine and dopa o-quinone with glutathione. NMR analysis of dopamine and dopa o-quinone-glutathione conjugates revealed that the addition of glutathione was at C-5 to form 5-S-glutathionyl-dopamine and 5-S-glutathionyl-dopa, respectively. Both conjugates were found to be resistant to oxidation by biological oxidizing agents such as O(2), H(2)O(2), and O(*-)(2), and the glutathione transferase-catalyzed reaction can therefore serve a neuroprotective antioxidant function.

Topics: Benzoquinones; Cysteinyldopa; Dihydroxyphenylalanine; Dopamine; Glutathione; Glutathione Transferase; Humans; Indolequinones; Indoles; Isoenzymes; Magnetic Resonance Spectroscopy; Models, Chemical; Oxidation-Reduction; Protein Binding; Quinones; Time Factors

Eumelanogenesis and phaeomelanogenesis diverge at an early stage in pigment formation, namely at the point where dopaquinone, the initial product of tyrosine oxidation by tyrosinase, undergoes one of two types of reaction: either (1) a reductive endocyclisation in which a Michael addition of the side-chain amino group takes place; or (2) a reductive addition of cysteine to give cysteinyldopa. In the former case, the product cyclodopa, is known rapidly to undergo a redox exchange reaction with dopaquinone to yield dopachrome, the precursor of the eumelanogenic pathway. In the second instance, cysteinyldopa is regarded as leading to the formation of benzothiazoles, which are characteristic of phaeomelanin. The precursor molecule of the phaeomelanic pathway is cysteinyldopaquinone. We have examined quantitatively the role of dopaquinone in the non-enzymatic oxidation of 5-S-cysteinyldopa using pulse radiolysis and have demonstrated that the redox exchange reaction between dopaquinone and 5-S-cysteinyldopa occurs spontaneously with a rate constant of 8.8 x 10(5) M(-1) sec(-1). This study has also enabled an improved estimate of < or = 4 x 10(7) M(-1) sec(-1) to be obtained for the rate constant of the reaction of dopaquinone with cyclodopa. Calculations utilising these figures and estimates of the rate constants for the other reactions in early melanogenesis, demonstrate that, whilst similar pathways are invoked, the phaeomelanic pathway predominates in the presence of cysteine, irrespective of the availability of dopaquinone and thus independently of the rate of tyrosinase-catalysed oxidation. This suggests that the balance between the formation of eumelanin and phaeomelanin is regulated principally by the availability of cysteine at the site of melanogenesis.

Topics: Benzoquinones; Cyclization; Cysteinyldopa; Dihydroxyphenylalanine; Kinetics; Melanins; Oxidation-Reduction; Spectrum Analysis

Human malignant melanoma represents a difficult therapeutic challenge to both medical scientists and practicing physicians. However, the biologic uniqueness of the tumor may provide opportunities for exploitation in therapeutics. This study proposed to undertake a systemic approach to the chemotherapy of malignant melanoma based upon the uniqueness of pigment-cell metabolic pathway pertaining to conversion of tyrosine and dopa with subsequent formation of melanin by tyrosinase and its related enzymes. The sulphur homologue of tyrosine, cysteinylphenol (CP), its amine derivative, cysteaminylphenol (CAP), and their N-acetyl and alpha-methyl derivatives have been synthesized and tested in in vivo and in vitro melanocytotoxicity and antimelanoma effects. These phenolic thioethers (PTEs) and phenolic thioether amine (amides) (PTEAs), which are substrates of tyrosinase, showed significant cytotoxicity that is selective to melanocytes and melanoma cells. Most previous attempts to impair the melanin pathway as a therapeutic strategy have been of limited success because they have been directed to catecholic compounds that are unstable and insufficient in lethality at physiologically tolerable doses. By contrast, our approach relies on phenolic compounds, PTEs and PTEAs, which are more stable than catechols and become toxic only after oxidation by tyrosinase. We found PTEA as the most promising agent for the future development of chemotherapeutic agents. The possible biologic, chemical, and pharmacologic reactions of these synthetic compounds within the melanoma cells are studied and discussed.

Topics: Benzoquinones; Catechols; Cysteinyldopa; Dihydroxyphenylalanine; Dopamine; Humans; Indoles; Melanins; Melanoma; Models, Chemical; Phenol; Phenols; Pigments, Biological; Tyrosine

The concentrations of dopa, cysteinyldopas, 5-S-glutathionyldopa, gamma-glutamyl-5-S-cysteinyldopa and 5-S-cysteinylglycinedopa, were analysed in homogenates of cultured human melanoma cells and in culture media. Cysteinyldopas were found to be the major catechol in the cells, with a molar concentration more than a hundred times that of dopa. 5-S-Glutathionyldopa was found in the same amount as dopa, while the quantity of 5-S-cysteinylglycinedopa was one order of magnitude less. gamma-Glutamyl-5-S-cysteinyldopa was not present in detectable amounts. In the medium the concentrations of dopa, 5-S-cysteinylglycinedopa and of 5-S-glutathionyldopa were about one half of those in the cells, while the concentration of cysteinyldopas was about 2%. The ratio between 2-S-cysteinyldopa and 5-S-cysteinyldopa when incubating dopa and cysteine with tyrosinase was identical with the ratio between the analogically synthetised isomers of glutathionyldopa. Consequently, from the calculation of these ratios in cells and media one cannot deduce whether cysteinyldopas arise from the direct addition of cysteine to dopaquinone, or from degradation of glutathionyldopa. Oxidation of 5-S-glutathionyldopa gives a red chromophore with maximum absorption at 480 nm which develops into a black pigment.

Topics: Benzoquinones; Cell Line; Cysteinyldopa; Dihydroxyphenylalanine; Humans; Melanoma; Pigmentation