Opiomelanins synthesis and properties

Opiomelanins represent a new class of synthetic pigments produced by the tyrosinase-catalyzed oxidation of opioid peptides and other tyrosine aminoterminal peptides. In contrast with dopamelanin, these polymers are fully soluble in hydrophilic media, due to the presence of the peptide moiety. Opiomelanins show paramagnetism as demonstrated by the EPR spectrum identical to that of dopamelanin. The presence of the aminoacidic chain linked to the melaninic moiety, influences the electron transfer properties of opiomelanins i.e. the ability to oxidize NADH. Like dopamelanin Tyr-Gly-melanin exhibits this behaviour whereas leuenkmelanin does not show any oxidizing potential. Opiomelanins UV-Vis spectrum is characterized by an absorption band at 330 nm which disappears upon acid hydrolysis, H202 treatment and under simulated solar illumination. Opiomelanins exhibit a fluorescence emission peaked at 440 and 520 nm if excited at 330 nm. These fluorescence bands are due to the oligomeric units and high molecular weight units, respectively. When opioid peptides are allowed to react with tyrosinase in the presence of an excess of cysteine, cysteinyldopaenkephalins are synthesized. These peptides are furtherly oxidized giving rise to pheoopiomelanins. Reactive oxygen species also are able to oxidize non enzymatically both enkephalins and cysteinyldopaenkephalins, producing the corresponding melanin pigments

PRODUCTION OF MELANIN PIGMENTS BY CHEMICAL AND ENZYMATIC OXIDATION OF TETRAHYDROISOQUINOLINES

Tetrahydropapaveroline (THP) oxidation was studied in various experimental conditions by absorbance spectroscopy. THP was found to be easily oxidized by mushroom tyrosinase, giving rise to the formation of a chromophore (THP-chrome) with absorption maxima at 308 and 470 nm. The oxidation further proceeds leading to the formation of a melanin-like pigment. The use of periodate as oxidant at pH 7.4 allows the visualization of the THP-chrome, as well. Other tetrahydroisocluinolines bearing a catechol moiety, such as salsolinol, laudanosoline and apomorphine, have been found to be easily oxidized in the same conditions, giving rise to pigmented derivatives. The products of THP oxidation are able to copolymerize with dopa or opioid peptides in the presence of tyrosinase, generating mixed-type melanins

MELANINS PRODUCTION FROM ENKEPHALINS BY TYROSINASE

Leu-enkephalin and Met-enkephalin are oxidized in vitro by mushroom and sepia tyrosinase giving rise to synthetic melanins whose production is dependent on incubation time and on enzyme concentration. The Enk-melanins formed are acid-insoluble brownish or reddish pigments showing a continous absorbance in the visible region when dissolved in basic solution. The presence of the amino acid chain makes them fully soluble in pH 7.4 0.05 M phosphate buffer and methanol

TYROSINASE-CATALYZED OXIDATION OF MET-ENKEPHALINS

Kinetic properties of the oxidation of several Met-enkephalins and of kyotorphin by tyrosinase are reported. The opioid peptides tested present an affinity for the enzyme equal or higher than tyrosine itself. Long lasting incubations of these opioid peptides with tyrosinase lead to the formation of DOPA, the presence of ascorbic acid improving the recovery of the aminoacid. Spectrophotometric measurements in the absence of ascorbic acid indicate the formation of dopachrome which reaches the maximum within 15-30 minutes

Catecholamines oxidation by xantine oxidase

Abstract: Dopamine and structurally related catecholamines in the presence of hydrogen peroxide are oxidized in vitro by xanthine oxidase producing the corresponding melanin pigments. The kinetic parameters of the reaction, measured as aminochrome formation, have been calculated. The rate of peroxidation depends on enzyme and hydrogen peroxide concentration. The optimum pH for the peroxidative activity of the enzyme is around 8.5. Activation of the peroxidative reaction is also elicited by catechol compounds through a redox cycle mechanism. Implications about the possible biochemical relevance of xanthine oxidase activity on catecholamines oxidation are discussed

Pheomelanin production by the lipoxygenase-catalyzed oxidation of 5-S-cysteinyl-dopa and 5-S-cysteinyl-dopamine

5-S-cysteinyl-dopa (cysdopa) and 5-S-cysteinyl-dopamine (cysdopamine) are oxidized in vitro by soybean lipoxygenase (LOX) in the presence of hydrogen peroxide giving rise to the corresponding pheomelanins. The reaction is activated by caffeic acid and other catechols, suggesting a cofactor role for these compounds. The activating effect is proportional to the concentration of the cofactor, with a saturation profile. The activation extent of the various cofactors is directly related to LOX affinity for the same compounds. The possible implications of the peroxidative action of LOX in Parkinson's disease and in aging are discussed

Some biochemical properties of melanins from opioid peptides

Opioid peptides are converted by mushroom tyrosinase into melanin-like compounds retaining the peptide moiety (opio-melanins). Opio-melanins, owing to the presence of the linked aminoacids and in contrast with DOPA-melanin, are soluble compound. The enkephalin-generated melanins are cleaved by carboxypeptidase A and pronase whereas aminopeptidase M cannot remove aminoacids from the pigment. Enkephalins, as well as other opioid peptides, (α-endorphin, kyotorphin, esorphins) if oxidized in presence of DOPA and tyrosinase are readily incorporated into DOPA-melanin. The resulting mixed-melanins (opio-melanin + DOPA-melanin) can be solubilized in hydrophylic solvents. Melanin from leuenkephalin exhibits paramagnetism as evidenced by an EPR spectrum identical to that of DOPA-melanin, but unlike the latter pigment, it does not appear to oxidize NADH, probably for the presence of the peptide moiety that exerts a hampering effect on the oxidizing capacity