Melanin like compound

Neurotransmitters, such as dopamine (DA) and epinephrine (EP), are catecholamines that undergo complex multistep oxidation processes in aqueous solution via coupled ET, proton transfer (2e , 2H at physiological pH), but with the complication of side reactions to form melanin-like compounds that can block electrode surfaces [193]. Such processes are expected to follow a classical scheme of squares [194], and are of considerable interest for the practical detection of neurotransmitters, as carbon electrodes have become the electroanalytical platform of choice [5, 60, 195]. This is due to a desirable range of properties including biocompatibihty, chemical inertness, and low background current that are responsible for lower detection hmits, wide potential windows, and low... 

A crnde extract of sweet potato Ipomoea-batatas (L.) Lam.) was nsed as a source of phenol oxidases (polyphenoloxidase, tyrosinase, catecholoxidase, EC 1.14.18.1). The extract was directly placed in the carrier of a FIA system with UVD, to promote oxidation of phenolic compounds to o-quinones that condense to form melanin-like pigments with a strong absorption at 410 nm. The determination of phenols in industrial wastewaters showed good agreement with conventional methods (correlation coefficient 0.9954) LOD was 10 p,M, with RSD <2.7% (w = 6). Under optimal storage conditions the enzymatic activity did not vary for at least five months . 

Allomelanins — These are structurally different compounds containing little or no nitrogen. They are considered polymers of phenolic compounds like catechol. Fnngi prodnce melanin pigments, predominantly dihydroxyphenylalanine (DOPA)-melanin and dihydroxynaphthalene (DHN)-melanin. ... 

There are a number of different types of UVR-absorbing compounds, most of which have functions apart from photoprotection against UVR damage. Myco-sporine-like amino acids are the most commonly encountered UVR-absorbing compounds in aquatic organisms however, other compounds that absorb in the UV-A and UV-B region include scytonemin, 3-hydroxykynurenine, melanin, various secondary metabolites and fluorescent pigments. 

Quinacrine, which is tricyclic and highly lipophilic, will easily penetrate cell membranes but hardly diffuse through the hydrophilic vitreous cavity. It also forms a stable complex with melanin and will therefore be retained in the RPE. Quinacrine must be considered a potent photosensitizer in the retina due to the absorption maximum of this drug in the visible region of the spectrum (Table 10.1). The more hydrophilic compounds chloroquine and hydroxychloroquine will more easily be transported to the lens. They are known to accumulate in the eye and induce toxic reactions. Primaquine is not likely to be distributed to the eye to any extent due to low distribution volume and fast elimination. 

Several of the antimalarial compounds seem likely to accumulate in the lipophilic or melanin-containing tissues of the skin. Adverse dermal effects observed after medication with antimalarials are probably a result of phototoxic reactions. 

The naphthalene derivatives (171, 172), from Daldinia concentrica (Ascomycetes), were significantly active towards C. elegans (10, 25 (ig/ml) and also showed antimicrobial and cytotoxic activity [6]. Since the black fruiting bodies of D. concentrica are highly melanized, it is likely that these compounds arise from melanin biosynthesis. Some naphthoquinones have been found to inhibit the motility and survival of H. contortus (LI). Plumbagin ( 173) was the most potent (ED 1 ig/ml) followed by 1, 4-naphthoquinone, juglone (174) and 1, 2-naphthoquinone [281]. 

Most reactions in secondary metabolic pathways are catalyzed by specific enzymes. Spontaneous, i.e., nonenzyme catalyzed reactions are significant in only a few cases, e.g., in the formation of some high molecular compounds like humic acids (D 3.3.1), melanins (D 22.1.3) and lignins (D 22.2.3), and in some cycliza-tions. There is also no evidence that enzymes of primary metabolism play a role in secondary metabolic pathways. 

In several instances polymeric substances arise during the transformation of secondary products, such as polymeric carbohydrates (D 1.4.1), humic acid-like polyphenols (D 3.3.1), rubber (D 6), sporopoUenins (D 6.5), polymeric products derived from 3-hydroxyanthranilic acid (D 8.4.1), melanins (D 22.1.3), lignins (D 22.2.3), and muramin (D 23.4). Many of these compounds are formed by oxidative polymerization catalyzed by phenoloxidases (C 2.3.1) and peroxidase... 

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