Cell melanin

FIGURE 5.6 Calcitonin receptor responses, (a) Real-time melanin dispersion (reduced light transmittance) caused by agonist activation (with human calcitonin) of transfected human calcitonin receptors type II in melanophores. Responses to 0.1 nM (filled circles) and lOnM (open circles) human calcitonin, (c) Dose-response curves to calcitonin in melanophores (open circles) and HEK 293 cells, indicating calcium transient responses (filled circles). 

N-ras, H-ras, K-ras Signal transduction pathway Inhibition of cell proliferation and colony formation change in morphology, enhanced melanin synthesis decrease of in vivo tumorigenicity... 

Neural cells convert tyrosine to epinephrine and norepinephrine (Figure 31—5). While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxai phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine P-oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-A -methyltransferase uti-hzes S-adenosyhnethionine to methylate the primary amine of norepinephrine, forming epinephrine (Figure 31-5). Tyrosine is also a precursor of triiodothyronine and thyroxine (Chapter 42). 

Guyon A, Banisadr G, Rovere C, Cervantes A, Kitabgi P, MeUk-Parsadaniantz S, Nahon JL (2005b) Complex effects of stromal cell-derived factor-1 alpha on melanin-concentrating hormone neuron excitabUity. Eur J Neurosci 21 701-710... 

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

Melanins are prodnced in mammals in two types of cells of different developmental origin (1) the melanocytes of the skin, hair, choroids and iris and (2) the retinal pigment epithelium (RPE). Specialized organelles of the melanocytes, the melano-somes, synthesize and store eumelanins and phaeomelanins. 

Sugumaran, M., Comparative biochemistry of eumelanogenesis and the protective roles of phenoloxidase and melanin in insects. Pigment Cell Res., 15, 2, 2002. Riley, P.A., Melanin, Int. J. Biochem. Cell. Biol, 29, 1235, 1997. 

Seagle, B.L. et al., Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis, Proc. Natl. Acad. Sci. USA, 102, 8978, 2005. 

Wang, Z., Dillon, J., and Gaillard, E. R., Antioxidant properties of melanin in retinal pigment epithelial cells, Photochem. PhotobioL, 82, 474, 2006. 

Kamei, H. et al.. Suppression of growth of cultured malignant cells by allomelanins, plant-produced melanins. Cancer Biother. Radiopharm., 12, 47, 1997. 

A cross section of excised human skin, histologically stained, is shown in Figure 6.11. It shows a layer structure of the tissue and the increased homogeneity in the bloodless stratum corneum layer, where the cell nuclei are absent, and where the potentially confounding melanin concentrations are... 

Figure 1.6 (A) Changes in c-Fos IR in hypocretin/orexin(HCRT+)- containing neurons in response to BIC treatment as a function of distance from the microdialysis probe. The percentage of double-labeled cells was greatest closer to the probe (grids 1 and 2, compared with 3 and 4) and was greater with higher concentrations or exposure times. Contralateral cells were not affected. (B) Much lower percentages of melanin-concentrating hormone (MCH+) exhibited c-Fos IR activation in response to BIC. From Alam et al. (2005).

Very recently, the method has been used for the synthesis of racemic linderol A [311], a natural product isolated from the fresh bark of Lindera umbellata. This compound is reported to exhibit potent inhibitory activity on the melanin biosynthesis of cultured B-16 melanoma cells in guinea pigs, without causing any cytotoxicity in the cultured cell or skin irritation [312]. 

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