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Repigmentation in Vitiligo
Published in Vineet Relhan, Vijay Kumar Garg, Sneha Ghunawat, Khushbu Mahajan, Comprehensive Textbook on Vitiligo, 2020
Bharat Bhushan Mahajan, Richa Nagpal
The major function of melanocytes is to synthesize melanin within specialized organelles called melanosomes and to transfer melanosomes to neighboring keratinocytes to provide protection against UV radiation. Two types of melanin are synthesized within melanosomes: eumelanin and pheomelanin. Eumelanin is dark, brown-black, and insoluble, whereas pheomelanin is light red-yellow, sulfur containing, and soluble. Melanins are indole derivatives of DOPA and are formed in melanosomes through a series of oxidative steps (Figure 14.1). The synthesis of both types of melanin involves a rate-limiting catalytic step in which tyrosine is oxidized by enzyme tyrosinase to L-DOPA, a reaction known as the Raper–Mason pathway. L-DOPA is further oxidized to dopaquinone.
Actions of Dopamine on the Skin and the Skeleton
Published in Nira Ben-Jonathan, Dopamine, 2020
Following the formation of dopaquinone, the melanin pathway divides into production of the black‐brownish eumelanin and red‐yellow pheomelanin. In the eumelanin pathway, dopachrome is either spontaneously converted to 5,6‐dihydroxyindole or is enzymatically converted to 5,6‐dihydroxyindole‐2‐carboxylic acid via enzymatic conversion by dopachrome tautomerase (DCT), also referred to as tyrosine‐related protein‐2 (TRP‐2). The two TRP enzymes, TRP‐1 and TRP‐2, share ~40% amino acid homology with tyrosinase. They reside within the melanosomes and, like tyrosinase, span the melanosomal membrane. It has been suggested that TRP‐1 increases the ratio of eumelanin to pheomelanin and also increases tyrosinase stability [27]. Finally, polymerization of indoles and quinones leads to eumelanin formation. The pheomelanin pathway branches from the eumelanin pathway at the L‐dopaquinone step and depends on the presence of cysteine, which is actively transported through the melanosomal membrane. Cysteine reacts with L‐dopaquinone to form cysteinyl‐dopa. The latter is converted to quinoleimine, alanine‐hydroxyl dihydrobenzothazine, and polymerizes to pheomelanin.
Hair Coloring
Published in Dale H. Johnson, Hair and Hair Care, 2018
Pheomelanins are now believed to be formed by a biosynthetic route closely related to that for the eumelanins (Figs. 1, 2). Dopaquinone [3] reacts with the amino acid cysteine to give cysteinyldopas [9], which can then cyclize to the various 1,4-benzothiazines [8]. In this route mixed melanins can be formed by reaction of dopa with cysteinyldopa (5), and the presence of cysteine or other thiols can trigger a change from eumelanin to pheomelanin formation. Both of these processes are suspected to occur in nature. A much expanded version of this whole process is described by Prota (6).
Biosensors for the detection of mycotoxins
Published in Toxin Reviews, 2022
Akansha Shrivastava, Rakesh Kumar Sharma
These biosensors work on the association between an enzyme and its substrate. Two main mechanisms involved in enzymatic biosensors are substrate detection (incorporation of the enzymatically converted substrate in the biosensor) and enzyme inhibition (enzyme activity determination in the presence and absence of inhibitor compounds) (Alonso-Lomillo et al. 2011, Karunakaran et al. 2015a). For example, the development of a biosensor for the detection of tyramine by tyrosinase enzyme, immobilization into orthophosphate calcium matrices by using glutaraldehyde was performed. The amperometric technique was used to detect the result of electrochemical reduction of the o-dopaquinone. An amperometric biosensor based on horseradish peroxidase, an oxidoreductase enzyme was developed to determine the toxic content of citrinin mycotoxin in rice samples. This method involved the use of carbon paste electrodes filled up with multi-walled carbon nanotubes fixed in mineral oil, horseradish peroxidase, and a redox mediator ferrocene (Zachetti et al. 2013, Asal et al. 2018).
Gene variations in Autism Spectrum Disorder are associated with alternation of gut microbiota, metabolites and cytokines
Published in Gut Microbes, 2021
Zhi Liu, Xuhua Mao, Zhou Dan, Yang Pei, Rui Xu, Mengchen Guo, Kangjian Liu, Faming Zhang, Junyu Chen, Chuan Su, Yaoyao Zhuang, Junming Tang, Yankai Xia, Lianhong Qin, Zhibin Hu, Xingyin Liu
Three metabolites in the putative causal relationship network were involved in tryptophan and tyrosine metabolism, i.e. Tyrosyl-Leucine, Dopaquinone, and Desaminotyrosine. The level of Dopaquinone was causally related to the variation of PGLYRP4 (rs148195147, c.G602A, p.R201Q) and the abundance of Faecalibacterium prausnitzii (Figure 5(b)). Also, the Faecalibacterium prausnitzii is associated with the level of the 2ʹ-Deoxyguanosine, a metabolite involved in the folate biosynthesis pathway. A variation located in LILRA6 (rs56257556, c.A931 G, p.N311D) was associated with Desaminotyrosine, Tyrosyl-Leucine, and an Alanine metabolites 3-(Uracil-1-yl)-L-alanine mediated by Bacteroides ovatus (Figure 5(c)). LILRA6 is a leukocyte immunoglobulin-like receptor and plays roles in the pathways of the innate immune system and class I MHC mediated antigen processing and presentation. The de novo mutation of LILRA6 was previously observed in ASD.58
A comprehensive review on tyrosinase inhibitors
Published in Journal of Enzyme Inhibition and Medicinal Chemistry, 2019
Samaneh Zolghadri, Asieh Bahrami, Mahmud Tareq Hassan Khan, J. Munoz-Munoz, F. Garcia-Molina, F. Garcia-Canovas, Ali Akbar Saboury
Tyrosinase (EC 1.14.18.1) has two activities in its catalytic cycle, see Figure 295,96, a monophenolase activity where it hydroxylates monophenols (e.g l-tyrosine) to o-diphenols (e.g. l-dopa) and a diphenolase activity where tyrosinase oxidises o-diphenols to o-quinones (o-dopaquinone). At the same time of these enzymatic reactions, there are different chemical reactions coupled where two molecules of o-dopaquinone react their-selves generating an o-diphenol molecule (L-dopa) and a dopachrome molecule.