Evaluation of selected dye compounds as inhibitors of enzymes, relevant to neurological disorders

Abstract

Neurological disorders such as Alzheimer’s disease, Parkinson’s disease and schizophrenia are disorders that affect a large part of the population, especially the elderly. The cause of these disorders is still not completely known but a deficiency of certain neurotransmitters in the central nervous system is often responsible for the symptoms of theses disorders. For this reason, enzymes that metabolise neurotransmitters are considered as targets for the treatment of neurological diseases. The inhibition of these enzymes leads to increased levels of specific neuotransmitters and thus relief of the symptoms. Sometimes metabolic by-products contribute to the neurodegenerative processes, thus the inhibition of these enzymes may protect against neurodegeneration. The aim of this study was to evaluate the phenothiazine compound, methylene blue, and 31 dyes that are structuarally related to methylene blue as potential inhibitors of enzymes that are relevant to neurological disroders. Methylene blue interacts with various molecular targets and inhibits enzymes relevant to disease states such as depression and Alzheimer’s disease. In this study methylene blue and the related dyes were evaluated as inhibitors of the following enzymes: acethylcholinesterase (AChE), butyrylcholinesterase (BuChE), D-amino acid oxidase (DAAO), diamine oxidase (DAO), catechol-O-methyltransferase (COMT) and xanthine oxidase (XO). As mentioned, most of these enzymes are relevant to neurological disorders, with AChE and BuChE inhibitors already being used as treatment for Alzheimer’s disease. DAAO metabolises D-serine, and thus plays a role in glutaminergic neurotransmission in schizophrenia and Alzheimer’s disease. DAO metabolises histamine and primary amines and produces, in the same way as monoamine oxidases (MAO), harmful by-products during the catalytic cycle. COMT inhibitors are already established as treatment for Parkinson’s disease by increasing the levels of dopamine in the central nervous system. XO contributes to the formation of reactive oxygen species in the brain, which can lead to the destruction of neurons and thus contribute to the pathology of neurological diseases. The results show that various dyes are inhibitors of AChE and BuChE. Twelve and sixteen dyes were potent inhibitors of AChE and BuChE, respectively. Two of these dyes displayed sub-micromolar IC50 values for AChE and five for BuChE. The most significant inhibitor was acridine orange with an IC50 value of 2.18 μM and 0.22 μM towards AChE and BuChE, respectively. With DAAO, the dyes generally displayed good inhibition, with sixteen potent inhibitors and five inhibitors with sub-micromolar IC50 values. The two most potent inhibitors were hematoxylin and phenoxazine, which displayed IC50 values of 0.3 μM and 0.319 μM, respectively. Inhibition of DAO was less potent than the previous two enzymes, but there were nine dye compounds that exhibited potent inhibition. However, no compound had a sub-micromolar IC50 value. The two most potent inhibitors were darrow red and Nile blue, which displayed IC50 values of 2.33 μM and 2.45 μM, respectively. None of the dye compounds other than purpurin displayed significant inhibition of COMT. Purpurin displayed an IC50 value of 8.2 μM for the inhibition of COMT, and is therefore a moderately potent inhibitor. Purpurin possesses the catechol group as part of its structure, which confirms that compounds with catechol and nitro-catechol moieties often inhibit the COMT enzyme. For XO, a series of compounds from an in-house library were tested for inhibition. Most compounds did not inhibit XO with only six diplaying activity. Juglone and menadione displayed potent inhibition towards XO with IC50 values of 3.81 μM and 8.54 μM, respectively. Interestingly, four of the six compounds that displayed activity are naphthoquinones which represents a new class of XO inhibitors. 1,4-Naphthoquinone displayed an IC50 value of 29.3 μM. Nitrocatcehol compounds were also tested and were found to inhibit XO, with tolcapone displaying an IC50 value of 40.9 μM. This study shows that several of the dye compounds inhibit AChE, BuChE, DAAO and DAO. Methylene green, new methylene blue N and Nile blue are particularly noteworthy because these compounds possess IC50 values less than 10 μM for all these enzymes, and often have sub-micromolar IC50 values. The literature shows that these three dyes are also potent inhibitors of MAO. Thus, it can be concluded that certain dye compounds inhibit a wide variety of enzymes that are relevant to the pathology of neurological disorders.