A silent agonist of α7 nicotinic acetylcholine receptors modulates inflammation ex vivo and attenuates EAE

Nicotinic acetylcholine receptors (nAChRs) are best known to function as ligand-gated ion channels in the nervous system. However, recent evidence suggests that nicotine modulates inflammation by desensitizing non-neuronal nAChRs, rather than by inducing channel opening. Silent agonists are molecules that selectively induce the desensitized state of nAChRs while producing little or no channel opening. A silent agonist of α7 nAChRs has recently been shown to reduce inflammation in an animal model of inflammatory pain. The objective of this study was to determine whether a silent agonist of α7 nAChRs can also effectively modulate inflammation and disease manifestation in an animal model of multiple sclerosis. We first evaluated the effects of various nAChR ligands and of an α7 nAChR-selective silent agonist, 1-ethyl-4-(3-(bromo)phenyl)piperazine (m-bromo PEP), on the modulation of mouse bone marrow-derived monocyte/macrophage (BMDM) numbers, phenotype and cytokine production. The non-competitive antagonist mecamylamine and the silent agonist m-bromo PEP reduced pro-inflammatory BMDM numbers by affecting their viability and proliferation. Both molecules also significantly reduced cytokine production by mouse BMDMs and significantly ameliorated disease in experimental autoimmune encephalomyelitis. Finally, m-bromo PEP also reduced chronic inflammatory pain in mice. Taken together, our results further support the hypothesis that nAChRs may modulate inflammation via receptor desensitization rather than channel opening. α7 nAChR-selective silent agonists may thus be a novel source of anti-inflammatory compounds that could be used for the treatment of inflammatory disorders.

The genetics of vitamin C loss in vertebrates

Vitamin C (ascorbic acid) plays important roles as an anti-oxidant and in collagen synthesis. These important roles, and the relatively large amounts of vitamin C required daily, likely explain why most vertebrate species are able to synthesize this compound. Surprisingly, many species, such as teleost fishes, anthropoid primates, guinea pigs, as well as some bat and Passeriformes bird species, have lost the capacity to synthesize it. Here, we review the genetic bases behind the repeated losses in the ability to synthesize vitamin C as well as their implications. In all cases so far studied, the inability to synthesize vitamin C is due to mutations in the L-gulono-γ-lactone oxidase (GLO) gene which codes for the enzyme responsible for catalyzing the last step of vitamin C biosynthesis. The bias for mutations in this particular gene is likely due to the fact that losing it only affects vitamin C production. Whereas the GLO gene mutations in fish, anthropoid primates and guinea pigs are irreversible, some of the GLO pseudogenes found in bat species have been shown to be reactivated during evolution. The same phenomenon is thought to have occurred in some Passeriformes bird species. Interestingly, these GLO gene losses and reactivations are unrelated to the diet of the species involved. This suggests that losing the ability to make vitamin C is a neutral trait.

Comparative studies of glutathione reductase and lipoamide dehydrogenase

1. Glutathione reductase and lipoamide dehydrogenase are structurally and mechanistically related flavoenzymes catalyzing various one and two electron transfer reactions between NAD(P)H and substrates with different structures. 2. The two enzymes differ in their coenzyme and functional specificities. Lipoamide dehydrogenase shows higher coenzyme preference while glutathione reductase displays greater functional specificity. 3. Binding preference of the two flavoenzymes for nicotinamide coenzymes is demonstrated by 31P-NMR spectroscopy. 4. The presence of arginines in glutathione reductase which is inactivated by phenyl glyoxal, is likely to be responsible for the NADPH-activity of glutathione reductase. 5. The substrate binding sites of the two enzymes are similar, though their functional details differ. 6. The active-site histidine of glutathione reductase functions primarily as the proton donor during catalysis. While the active-site histidine of lipoamide dehydrogenase stabilizes the thiolate anion intermediate and relays a proton in the catalytic process.

Multifunctional activities of yeast glutathione reductase

Yeast glutathione reductase exists in a single molecular form which exhibits preferred NADPH and weak NADH linked multifunctional activities. Kinetic parameters for the NADPH and NADH linked reductase, transhydrogenase, electron transferase and diaphorase reactions have been determined. The functional preference for the NADPH linked reductase reaction is kinetically related to the high catalytic efficiency and low dissociation constants for substrates. NADP+ and NAD+ may interact with two different sites or different kinetic forms of the enzyme. The active site disulfide and histidine are required for the reductase activity but are not essential to the transhydrogenase, electron transferase and diaphorase activities. Amidation of carboxyl groups and Co(II) chelation of glutathione reductase facilitate the electron transferase reaction presumably by encouraging the formation of an anionic flavosemiquinone.

Dye-sensitized photo-oxidation of enzymes

Heart lipoamide dehydrogenase, liver alcohol dehydrogenase and egg-white lysozyme are photo-oxidized in the presence of various dye sensitizers. The photodynamic process is preceded by the binding between the enzyme and the sensitizers. Among the commonly used dyes, halogenated xanthines and thiazine are effective sensitizers for the photo-inactivation of these three enzymes. Histidine residues are the primary target for the sensitized photo-oxidation that inactivates lipoamide dehydrogenase and alcohol dehydrogenase. However, the destruction of tryptophan residues is responsible for the photo-inactivation of lysozyme. The deuterium medium effect and the quenching effect by various scavengers of the potential photo-oxidative intermediates implicate the participation of the mixed type I-type II mechanism, with the involvement of singlet oxygen being of greater importance, in the photo-inactivation of the enzymes.

Triazene metabolism. I. The effect of substituents in the aryl group on the kinetics of enzyme-catalysed N-demethylation of 1-aryl-3,3-dimethyltriazenes

A series of antitumor dimethylaryltriazenes (ArN==N . NMe2) have been studied with respect to enzyme catalysed N-demethylation by liver microsomes and the Km values determined by Lineweaver-Burk treatment. The substituent in the aryl group of the triazene does not significantly effect the magnitude of Km, which is of the same order of magnitude as the Km for aminopyrine. On the other hand, dimethyltriazenes appear to have lower Km values for demethylation than the structurally similar dimethylnitrosamines. Monomethyltriazenes, the proposed active metabolites of the dimethyltriazenes, do not undergo appreciable demethylation in the presence of microsomes and it appears that the dimethyltriazenes only demethylate once during metabolism. Spontaneous formaldehyde release from the hydroxymethyltriazene in the presence of the Nash reagent prevented an analogous study of the metabolism of these compounds.

Triazene metabolism. II. Transportability and enzyme inhibitory characteristics of metabolites of antitumor dimethyltriazenes

A series of dimethyltriazenes (Ar . N==N . NMe2), monomethyltriazenes (Ar . N==N . NH . Me), and triazene carbinolamines (Ar . N==N . NMe . CH2OH) have been studied for their inhibitory effects on the enzyme, hog liver esterase (EC 3.1.1.1). p-Nitrophenyl acetate was used as the model substrate and the rate of hydrolysis was followed spectrophotometrically at 400 nm, the absorption maximum of the p-nitrophenylate ion. Only triazenes with an ester group in the aryl moiety exhibited significant inhibition. Preincubation of the enzyme with the monomethyltriazene (McO2C . C6H4 . N==N . NHMe) or the methyloltriazene (MeO2C . C6H4 . N==NMe . CH2OH) gave enhanced inhibition, which was not observed when the dimethyltriazene (MeO2C . C6H4 . N==N . NMe2) was preincubated with the enzyme. Inhibition of enzyme activity by the monomethyltriazene was shown to be essentially irreversible, whereas the model substrates, methyl benzoate and methyl p-aminobenzoate, gave reversible inhibition in the same assay. The inhibition by the N-methyloltriazene was only partially reversible. . The results are discussed with relevance to the role of the monomethyltriazenes and N-methyloltriazenes as possible transportable metabolites of the antitumour dimethyltriazenes.