Sergey O. Sablin

Antihistamine agent Dimebon as a novel neuroprotector and a cognition enhancer.

Abstract: Dimebon, launched earlier in Russia as an antihistamine drug, was evaluated as a representative of a new generation of anti‐Alzheimers drugs that have two beneficial actions: (1) to alleviate symptoms, and (2) to prevent progression of the disease. The drug demonstrated cognition and memory‐enhancing properties in the active avoidance test in rats treated with the neurotoxin AF64A, which selectively destroys cholinergic neurons. Dimebon protected neurons in the cerebellum cell culture against the neurotoxic action of β‐amyloid fragment (Aβ25−35, EC50= 25 μM). In vitro, Dimebon displayed Ca2+‐blocking properties (IC50= 57 μM, on isolated rat ileum intestine) and pronounced anticholinesterase activity (IC50= 7.9 μM and 42 μM for butyrylcholine esterase and acetylcholine esterase, respectively). It also exhibited strong anti‐NMDA activity in the prevention of NMDA‐induced seizures in mice (EC50= 42 ± 6 mg/kg i.p.). A beneficial effect of Dimebon in the therapy of Alzheimers disease was demonstrated in a pilot clinical trial performed in the Moscow Center of Gerontology. Fourteen patients who participated in the trial were evaluated for their state of personality and for the severity of the disease. The evaluation included orientation (space, place, time, and patient personality), memory for the past and present, life in present, speech, irritability, and so forth. During and after the eight‐week therapy with Dimebon, cognitive and self‐service functions of patients improved significantly, and psychopathic symptoms, anxiety, depression, tearfulness, and headache were substantially diminished. The results of these studies suggest Dimebon as a new candidate for the therapy of Alzheimers‐like disorders.

Mitochondria as a Target for Neurotoxins and Neuroprotective Agents

Abstract: Mitochondrial permeability transition pores represent a multiprotein complex that includes components of both inner and outer membrane. The pores regulate transport of ions and peptides in and out of mitochondria, and their regulation is associated with a general mechanism for maintaining Ca2+ homeostasis in the cell and apoptosis. Various pathologic factors may induce a pathologic activation of the permeability transition and an irreversible opening of mitochondria pores. This event is a major step in the development of neurotoxicity and neurodegeneration. This paper explores the effect of MPP+ and β‐amyloid fragment 25‐35, neurotoxins that are known to generate Parkinsons‐like syndrome and Alzheimers disease, on the regulation of the mitochondrial pores. Both neurotoxins induce opening of mitochondrial pores, which is prevented by cyclosporin A, a specific inhibitor of the permeability transition. The effect of MPP+ and β‐amyloid may be also prevented by an endogenous precursor of melatonin, N‐acetylserotonin, by an anti‐Alzheimers medication tacrine, and by dimebon, which is in development as an agent for the therapy of Alzheimers disease and other types of dementia. The paper illustrates that the effect on mitochondrial pores is an important aspect of the mechanism of neurotoxicity. Substances that may prevent opening of mitochondrial pores induced by neurotoxins may preserve the mitochondrial function and, thus, may have potential as neuroprotective agents.

Effect of Methylene Blue and Related Redox Dyes on Monoamine Oxidase Activity; Rat Pineal Content of N-Acetylserotonin, Melatonin, and Related Indoles; and Righting Reflex in Melatonin-Primed Frogs

The ability of methylene blue (MB) to inhibit the nitric oxide–induced stimulation of N‐methyl‐d‐aspartate receptors has been suggested as a possible mechanism of MBs clinical antidepressant action. This study evaluated the alternative/additional mechanisms of the antidepressant effect of MB on biochemical and behavior levels. Selective inhibition of monoamine oxidase type A (MAO‐A) is widely accepted as a major mechanism of the clinical antidepressant effect. MB and the related redox dyes toluidine blue O (TBO), thionine (TN), brilliant cresyl blue (BCB), and toluylene blue (TB) were reversible competitive inhibitors of both MAO‐A and MAO‐B and were highly selective toward MAO‐A. TBO was the most potent inhibitor, followed by TN, BCB, MB, and TB. The dyes studied increased rat pineal N‐acetylserotonin (NAS) and melatonin content, in accordance with our previous observations of the stimulating effect of selective inhibition of MAO‐A on pineal melatonin biosynthesis. The redox dyes exerted antidepressant‐like activity in frogs; that is, they suppressed the righting reflex in melatonin‐primed frogs. This studys results indicate that selective inhibition of MAO‐A might mediate the clinical antidepressant effect of MB through NAS stimulation and melatonin biosynthesis.

Reactivation of NADH Dehydrogenase (Complex I) Inhibited by 1-Methyl-4-(4′-Alkylphenyl)pyridinium Analogues: A Clue to the Nature of the Inhibition Site

Abstract: Expression of the neurotoxicity of 1‐methyl‐4‐phenyl‐1.2,3,6‐tetrahydropyridine, following oxidation to l‐methyl‐4‐phenylpyridinium ion (MPP+), is believed to involve inhibition of mitochondrial electron transport from NADH dehydrogenase (complex l) to ubquinone. MPP+ and its analogues have been shown to Mock electron transport at or near the same site as two powerful inhibitors of mitochondrial respiration, rotenone and piericidin A. All three types of inhibitors combine at two sites on NADH dehydrogenase, a hydrophilic and hydrophobic one, and occupancy of both sites is required for complete inhibition. Tetraphenylboron anion (TPB−) in catalytic amounts is known to increase the effectiveness of positively charged MPP+ analogues in blodclng mitochondrial respiration. A part of this effect involves facitation of the entry of MPP+ oongeners into the hydrophobic site by ion pairing, as has been demonstrated in studies with submitochondrial particles (electron transport particles). This communication documents the fact that TPB−, when present in molar excess over the MPP+ analogues, reverses the inhibition. This seems to involve again strong ion pairing. removal of the inhibitory analogue from one to the two binding sites, and concentration of the inhibitor in the membrane, so that only the hydrophobic binding site remains occupied, resulting in lowering of the inhibiti to 30–40%.

Chapter 3 Redox properties of the flavin cofactor of monoamine oxidases A and B and their relationship to the kinetic mechanism

Publisher Summary This chapter discusses the redox properties of the flavin cofactor of monoamine oxidases (MAO) A and B and their relationship to the kinetic mechanism that they follow. The purified enzyme preparations used in this study were the human liver MAO A expressed in yeast and bovine liver MAO B. The redox potentials in the absence of substrate were determined by the method developed by Massey. The redox potentials of cysteinyl-FAD in unliganded MAO A and B have been determined. The spectral changes were observed in a mixture of MAO B and a reference dye, indigo disulfonate. Reduction of the dye is characterized by the disappearance of the peak at 610 nm observed for the oxidized dye and the appearance of an equally prominent peak at 370 nm for the reduced dye. These changes are superimposed on the decrease at 456 nm observed for the reduction of cysteinyl-FAD from the oxidized form to a semiquinone. The redox potentials of the cysteinyl-FAD in unliganded MAO A and B are close to that for free flavin. It has been concluded that the redox potential in an enzyme-substrate complex is positively shifted towards the potential of an amine substrate. This shift is different for each substrate. The values for the redox potentials for MAO in the presence of physiological substrates remain to be determined. The rate of reduction of the flavin by substrate correlates with the redox potential—that is, an increase in the potential of the flavin favors electron transfer from amine to flavin.

Biochemical basis of structural electivity of MPTP-like neurotoxicity.

The key steps of the mechanism of neurodegenerative action of MITP are the oxidation of tetrahydropyridine fi;lgment to pyridinium metabolite catalyzed by monoamine oxidases (MAO) and its interaction with DA-uptake system. In the present paper we have studied the structural kinetic regularities defining the efficiency of these processes for the series of MAIT and h4TS analogues of MPTP (TABLE 1).