Elsevier

Inorganica Chimica Acta

Volume 336, 28 July 2002, Pages 163-167
Inorganica Chimica Acta

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The interaction of methylmercury with lysosomes from rat liver

https://doi.org/10.1016/S0020-1693(02)00868-XGet rights and content

Abstract

The interactions of methylmercury with rat liver lysosomes were investigated. The results indicate that methylmercury enhances the proton permeability in membranes. Since this uncoupling mechanism necessarily implies the transport of a charged ion (in this case CH3Hg+) through the membrane, this fact can help to explain why a preferential target of methylmercury are nerve cells, whose plasma membrane potential has the highest value, when compared with other cells.

Methlmercury enters inside lysosomes as electroneutral CH3Hg OH compound, the driving force being the acid inside pH. Once inside methylmercury is extruded as CH3Hg+ cation since the potential is positive inside. The balance of the cyclic mechanism is, at any circle, the transport of a proton through the membrane.

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Introduction

The presence of methylmercury in natural waters is a problem of great environmental concern since this compound possesses a high toxicity very well documented in animals and humans [1], [2], [3], [4], [5], [6].

Methylmercury is prevalently neurotoxic and studies have been performed in cells and in subcellular structures [7], [8], [9], [10] in order to establish the molecular mechanism (or mechanisms) which is responsible for the toxicity in whole animals.

Methylmercury is reported to increase cytosolic Ca++ concentration in rat cerebrum synaptosomes [7], in PC12 cells [8], and in rat T lymphocytes [9]. In other cellular systems, the inhibition of ATP synthesis and apoptosis have also been reported [10], [11].

Among the subcellular structures, mitochondria have been extensively investigated since a damage on mitochondria, which produce ATP for the cell, gives rise to a corresponding cell damage [12], [13], [14], [15], [16], [17]. In mitochondria, methylmercury interacts with all the mechanisms, which are involved in ATP synthesis. In fact, it has been reported that methylmercury inhibits the respiratory chain [14], [15], and, in the same way as uncoupler, it collapses Δψ and causes a Ca++ release [14], [15], [16]. Furthermore, it induces a cytochrome c release [17] and the opening of a transition pore [18]. In mitochondria, therefore, it is difficult to establish the effective toxic effect (the lowest dose which causes a failure in ATP synthesis) since many interaction mechanisms are present. Furthermore, the effective toxic effect in cells and, in particular, the reason why nerve cells are the preferential target for methylmercury has not yet been established.

In this paper our attention is focused on lysosomes. Up to now, the interactions of methylmercury with these organelles have never been examined and our aim is to verify whether a particular transport system of this compound is present in the lysosomal membrane.

Moreover, a study of a membrane transport mechanism in lysosomes offers some advantages since, in this biological system, many functions such as the respiratory chain, selective transition pore and antiporters are absent or present in a limited number. This facilitates the approach and could permit to propose a general peculiar behaviour also for other membranes. The results, in reality, indicate that methylmercury behaves as an uncoupler. This behaviour, being a membrane property, could explain not only one aspect of the toxicity of methylmercury since uncouplers are toxic compounds, but in particular the prevalent neurotoxicity of the compound.

Section snippets

Materials and methods

Rat liver lysosomes were prepared according to Sawant et al. [19] and resuspended before use at 0 °C in a medium (mother solution) containing 0.25 M sucrose, 10 mM Mops-Hepes pH 7.4 and 5 mM MgSO4. Acridine Orange (AO) (Merck) was purified according to Pal and Schubert [20]. Aliquots of lysosomes were withdrawn from the mother solution and added to the operating medium. The uptake of AO by lysosomes was monitored spectrophotometrically at room temperature using a Jenway 6400 spectrophotometer,

Results and discussion

In lysosomes the pH in the matrix is acid, about 5.2 as a consequence of a Donnan effect [23]. The lysosomes possess an ATP-driven proton pump, which, in the presence of ATP, pumps protons into the matrix. The acidification in lysosmes can easily be monitored using the AO dye [24]. This dye is a weak permeant base and, following the classical mechanism of weak bases, it crosses the membrane as an electroneutral compound and accumulates in the matrix, since the pH is acid inside. The

Supplementary material

The material is available from the authors on request.

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