NoteA new proposal regarding the transport mechanism of mercury in biological membranes
Graphical abstract
The mechanism for the uptake of Hg2+ in biological membranes. The mercury crosses the lysosomal membrane in the form of the electroneutral Hg(OH)2 compound. The mechanism is similar to that proposed for the uptake of weak permanent bases and the driving force for the uptake is the acidic pH (4.2) in the lysosomal membrane.
Introduction
The interactions of Hg2+ with biological systems have been widely studied, as the compound is very toxic [1], [2], [3]. The results indicate that Hg2+ interacts with many proteins, in particular with the sulfur groups [4], [5] and uncouples the oxidative phosphorylation in mitochondria [3]. However, the molecular causes responsible for the toxic effects in whole organisms have never been explained. The aim of this study is not to give new elements as an explanation for the toxicity of mercury, but to clarify the transport mechanism of Hg2+ within the biological membranes since the transport mechanism would play an essential and propaedeutic part in any subsequent toxicological studies. The above problem was examined regarding liposomes in a previous article [6]. The experimental results indicate that Hg2+ behaves as a Cl−/OH− exchanger by means of an eletroneutral process. These results indicate that the Hg(OH)2 and HgCl2 compounds are transported through the membrane. In this paper, we have examined the Hg2+ transport system in Lysosomes extracted from rat liver. The results are in agreement with those found in artificial membranes [6], as the experimental evidence suggests the transport of Hg(OH)2 through the membrane.
Section snippets
Materials and methods
Lysosomes from rat liver have been prepared following the procedure described by Savant et al. [7]. The protein concentration has been determined by the Lowry method [8]. The lysosomes, after the last centrifugation of the preparation, have been resuspended in a medium containing: 0.1 M sucrose, 50 mM K2SO4, 20 mM Hepes, pH 7.4, 2.5 mM MgSO4 and 500 μM EGTA and stored at 0 °C. The protein concentration of the resuspended lysosomes was 20 mg/ml. The dye acridine orange (AO), 1 mM, was used in
Results and discussion
The pH in the matrix is about 4, in energized lysosomes, as a consequence of three factors:
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The preexistence of a pH of 5.2 (before the addition of ATP) due to a Donnam effect [9].
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The presence of an ATP-driven proton pump (Vacuolar ATPase) [10], [11], [12], [13].
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The presence of a selective Cl− channel which, in a chloride medium, allows for the uptake of chloride ions in the matrix [10], [11], [12].
The driving force for the acidification of energized lysosomes is the vacuolar proton pump, which
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