HOW ZINC ENTERS MITOCHONDRIA

G. Beutner, J.J. Senn, R.A. Gross and S-S. Sheu
University of Rochester, SMD, Department of Pharmacology and Physiology, Rochester, NY  14642

Zinc plays a crucial role in maintaining cellular function, but micromolar intracellular concentrations of Zn2+ are toxic.  Addition of 0.5 to 50 mM Zn2+ to isolated liver mitochondria resulted in swelling and the release of cytochrome c.  Maximal swelling was observed with 10 mM Zn2+. Mitochondrial swelling was inhibited N-methyl-valin cyclosporin. Incubation with 5 to 10 mM Ca2+ for 5 minutes prior to the addition of 1 to 5 mM Zn2+ resulted in enhanced swelling compared to what was observed with either Zn2+ concentration alone. This enhanced effect was also observed when mitochondria were first preincubated with Zn2+ followed by the addition of Ca2+. This suggests that both uptake mechanisms do not interfere and Ca2+ is not able to inhibit Zn2+ uptake or vice versa. 

To address the question how Zn2+ enters mitochondria different inhibitors of possible ion transport proteins were used. Ruthenium red (RR) inhibited Zn2+ induced swelling, but the concentration necessary for inhibition of Zn2+ induced swelling was much higher than for Ca2+ induced swelling.  RR did not inhibited Zn2+ uptake as measured with the Zn2+ specific fluorescence dye Newport Green. Mg2+, which inhibits the calcium uniporter and the permeability transition pore, did not prevent Zn2+-mediated swelling and Zn2+ uptake. However, CGP37157, a specific inhibitor of the sodium-calcium exchanger suppressed Zn2+ mediated, but not Ca2+ mediated swelling. Finally, in sodium-free medium, Zn2+ resulted in a swelling but to a lesser degree and this effect was not to inhibit with CGP37157. These data indicate that mitochondrial zinc uptake may occur by the sodium-dependent mechanism.

Supported by AHA grant 9920244T and NS19163.

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