Yeast mitochondrial calcium uptake: regulation by polyamines and magnesium ions.

Spermine, spermidine, and magnesium ions modulate the kinetic parameters of the Ca2+ transport system of Endomyces magnusii mitochondria. Mg2+ at concentrations up to 5 mM partially inhibits Ca2+ transport with a half-maximal inhibiting concentration of approximately 0.5 mM. In the presence of 2 mM MgCl2, the S0.5 value of the Ca2+ transport system increases from 220 to 490 microM, which indicates decreased affinity for the system. Spermine and spermidine exert an activating effect, having half-maximal concentrations of 12 and 50 microM, respectively. In the case of spermine, the S0.5 value falls to 50-65 microM, which implies an increase in the transport system affinity for Ca2+. Both Mg2+ and spermine cause a decrease of the Hill coefficient, giving evidence for a smaller degree of cooperativity. Spermine and spermidine enable yeast mitochondria to remove Ca2+ from the media completely. In contrast, Mg2+ lowers the mitochondrial buffer capacity. When both Mg2+ and spermine are present in the medium, their effects on the S0.5 value and the free extramitochondrial Ca2+ concentration are additive. The ability of spermine and Mg2+ to regulate yeast mitochondrial Ca2+ transport is discussed.