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.

Polyamines improve Ca2+ transport system of the yeast mitochondria.

Spermine at concentrations of 12-100 microM considerably activates the Ca2+ transport system of the Endomyces magnusii yeast mitochondria. As a result, in the presence of spermine the mitochondria are able to decrease extramitochondrial Ca2+ to the physiological level. At Ca2+ concentrations up to 200 microM, spermine enhances the initial rate of Ca2+ uptake (a half-maximal effect at 12 microM spermine). The Ca2+ concentrations required for half-maximal Ca2+ uptake rate to be achieved were 160 and 60 microM Ca2+ without and with spermine, respectively. Spermidine is shown to be less effective (a half-maximal effect at 50-100 microM spermidine). The polyamines do not change the parameters of energy coupling of mitochondria. The data obtained enabled the yeast mitochondria to be considered to take part in regulation of cytoplasmic and matrix Ca2+.