Hyperbolic kinetics of the electrophoretic carrier of Ca2+ uptake in liver mitochondria

The use of a fast-responding Ca2+-selective electrode has allowed the simultaneous measurement of the true Ca2+-activity in the medium and of the Ca2+-transport rate, eliminating the uncertainties connected with the use of indirect methods of estimating Ca2+ transport rates (e.g. O2-stimulation, K+-efflux). The results on the kinetics of Ca2+ uptake by rat liver mitochondria obtained in this study are qualitatively similar to those obtained by Bragadin et al. [Bragadin, M., Pozzan, T. & Azzone, G. F. (1979) Biochemistry, 18, 5973--5978] by monitoring the K+-efflux from mitochondria. In the valinomycin-driven Ca2+ uptake, the Ca2+ translocase is the limiting factor, whereas in the system driven by succinate extraneous limiting factors play a role. Thus, liver mitochondria, when studied under appropriate non-limiting conditions, exhibit a hyperbolic dependence on the Ca2+ uptake rate on Ca2+-activity analogous to that of mitochondria from other tissues. The sigmoidal dependence of the uptake rate on Ca2+-activity in liver mitochondria, observed under conditions which produce hyperbolic kinetics in mitochondria from other tissues, can thus be attributed to factors which are extraneous to the Ca2+ transport system per se.