Slow Ca²⁺‐induced inactive/active transition of the energy‐dependent Ca²⁺ transporting system of rat liver mitochondria: clue for Ca²⁺ influx cooperativity

Rat liver mitochondria essentially free of endogenous Ca2+ show low initial rate of energy-dependent Ca2+ uptake. Preincubation of mitochondria under de-energized conditions in the presence of small amounts of external Ca2+ results in a 8-10-fold time-dependent increase of energy-dependent Ca2+ uptake. Ca2+-dependent activation of the Ca2+-transporting system follows first-order kinetics (t1/2 approximately 1 min in the presence of 5 microM Ca2+ at 20 degrees C). Ca2+-activated mitochondria demonstrate a simple hyperbolic initial rate-Ca2+ concentration dependence, whereas strong apparent cooperativity is observed in the velocity-substrate curves for Ca2+-depleted mitochondria. It is concluded that apparent cooperativity of the energy-dependent Ca2+ uptake is due to slow (as compared with the 'turnover number') activation of a Ca2+-specific uniporter which is inactive in the absence of external Ca2+.