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 ( in the presence of 5 μM Ca2+ at 20°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+.