The cause of superstoichiometric CA2+ uptake and H+ ejection in L1210 mouse ascites tumor mitochondria

Mitochondria from L1210 mouse leukemia cells show superstoichiometric → H⁺/2e^- ejection and → Ca²⁺/2e^- uptake ratios in the absence of phosphate. Kinetic analysis of Ca²⁺-induced respiratory jumps shows a very rapid, early burst of H⁺ ejection and Ca²⁺ uptake dependent on energy-coupled electron transport but with extraordinarily high stoichiometry (→H⁺/2e^- > 20; →Ca²⁺/2e^- > 10), followed by a slower phase with normal stoichiometry (→H⁺/2e^-=4-5; →Ca²⁺/2e^-=4.0). The early burst of H⁺ ejection, which has a higher K_M for Ca²⁺ than respiratory stimulation, accounts for the superstoichiometry phenomenon; it requires energization by state 4 respiration.