Passive Interactions of Ca2+and other Multivalent Cations with the Membranes of Isolated Corn Mitochondria

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Abstract
The presence of Ca2+ in a hypo-osmotic reaction medium reduces succinate: cytochrome c reductase activity and the release of outer membrane-specific antimycin A-insensitive NADH: cytochrome c reductase. The action of Ca2+ is non-competitive and approximately 30 mmol m−3 Ca2+ affords half-maximal (I50) protection. The effect of a range of inorganic and organic multivalent cations on succinate: cytochrome c reductase activity suggests that the action of Ca2+ is non-specific and probably involves Ca2+ binding to outer membrane component(s) which may be proteins. Valinomycin- or gramicidin-induced passive swelling of isolated corn mitochondria in isotonic K.C1 is also non-competitively inhibited by up to 50% with Ca2+. Half-maximal inhibition (I50) occurs at 0-35 mol m−3 Ca2+ for valinomycin and 1-0 mol m−3 Ca2+ for gramicidin. Other divalent cations, Mg2+, Sr2+ and Ba2+, seem to inhibit similarly while the trivalent cations La3+ and Ho3+ show a maximum inhibition of up to 85%, with an I50 of 0.1 mol m−3 for valinomycin. It is suggested that non-specific cation binding may reduce membrane fluidity thereby slowing down the rate of ionophore penetration through the inner membrane.