Mitochondria isolated from the ascites form of L1210 mouse leukemia cells readily accumulate Ca2+ from the suspending medium and eject H+ during oxidation of succinate in the presence of phosphate and Mg2+, with normal stoichiometry between Ca2+ uptake and electron transport. Ca2+ loads up to 1600 ng atoms per mg of protein are attained. As is the case in mitochondria from normal tissues, Ca2+ uptake takes precedence over oxidative phosphorylation. However, Ca2+ transport by the L1210 mitochondria is unusual in other respects, which may have general significance in tumor cells. The apparent affinity of the L1210 mitochondria for Ca2+ in stimulation of oxygen uptake is about 3 times greater than in normal liver mitochondria; the maximal rate of Ca2+ transport is also considerably higher. When Ca2+ pulses are added to L1210 mitochondria in the absence of phosphate or other permeant anions, much larger amounts of Ca2+ are bound and H+ ejected per atom of oxygen consumed than in the presence of phosphate; up to 7 Ca2+ ions are bound per pair of electrons passing each energy conserving site of the electron transport chain. Such 'superstoichiometry' of Ca2+ uptake can be accounted for by two distinct types of respiration dependent interaction of Ca2+ with the L1210 mitochondria. One is the stimulation of oxygen consumption, which is achieved by relatively low concentrations of Ca2+ [K(m)= 8μM] and is accompanied by binding of Ca2+ up to 40 ng atoms per mg of protein. The second process, also dependent on electron transport, is the binding of further Ca2+ from the medium in exchange with previously stored membrane bound protons, in which the affinity for Ca2+ is much lower [K(m)= 120 μM].
|Original language||English (US)|
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - 1973|
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