Isolation of a fraction with Ca2+ ionophore properties from rat liver mitochondria.
Isolation of a small protein with properties of a Ca2+ ionophore from calf heart mitochondria has recently been reported [A. Y. Jeng and A. E. Shamoo, 1980, J. Biol. Chem. 255, 6897, 6904]. We have isolated a fraction with similar physical and chemical properties from rat liver mitochondria. In particular, the hepatic preparation is able to bind Ca2+ with high affinity in such a fashion that the resultant complex is soluble in a hydrophobic phase. It will also transport Ca2+ through a stirred organic phase (Pressman cell). Interaction of the liver preparation with Ca2+ is sensitive to inhibitors of mitochondrial Ca2+ uptake. The hepatic preparation contains both protein and lipid components. The phospholipid components were identified and the behavior of a similar mixture of commercially available phospholipids was compared to that of the ionophore fraction from rat liver mitochondria. All of the Ca2+ binding properties of the rat liver preparation could be mimicked by the lipids. In a preliminary experiment, reduction of the phospholipid content of the preparation to less than one lipid phosphate per protein molecule (assuming a molecular weight of 3000 by analogy with the calf heart case) resulted in a protein that was unable to bind Ca2+. We, therefore, suggest that the ability of the preparation to interact with Ca2+ is due to the constituent phospholipids. Measurements of phospholipid-Ca2+ interactions in the model systems and under the conditions of low (microM) Ca2+ and phospholipid concentration utilized here demonstrated an affinity for Ca2+ (Ks approximately 1 microM) and a cation selectivity that have not previously been reported.
D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000302:isolation & purification* / Q000494:pharmacology; D002413:Cations, Divalent; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D007476:Ionophores / Q000302:isolation & purification*; D008297:Male; D008930:Mitochondria, Liver / Q000378:metabolism*; D010743:Phospholipids / Q000302:isolation & purification / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains
D002135:Calcium-Binding Proteins; D002413:Cations, Divalent; D007476:Ionophores; D010743:Phospholipids; C025673:calciphorin; D002118:Calcium