Metal ions bound to phospholipids after isolation by silicic acid column chromatography

Abstract

(1) Total rat-liver lipid has been purified by gel filtration and fractionated on a silicic acid column. Composition of the obtained phospholipid fractions was determined by silicic acid thin-layer chromatography. The metal ion content (Na, K, Ca and Mg) was determined for crude rat-liver lipid, gel filtered liver lipid and for each of the phospholipid fractions obtained by the silicic acid column chromatography. (2) The acidic phospholipids (cardiolipin and phosphatidyl inositol) contained about 1 equivalent of metal ion per atom of phosphorus. Metal ion content of the phosphatidyl ethanolamine fraction indicated that this phospholipid contained 0.32 equivalent of metal ion per atom of phosphorus. Data for the lecithin fraction did not allow to decide if metal ions are bound to this phospholipid. (3) The total metal ion content (equiv metal ion:atom P) of the lipid recovered from the silicic acid column was more than twice as high as that of the gel filtered lipid and its composition was quite different. The amounts of K and Ca bound to the lipids were most drastically altered. Thus, K constituting about 20% of the metal ion equivalents bound to the gel filtered lipid was completely absent from the phospholipids recovered in the chromatography. In contrast, Ca which in the gel filtered lipid is present only in trace amounts made up more than 30% of the metal ion equivalents bound to all the phospholipid recovered in the chromatography. Also the amounts of Na and Mg bound to the lipids were profoundly changed by the chromatography. (4) When individual phospholipids with known cation composition were chromatographed on silicic acid, ion exchange reactions took place which were consistent with the results obtained for gel filtered rat-liver lipid. (5) Na, K, Ca and Mg could be extracted from the silicic acid with water or diluted HCl. The amounts of these metal ions in the silicic acid were compatible with the observed changes in the amounts of lipid-bound metal ions. (6) The obtained data indicate that cardiolipin, phosphatidyl ethanolamine and phosphatidyl inositol upon isolation by SiO 2 chromatography are subject to ion exchange which profoundly alters their cation composition.