Cardiolipin and Other Phospholipids in Ox Liver By MARJORIE

The objects of this work were to find whether the polyglycerophosphatide detected in liver extracts had the same structure as cardiolipin isolated from ox-heart muscle, and to examine the fatty acid composition of the phospholipids. The molar ratios glycerol:phosphorus:fatty acid reported for the glycerophosphatide (McKibbin & Taylor, 1952; Dawson, 1958; Marinetti, Erbland, Albrecht & Stotz, 1957) differed from those found for cardiolipin by Pangborn (1947), who considered cardiolipin to be a fully acylated polyglycerophosphatide with four glycerol, three phosphoric acid and six fatty acid residues. However, evidence in favourofthestructurebis(diacylglycerophosphoryl)glycerol (diphosphatidylglycerol) for cardiolipin was obtained by oxidative degradation of the deacylated polyglycerophosphate, and by the identification of diglycerides and glycerol diphosphate as the main products of hydrolysis of cardiolipin in 90% acetic acid (Macfarlane, 1958; Macfarlane & Wheeldon, 1959). The amount of polyglycerophosphatide isolated by McKibbin & Taylor (1952) was about 0-5% of the total phospholipid. Hanahan, Dittmer & Warashina (1957) fractionated the phospholipids of rat and ox liver on silicic acid and found about 2% of the total phosphorus in a fast-running nitrogen-low fraction and about 7 % as inositide. While the present work was in progress, Faure, Morelec-Coulon, Mar6chal & LeBorgne (1959) isolated from ox liver, by solvent fractionation, a polyglycerophosphatide with an iodine value and a serological activity similar to that of cardiolipin, and with molar ratios of glycerol: phosphorus: fatty acid of 3: 2: 4; they made no comment on the structure and considered that the acidic phospholipid fraction contained phosphatides other than cardiolipin and phosphatidylinositol. The amount of cardiolipin and inositide obtainod by these authors was apparently equivalent to about 1-1 and 1-4% respectively of the total phospholipid phosphorus, on average figures for water and lipidphosphate content of liver. At the same time Hubscher & Clark (1959) reported briefly the isolation from ox liver of phosphatidic acid, in a yield of 0-5-1.0% of the total lipid phosphorus; this is of considerable interest since, although other workers have not found phosphatidic acid in tissue extracts, Hokin & Hokin (1958) detected its presence by heavy labelling with 32p in vivo. In an attempt to resolve some of the discrepancies, the acidic phospholipids were fractionated by chromatography on silicic acid and also by solvent fractionation of the free acids on the lines used by Faure et al. (1959). By both methods, described below, cardiolipin was isolated in amounts equivalent to about 1-2% of the total phospholipid phosphorus, but unequivocal evidence for the presence of naturally occurring phosphatidic acid was not obtained. The discrepancy between these results and those of Hubscher & Clark (1959, 1960) is discussed later.