Isolation and characterization of cytidine diphosphate diglyceride from beef liver

Cytidine diphosphate diglyceride was isolated from beef liver by a combination of silicic acid column, DEAE-cellulose column, and this layer chromatography. The product (5.8 to 17.4 mumol/kg of liver) contained cytidine/phosphate/fatty acids in the molar proportions 1.05/2.0/2.05 (theoretical, 1.0/2.0/2.0) (average for three preparations). The liponucleotide was split quantitatively by a partially purified hydrolase from Escherichia coli, specific for CDP-diglyceride, (Raetz, C. R. H., Hirschberg, C. B., Dowhan, W., Wickner, W. T., and Kennedy, E. P. (1972) J. Biol. Chem. 247, 2245-2247) into phosphatidic acid and a water-soluble nucleotide that was chromatographically identical with CMP. No dCMP was located in these hydrolysates. The liver liponucleotide was more effective than a synthetic preparation of CDP-diglyceride in promoting the formation of phosphatidylinositol with guinea pig brain microsomes. The fatty acid composition of CDP-diglyceride was compared with metabolically related phospholipids from beef liver. The liponucleotide had a similar composition to phosphatidylinositol, characterized by a high level of stearate and with arachidonate as the major unsaturated fatty acid. The content of arachidonate in both lipids was significantly higher than that in phosphatidic acid. The profile of fatty acids of cardiolipin was quite unlike that of CDP-diglyceride. These findings suggest several alternatives for the metabolic origins of beef liver CDP-diglyceride: (a) CDP-diglyceride is formed from an atypical pool of phosphatidic acid, (b) the enzyme is selective for arachidonoyl-containing species of phosphatidic acid, (c) the liponucleotide may also be derived from phosphatidylinositol by the back-reaction of CDP-diglyceride: inositol phosphatidyltransferase.