Synthesis of molecular classes of cytidine diphosphate diglyceride by rat liver in vivo and in vitro.

Abstract

Cytidine diphosphate (CDP) diglyceride isolated from pooled rat liver was shown like phosphatidyl inositol to contain stearate and arachidonate as the major fatty acids. On the other hand palmitate and oleate were the predominant fatty acids of total liver phosphatidic acid.Labelling of molecular classes of phosphatidic acid, CDP-diglyceride, and phosphatidyl inositol was examined in liver at various time intervals after either intraportal or intrajugular injections of [3H]glycerol. In general the major flux of radioactivity was through the oligoenoic classes [Δ1 and Δ2] of phosphatidic acid. In contrast the labelling of oligoenoic classes of CDP-diglyceride was less and of polyenoic classes [Formula: see text] significantly enhanced, raising the possibility that there may be some discrimination in selection of species of phosphatidic acid for liponucleotide synthesis. After intrajugular injections of isotope the monoenoic classes of phosphatidyl inositol were most highly labelled but between 5 and 240 min there was a progressive decrease of radioactivity in this class and increase in the tetraenoic species, presumably as a result of deacylation and reacylation reactions. The backflow of radioactivity from phosphatidyl inositol was deemed not a likely explanation for the observed labelling pattern of CDP-diglyceride.The conversion of sonicated dispersions of [3H]phosphatidic acid, labelled in the glycerol moiety, to CDP-diglyceride by rat liver microsomes was examined. There was slightly less label in the oligoenoic classes and slightly more in trienoic and polyenoic (> Δ4) classes of CDP-diglyceride as compared with phosphatidic acid. These differences were much smaller in vitro than those observed in vivo. The in vitro data provide no convincing evidence for a high enough selectivity to explain the differences in arachidonate content between phosphatidic acid and the liponucleotide.