THE INCORPORATION OF α-GLYCEROPHOSPHATE-32P INTO THE LIPIDS OF RAT BRAIN PREPARATIONS: II. ON THE BIOSYNTHESIS OF MONOPHOSPHOINOSITIDE

Previous investigations conducted in this laboratory showed a number of differences in the cytosine nucleotide requirement for the incorporation of α-glycerophosphate (α-G32P) into the monophosphoinositide of rat brain preparations compared to the pathway described by Paulus and Kennedy, where α-glycerophosphate → phosphatide acid → CDP-diglyceride → monophosphoinositide, and CTP is specifically required. Experiments were carried out with rat brain preparations to determine the nature of the mechanism whereby CDP-choline is as effective as or more effective than CTP in stimulating the incorporation of α-G32P into monophosphoinositide. Isotope dilution experiments in which unlabeled phosphatidic acid and CDP-diglyceride were used, yielded results consistent with the view that both of these compounds are intermediates in the incorporation of a-G32P into monophosphoinositide stimulated by either CTP or CDP-choline. Time-course experiments where cytosine nucleotides were added either at the beginning or after 20 minutes produced a pattern of labeling which could be fitted into the above interpretation, provided that newly formed radioactive molecules of phosphatide acid could be used selectively and CTP in some way inhibits phosphatide acid formation or accumulation. The latter could account for the observation that monophosphoinositide becomes far more actively labeled than phosphatidic acid in the presence of added CTP.