The CMP-stimulated production of diacylglycerol and CDPdiacylglycerol in neuronal nuclei labelled with radioactive arachidonate

Phosphatidylinositol synthetase activities in neuronal nuclei and microsomal fractions isolated from immature rabbit cerebral cortex

The synthesis of phosphatidylinositol was studied using a nuclear fraction N1, a microsomal fraction P3, rough (R) and smooth (S) microsomal fractions and a microsomal fraction P derived from isolated nerve cell bodies. Each fraction was prepared using cerebral cortices of 15-day-old rabbits. In assays using CDP-diacylglycerol (prepared from egg phosphatidylcholine) and myo[3H]inositol at pH 7.4, fraction N1 had the highest maximal specific rates of phosphatidylinositol synthetase (EC 2.7.8.11) (expressed per mumol phospholipid in the fraction). However the three microsomal fractions achieved maximal specific activities at liponucleotide concentrations close to 50 microM, while fraction N1 required 200 microM concentrations. In certain cases (25-120 microM CDP-diacylglycerol, and at higher pH values) fraction R had specific activities which equalled or surpassed those of N1. However, with respect to inositol, fraction N1 had a distinctly lower Km than was shown for fractions R or P3. Each of the microsomal fractions and N1 required Mg2+ for the reaction, but for N1, maximal rates could be sustained at 0.1 mM, while for the microsomal fractions the optimal Mg2+ concentration was 1 mM. For each fraction Mn2+ could not replace Mg2+ in the reaction and Mn2+ was inhibitory. The optimal pH for the reaction was between 8.0 and 9.0. Phosphatidylinositol synthetase could also be shown using fraction N1 enriched in endogenous CDP-diacylglycerol. The relatively high specific activities of fraction N1, and the differences found between N1 and the microsomal fractions, for optimal CDP-diacylglycerol and Mg2+ concentrations and for Km values for inositol, support the existence of a neuronal nuclear phosphatidylinositol synthetase.

The incorporation of fatty acids into triacylglycerols of isolated neuronal nuclear envelopes: the influence of thiol reducing reagents and chromatin

Using [3H]arachidonate, ATP, coenzyme A, MgCl2, EGTA and CMP, triacylglycerols were labelled in an isolated neuronal nuclear fraction, N1 (from immature rabbit cerebral cortex). When the radioactive nuclear fraction N1 was subfractionated, 75% of the labelled triacylglycerol product was located in the nuclear envelope fraction, E, indicating that the fatty acid incorporation was taking place at the nuclear membrane. However, when nuclear envelope fraction E was first isolated and then incubated with radioactive fatty acid, a significant incorporation into triacylglycerol was found only when nuclear envelope fractions had been prepared in the presence of dithiothreitol or mercaptoethanol. The use of the thiol compounds during the isolation of nuclear envelope fraction E led to specific incorporation rates (based on phospholipid content) which were at best 45-56% of the corresponding values seen for the parent nuclear N1 fraction. This was seen for nuclear envelope fractions isolated by two different procedures. Specific rates for acyl-CoA synthetase and diacylglycerol generation (by cholinephosphotransferase) were measured in nuclear envelope fractions and found to be similar to specific rates for these enzymes in the nuclear N1 fraction. The deficiency in triacylglycerol labelling in nuclear envelope fractions was likely due to impaired diacylglycerol acyltransferase activity. Higher specific rates of triacylglycerol labelling (82-90% of N1 values) were seen in nuclear envelope fractions assayed very shortly after preparation and in another subfraction of nuclear fraction N1 which contained small amounts of phospholipid and high concentrations of nucleates and protein. These data suggest that triacylglycerol formation may be maintained by the presence of chromatin, while in its absence there is a loss of acylation activity in nuclear envelope fractions.

Mechanism of arachidonic acid liberation during ischemia in gerbil cerebral cortex

Once brain ischemia was induced in the gerbil cerebral fronto-parietal cortex, serial changes occurred in energy metabolites and various lipids. The amounts of inositol-containing phospholipids began to decrease immediately after energy failure, followed by an increase in the amount of 1,2-diacylglycerol with a subsequent liberation of arachidonic acid and other free fatty acids. The fatty acid compositions of inositol-containing phospholipids, of 1,2-diacylglycerols produced by ischemia, and of free fatty acids liberated during ischemia were quite similar. The amount of stearic acid liberated was much larger than that of arachidonic acid between 30 s and 1 min of ischemia. On the other hand, there was no significant decrease in the amount of the other phospholipids except for phosphatidic acid. Furthermore, there was also no change in the fatty acid composition of phosphatidylcholine or phosphatidylethanolamine throughout 15 min of ischemia. The amount of cytidine-monophosphate reached a peak (36.7 nmol/g wet wt) at 2 min of ischemia. These results indicated that arachidonic acid was predominantly liberated from inositol-containing phospholipids by phospholipase C, and by the diglyceride lipase and monoglyceride lipase system rather than from phosphatidylcholine or phosphatidylethanolamine by phospholipase A2 or plasmalogenase or choline phosphotransferase during the early period of ischemia.