The de novo synthesis of molecular species of phosphatidylinositol from endogenously labeled CDP diacylglycerol in alveolar macrophage microsomes.

The de novo synthesis of molecular species of phosphatidylinositol (PI) from endogenously labeled CDP diacylglycerol (CDP-DG) and phosphatidic acid (PA), with [14C]-glycerol 3-phosphate, in microsomes of macrophages was studied using a recently developed HPLC technique. Endogenously labeled PA, CDP-DG, and PI were sequentially formed from labeled glycerol 3-phosphate through the addition of CoA, CTP, and then inositol into microsomes. The rate of formation of CDP-DG from endogenously labeled PA was low as compared with those of PA and PI. The low rate of CDP-DG synthesis suggests that it may be the rate-limiting step in the de novo synthesis of PI. Analysis of newly synthesized molecular species of PI by HPLC revealed that large proportions of radioactivity were associated with the 16:0-18:1, 16:0-18:2, 18:1-18:2, and 18:2-18:2 species, and a small amount, 2-3%, of radioactivity was associated with the 18:0-20:4 species. The profiles of newly synthesized PA and CDP-DG species were quite similar to those of PI species. This suggests that the enzymes involved in the formation of PI species from glycerol 3-phosphate show little specificity toward different molecular species of substrates. The results of the present study also suggest that free fatty acid composition in microsomes greatly affect the composition of the molecular species of PI synthesized through the de novo pathway, since the proportion of fatty acids utilized for the de novo synthesis of PI species was similar to that of free fatty acids in the microsomal membrane.