BIOSYNTHESIS OF THE MOLECULAR-SPECIES OF PHOSPHATIDYLINOSITOL FOUND IN MICROSOMES AND PLASMA-MEMBRANES FROM ETIOLATED MAIZE COLEOPTILES

Plasma membranes of high purity have been prepared by the two phase pa rtition method from etiolated maize coleoptiles. By combining HPLC and GLC, the same six molecular species of phosphatidylinositol (PI) have been found in microsomes and plasma membranes; 16:0/18:2 PI represent ed more than 75% of total PI in both membranes. PI was labelled in vit ro from [C-14]inositol with coleoptile microsomes; 16:0-18:2 PI was th e molecular species most heavily labelled (80% of total radioactivity) . In contrast, the in vitro labelling of PI with plasma membrane prepa rations represented only 5% of the labelling found in microsomes after 20 min of incubation; however 23 and 12% of the labelling found in mi crosomes have been measured in vitro with plasma membranes incubated f or 2 or 5 min respectively. This PI synthase activity (EC 2.7.8.11) of plasma membranes could be accounted for either by endoplasmic reticul um (ER) membrane contamination or by the existence of a special PI syn thase, bound to plasma membranes, and active in the recycling of diacy glycerol and inositol produced during the polyphosphoinositide cycle. Nevertheless a transfer of newly synthesized PI from ER to plasma memb ranes should be postulated for the bulk of PI located in plasma membra nes.