The distribution of phosphatidylinositol in microsomal membranes from rat liver after biosynthesis de novo. Evidence for the existence of different pools of microsomal phosphatidylinositol by the use of phosphatidylinositol-exchangeprotein

The phosphatidylinositol-exchange protein from bovine brain was used to determine to what extent phosphatidylinositol in rat liver microsomal membranes is available for transfer. The microsomal membranes used in the transfer reaction contained either phosphatidy1[2-³H]inositol or ³²P-labelled phospholipid. The ³²P-labelled microsomal membranes were isolated from rat liver after an intraperitoneal injection of [³²P]PI. The ³H-labelled microsomal membranes and rough- and smooth-endoplasmic-reticulum membranes were prepared in vitro by the incorporation of myo-[2-³H]inositol into phosphatidylinositol by either exchange in the presence of Mn²⁺ or biosynthesis de novo in the presence of CTP and Mg²⁺. Tryptic or chymotryptic treatment of the microsomes impaired the biosynthesis de novo of phosphatidylinositol. It was therefore concluded that the biosynthesis of phosphatidylinositol and/or its immediate precursor CDP-diacylglycerol takes place on the cytoplasmic surface of the microsomal membrane. Under the conditions of incubation 42% of the microsomal phosphatidy[2-³H]inositol was transferred with an estimated half-life of 5 min; 38% was transferred with an estimated half-life of about 1 hr; the remaining 20% was not transferable. Identical results were obtained irrespective of the method of myo-[2-³H]inositol incorporation. Both measurement of phosphatidylinositol phosphorus in the microsomes after transfer and the transfer of microsomal [³²P]phosphatidylinositol indicate that phosphatidy1[2-³H]-inositol formed by exchange or biosynthesis de novo was homogeneously distributed throughout the microsomal phosphatidylinositol. The authors present evidence that the slowly transferable pool of phosphatidylinositol does not represent the luminal side of the microsomal membrane; hence they suggest that this phosphatidylinositol is bound to membrane proteins.