The occurrence and metabolism of protein-bound phosphoinositides in several lipid-protein complexes from brain

Abstract

Phosphoinositides have been found to occur bound to brain proteins not only in phosphatidopeptides but also in proteolipids and in a water-soluble viscous protein fraction extracted from the lipid-free tissue residue with dilute aqueous acid. In these fractions, the incorporation of 32P into the inositol phosphatide occurs at the relatively rapid rate previously demonstrated for phosphatidopeptides. The incorporation of labeled phosphate in vivo into phosphatidopeptides has been studied in rats of different ages and in adult rats at different times after injection. Comparison of this incorporation with the parallel incorporation into water-soluble phosphate fractions and the total lipid fraction suggested that the incorporation is not limited by the rate of passage of labeled phosphate into the brain from the blood. Maximum specific activity occurs at about 4 h. The pattern of incorporation at different ages and times indicated that, despite the fact that these protein-bound phosphoinositidesare more abundant in white matter, they are not a stable constituent of myelin. The data presented here correlate reasonably well with previously published data for the phosphorus metabolism of the phosphoinositides obtained from total lipid extracts. It is suggested that and protein-bound phosphoinositide acts as a bridge between the protein and less closely associated lipids in some of the membrane structures of brain white matter and that the phosphorus metabolism may reflect membrane activity during function.