The incorporation of radioactive fatty acids into the phospholipids of nerve-cell-body membranes in vivo. Evidence for highly labelled neuronal nuclei

Abstract

1. Nerve cell bodies were isolated in bulk from cerebral cortices of 15 day-old rabbits after intrathecal injections of [3H]plamitate, [3H]oleate or [3H]arachidonate and [14C]glycerol. 2. Nuclear, microsomal and two mitochondrial fractions were isolated from homogenates of the radioactively labelled nerve cell bodies by using differential and discontinuous-gradient centrifugation. 3. After 7.5min in vivo, a high percentage (>80%) of the total 3H-labelled fatty acid radioactivity was found in the membrane fractions of the nerve cell bodies, whereas after 60min in vivo 50% of the total [14C]glycerol radioactivity was found in the high-speed supernatant. 4. The specific radioactivities of phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, and the radioactivity in neutral lipid and non-esterified fatty acid fractions were determined in the four subfractions, as were the distributions of several marker enzymes and nucleates. 5. With respect of 3H-labelled fatty acid, the phospholipids of the nuclear fraction had the highest specific radioactivities of the four subfractions. However, for [14C]glycerol labelling, generally the 14C specific radioactivities for individual phospholipids were comparable in the four subfractions. This latter observation suggests transport of phospholipids synthesized de novo between membranes of the nerve cell body. 6. Double-labelling experiments demonstrated that individual phospholipids and the combined neutral lipids of the nuclear fraction had higher labelling ratios of 3H-labelled fatty acid/[14C]glycerol than did the corresponding lipids of the microsomal or mitochondrial fractions. 7. On the basis of the labelling results and the marker studies, it is proposed that it is indeed the nuclei of the nuclear fraction that have these lipids highly labelled with 3H-labelled fatty acid, and the existence of nuclear acyl transferases that are responsible for this fatty acid incorporation is suggested.