Abstract— Labeled malonic acid ([1‐14C] and [2‐14C]) was injected into the left cerebral hemisphere of anesthetized adult rats in order to determine the metabolic fate of this dicarboxylic acid in central nervous tissue. The animals were allowed to survive for 2, 5, 10. 15 or 30min. Blood was sampled from the torcular during the experimental period and labeled metabolites were extracted from the brain after intracardiac perfusion. There was a very rapid efflux of unreacted malonate in the cerebral venous blood. Labeled CO2 was recovered from the venous blood and the respired air after the injection of [1‐14C]malonate but not after [2‐14C]malonate. The tissue extracts prepared from the brain showed only minimal labeling of fatty acids and sterols. Much higher radioactivity was present in glutamate, glutamine, aspartate, and GABA. The relative specific activities (RSA) of glutamine never rose above 1.00. Aspartate was labeled very rapidly and revealed evidence of 14CO2 fixation in addition to labeling through the Krebs cycle. GABA revealed higher RSA after [1‐14C]malonate than after [2‐14C]malonate. Sequential degradations of glutamate and aspartate proved that labeling of these amino acids occurred from [1‐14C] acetyl‐CoA and [2‐14C] acetyl‐CoA, respectively, via the Krebs cycle. Malonate activation and malonyl‐CoA decarboxylation in vivo were similar to experiments with isolated mitochondria. However, labeled malonate was not incorporated into the amino acids of free mitochondria. The results were compared to data obtained after intracerebral injection of [1‐14C]acetate and [2‐14C]acetate.

In-vitro fatty acid biosynthesis was studied in normal rat sciatic nerve and during wallerian degeneration. Normal nerve incorporated 1,3-C14-malonyl-CoA and 1-C14-acetyl-CoA into fatty acids by a de-novo biosynthetic pathway. The reaction product with highest radioactivity was palmitic acid, and the free fatty acids of nerve contained 90% of the total fatty acid label. During wallerian degeneration, there was a rapid increase of fatty acid biosynthesis that reached a peak between 16 and 24 days after nerve section. Values declined to normal levels at approximately 50 days.

Labeled palmitic acid ([ 16-14C]palmitate) (0.5 pCi) was injected into rat sciatic nerves in vivo to characterize thc incorporation of this fatty acid into complex peripheral nerve lipids after time lapses of 1 min to 2 weeks. For the first 30 min after intraneural injection, the label was concentrated in nerve diglycerides. Thereafter, the relative diglyccride label declined rapidly, and phospholipid radioactivity rose steadily. After 120 min, phospholipids contained over 70% of the total lipid radioactivity. Among the phospholipids, phosphatidylcholine had the largest percentage of total phospholipid label, and acylation of lysophosphatidylcholine accounted for approximately 75% of this label. With time, there was conversion of I 16-14C]palmitate to other long-chain fatty acids by elongation and desaturation. Phosphatidic acid was labeled also, suggesting the operation of the de novo biosynthetic mechanism. However, the specific radioactivity of 1,2-diacylglycerol was much higher than that of phosphatidic acid, suggesting phosphorylation of diglycerides by diglyceride kinase. After nerve section and survival of 2 h to 50 days, the injection of [16-''C]palmitate into the degenerating distal segment revealed an overall decline of phospholipid labeling and a commensurate increase of triglyceride radioactivity. Phosphatidylcholine in degenerating nerve contained a larger percentage of the fatty acid label than that in normal nerve. Almost all of the labeling was due to acylation of lysophosphatidylcholine, implying a much smaller contribution of the de novo pathway. Phosphatidylethanolamine and phosphatidylserine showed a relative loss of radioactivity. The changes were apparent at 1 day, but not at 2 h, suggesting loss of homeostatic control, presumably by interruption of axonal flow. An incidental observation was the stimulation of phosphatidylcholine biosynthesis by acylation of lysophosphatidylcholine in the contralateral unoperated sciatic nerve. Key Words: Fatty acids-Peripheral nerve lipids-Wallerian degeneration-Sciatic nerve (rat). Koeppen A. H. et al. Fatty acid incorporation in normal and degenerating rat sciatic nerve in vivo.

Growing albino rats between the ages of birth and 50 days received single intracranial injections of either [2-"C]malonate or [ 1-"C]malonate. After 1 h, 90% of the nonmalonate radioactivity was recovered in the free amino acids. Fatty acids and sterols accounted for the remainder. Most of the fatty acid radioactivity resided in palmitate and stearate. The label of the polar lipid fraction was concentrated in choline phosphoglyceride (CPG) (65%). Over half of the CPG-fatty acid radioactivity was present in the 2-acyl group. Fatty acid biosynthesis occurred predominantly by a d~ m i ' o mechanism although some elongation was also present. The incorporation of the precursor into amino acids increased rapidly after birth and reached stable levels at 14 days. Fatty acid labeling also increased with brain growth but the fatty acid-amino acid ratio declined steadily from birth to 50 days. Synthesis of malonyl-CoA is the rate-limiting step in the biosynthesis of amino acids, fatty acids, and sterols from free malonic acid.