Free malonate, malonyl-coenzyme A (malonyl-CoA), and acetyl-CoA were assayed in rat brain at developmental ages from the 20th day of gestation to 60 days of postnatal life. The determination of malonate was based on its conversion to malonyl-CoA and decarboxylation to acetyl-CoA by enzyme extracts from Pseudomonas fluorescens. The resulting acetyl-CoA reacted with [4-14C]oxaloacetate to form [5-14C]citrate, which was isolated by TLC. Malonyl-CoA in perchloric acid extracts from brain was converted to acetyl-CoA by rat liver mitochondrial malonyl-CoA decarboxylase (EC 4.1.1.9). Acetyl-CoA derived from this step was assayed by a modified CoA-cycling procedure. Brain acetyl-CoA was also assayed by CoA cycling. Prenatal brain contained no free malonate but malonyl-CoA was present. The acetyl-CoA level was relatively high just prior to birth and declined slightly with growth. Malonate concentrations after birth rose rapidly to reach 192 nmol/g wet weight at 60 days. Adult levels for malonyl-CoA and acetyl-CoA were 1.83 and 1.90 nmol/g wet weight, respectively. The origin and natural role of free malonate in brain are not known but deacylation of malonyl-CoA by reversal of the malonyl-CoA synthetase reaction is postulated. Rat liver and kidney also contain substantial concentrations of free malonate.

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.

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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.