Lipid Labeling After Intracerebral Injection of Radioactive Malonic Acid

Growing albino rats between the ages of birth and 50 days received single intracranial injections of either [2‐14C]malonate or [1‐14C]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 de novo 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.