[2‐³h]glycerol as a Precursor of Phospholipids in Rat Brain: Evidence for Lack of Recycling

When [2‐ 3 H]glycerol was injected intracranially into young rats, it was presented as a pulse label, leaving the brain rapidly and giving up much of its labelled hydrogen to water. [2‐ 3 H]glycerol was efficiently incorporated into brain lipids, especially into choline and ethanolamine phospholipids. Following injection of a mixture of [ 3 H]‐ and [ 14 C]‐labelled glycerol, the ratio of 3 H to 14 C in the phospholipids of both whole brain and the microsomal fraction decreased as a function of time after injection. This finding indicated less recycling of the tritium label. This lack of recycling was further indicated by the finding that 94 per cent of the tritium label of phosphatidyl choline was in the glycerol portion of the molecule rather than in the fatty acids. At 2 weeks following injection with [ 3 H]glycerol, 93 per cent of the total radioactivity in brain appeared in the lipid fraction. In contrast, following injection with [ 14 C]glycerol, only 57 per cent of the radioactivity appeared in lipid, with about 20 per cent in protein.