Turnover of Phosphatidylcholine in Microsomes and Myelin in Brains of Young and Adult Rats

We have tested the hypothesis that the turnover of phosphatidylcholine in subcellular fractions of rat brain is a function of the age at which this lipid is deposited. Rats, 60 days of age, were injected intracranially with [2‐3H]glycerol and either [methyl‐14C]choline (to label the base moiety) or [U‐14C]glucose (to label acyl moieties). Littermates were killed up to 90 days after injection and brain microsomes and myelin isolated. Lipids were extracted and the phosphatidylcholine was isolated by 2‐dimensional TLC and hydrolyzed to its constituent moieties. The 3H in the glycerol backbone and 14C in the choline or acyl residues was quantitated. The microsomal and myelin 3H/14C ratios decreased with time with either set of precursors, indicating that labeled choline and acyl moieties were reutilized more efficiently than the glycerol backbone. The various precursors exhibited first order decay curves with half‐lives for the glycerol backbone of 6 and 11 days for the microsomal and myelin fractions respectively. These results contrast with those previously obtained with identical experimental procedures when 17‐day‐old animals were injected. In that study, although much of the phosphatidylcholine turned over rapidly as for the older animals, by 2 weeks after injection most of the remaining phosphatidylcholine was turning over more slowly with a half‐life of 13 and 25 days for microsomes and myelin respectively (Miller et al., 1977). The base and acyl moieties also had a corresponding shorter half‐life in older animals relative to the slow turnover phase in younger rats.