Metabolism of palmitic acid in the subcellular fractions of mouse brain.

After an intracerebral injection of [(14)C]palmitic acid to C57BL/10J mice, the radioactivity in the brains decreased rapidly with time. The incorporated radioactivity was primarily in the 16:0 acyl groups of the diacyl phosphoglycerides at 1 and 3 days after injection. At longer times, increasing proportions of the radioactivity were found in cerebrosides, alkenyl groups, and other acyl groups. The specific radioactivities of the phosphoglycerides were highest in the microsomal fraction at 1 day after injection. The exchange of the diacyl glycerophosphorylcholines and diacyl glycerophosphorylethanolamines between the microsomes and the myelin required 8-14 days. When calculated on the basis of the radioactivity in the 16:0 acyl groups, the half-lives for both of these phosphoglycerides were 6-8 days in all subcellular fractions during the period from 14 to 30 days after injection. The radioactivity in the total lipids from the purified myelin fraction did not decline until more than 14 days after injection because of the reutilization of labeled 16:0 acyl groups for lipid biosynthesis. Recycling of the acyl groups explains the long half-lives reported for myelin phosphoglycerides after injection of [(14)C]acetic acid. Lipids with a relatively high specific radioactivity were lost from the myelin fraction during the purification procedure. The most likely source of these lipids is the most recently formed myelin that is not consolidated into the myelin sheath.