Ketone-body utilization and lipid synthesis by developing rat brain-a comparison between in vivo and in vitro experiments.

The distribution of ketone bodies between oxidation and lipid synthesis was analysed in homogenates of developing rat brain. The capacity for lipid synthesis of homogenized or minced brain preparations was compared with rates of lipid synthesis in vivo, assessed by incorporation of (3)H from (3)H(2)O into fatty acids and cholesterol. Brain homogenates of suckling rats (but not those of adults) incorporated label from [3-(14)C]ketone bodies into lipids, but this process was slow as compared to (14)CO(2) production (< 5%) and much slower than the total rate of ketone-body utilization (< 0.5%). Study of (3)H(2)O incorporation demonstrated that the rates of lipogenesis and cholesterogenesis are at least one order of magnitude higher in vivo than in vitro. Maximal rates of (3)H incorporation into fatty acids (3 ?mol/g brain . h) and into cholesterol (0.6 ?mol/g brain . h) were found during the third postnatal week. Adult rats still incorporated (3)H into brain fatty acids at an appreciable rate (1 ?mol/g brain . h), whereas cholesterogenesis was very low. It is concluded that in vitro measurements of lipid synthesis severely underestimate the rates that occur in developing rat brain in vivo. The high rate of (3)H incorporation into lipids by developing and adult rat brain as compared to the amounts of these lipids present in the brain suggests an important contribution of endogenous lipid synthesis during brain development and an appreciable rate of fatty acid turnover during brain growth, but also in the adult brain.