ONTOGENY AND CELLULAR-LOCALIZATION OF THE PYRUVATE RECYCLING SYSTEM IN RAT-BRAIN

The ontogeny of the cerebral pyruvate recycling pathway and the cellul ar localization of associated enzymes, malic enzyme (ME) and phosphoen olpyruvate carboxykinase (PEPCK), have been investigated using a combi nation of C-13 NMR spectroscopy, enzymatic analysis, and molecular bio logy approaches. Activity of the pathway, using [1,2-C-13(2)]acetate a s a substrate, was detected by C-13 NMR in brain extracts 3 weeks afte r birth, increasing progressively up to the third month of age. In who le-brain homogenates, ME activity increased to adult levels with the s ame time course as the recycling pathway. PEPCK activity was low durin g the first 2 weeks of life and decreased further toward adulthood. ME and PEPCK activity were found in primary cultures of astrocytes and i n synaptosomal fractions of adult brain. Primary cultures of cortical neurons showed PEPCK activity but no detectable ME activity. The cytos olic ME gene was expressed in primary cultures of neurons and in astro cytes as well as in the neonatal and adult brain. The PEPCK gene was e xpressed both in primary cultures of cortical neurons and in astrocyte s, but the level of its expression in the neonatal and adult brain was undetectable.