d-β-HYDROXYBUTYRATE: A MAJOR PRECURSOR OF AMINO ACIDS IN DEVELOPING RAT BRAIN

Abstract— D-β-hydroxybutyrate (β-OHB) was compared to glucose as a precursor for brain amino acids during rat development. In the first study 3-¹⁴C]β-OHB or 2-¹⁴C]glucose was injected subcu-taneously (01 μCi/g body wt) into suckling rats shortly after birth and at 6. 11, 13, 15 and 21 days of age. Blood and brain tissue were obtained 20 min later after decapitation. The specific activity of the labelled precursor in the blood and in the brain tissue was essentially the same for each respective age suggesting that the labelled precursor had equilibrated between the blood and brain pools before decapitation. 3-¹⁴C]β-OHB rapidly labelled brain amino acids at all ages whereas 2-¹⁴C]glucose did not prior to 15 days of age. These observations are consistent with a maturational delay in the flux of metabolites through glycolysis and into the tricarboxylic acid cycle. Brain glutamate, glutamine, asparate and GABA were more heavily labelled by 3-¹⁴C]β-OHB from birth-15 days of age whereas brain alanine was more heavily labelled by 2-¹⁴C]glucose at all ages of development. The relative specific activity of brain glutamine/glutamate was less than one at all ages for both labelled precursors suggesting that β-OHB and glucose are entering the‘large’glutamate compartment throughout development. In a second study, 6 and 15 day old rats were decapitated at 5 min intervals after injection of the labelled precursors to evaluate the flux of the ¹⁴C]label into brain metabolites. At 6 days of age, most of the brain acid soluble radioactivity was recovered in the glucose fraction of the 2-^,4C]glucose injected rats with 72, 74, 65 and 63% after 5, 10, 15 and 20 min. In contrast, the 6 day old rats injected with 3-¹⁴C]β-OHB accumulated much of the brain acid soluble radioactivity in the amino acid fraction with 22, 47, 57 and 54% after 5, 10, 15 and 20 min. At 15 days of age the transfer of the ¹⁴C]label from 2-¹⁴C]glucose into the brain amino acid fraction was more rapid with 29, 40, 45, 61 and 73% of the brain acid soluble radioactivity recovered in the amino acid fraction after 5, 10, 15, 20 and 30 min. There was almost quantitative transfer of ¹⁴C]label into the brain amino acids of the 15-day-old 3-¹⁴C]β-OHB injected rats with 66, 89, 89, 89 and 90% of the brain acid soluble radioactivity recovered in the amino acid fraction after 5, 10, 15, 20 and 30 min. The calculated half life for /?-OHB at 6 days was 19 8 min and at 15 days was 12-2 min. Surprisingly, the relative specific activity of brain GABA/glutamate was lower at 15 days of age in the 3-¹⁴C]β-OHB injected rats compared to the 2-¹⁴C]glucose injected rats despite a heavier labelling of brain glutamate in the 3-¹⁴C]β-OHB injected group. We interpreted these data to mean that β-OHB is a less effective precursor for the brain glutamate ‘subcompartment’ which is involved in the synthesis of GABA.