Generation of ketone bodies from leucine by cultured astroglial cells

To elucidate the significance of branched-chain amino acids (BCAAs) for brain energy metabolism, the capacity to use BCAAs for oxidative metabolism was investigated in astroglia-rich primary cultures derived from newborn rat brain. The cells selectively removed BCAAs from the culture medium, the disappearance following first-order kinetics. The BCAAs disappeared rapidly in spite of the presence of sufficient glucose as substrate for the generation of energy. Taking into consideration that the ketogenic amino acid leucine could be degraded only to acetyl-CoA and acetoacetate, and with the knowledge that astroglial cells have the capacity to secrete ketone bodies, this amino acid was chosen for further metabolic studies. After incubation of the cells with leucine, acetoacetate, D-beta-hydroxybutyrate, and alpha-ketoisocaproate were found to have accumulated in the culture medium. Identification of the radioactive metabolites generated from [4,5-3H]leucine established that the source of the substances released was indeed leucine. These results indicate that, at least in culture, astroglial cells degrade leucine via the known metabolite alpha-ketoisocaproate, to acetoacetate, which can be further reduced to D-beta-hydroxybutyrate. It is hypothesized that upon release from brain astrocytes, the ketone bodies could serve as fuel molecules for neighboring cells such as neurons and oligodendrocytes. In view of these and other results, astrocytes may be considered the brain's fuel processing plants.