Inhibition of glutamate uptake into synaptic vesicles of rat brain by the metabolites accumulating in maple syrup urine disease

Maple syrup urine disease is an inherited metabolic disorder characterized by tissue accumulation of branched-chain amino acids and their correspondin g keto acids in the affected children. Although this disorder is predominan tly characterized by neurological symptoms, only few studies were carried o ut to investigate its neuropathology. In this study we investigated the eff ect of the metabolites accumulating in maple syrup urine disease on the in vitro uptake of [H-3]glutamate by synaptic vesicles of rat brain. Synaptic vesicle preparations from whole brain of male adult Wistar rats (200-250 g) were incubated with the branched-chain amino acids and their corresponding keto acids at final concentrations ranging from 0.25 to 10 mM for the dete rmination of glutamate uptake. Glutamate uptake was significantly inhibited by L-leucine, L-isoleucine, L-2-ketoisocaproic acid and L-2-keto-3-methylv aleric acid by approximately 60%, whereas L-valine and L-2-ketoisovaleric a cid showed no effect. We also verified that the metabolites probably act by competitive inhibition. Therefore, it is possible that extracellular gluta mate levels may be increased in maple syrup urine disease and that excitoto xicity may be involved in the neuropathology of this disorder. (C) 2000 Els evier Science B.V. All rights reserved.