Chloride-dependent inhibition of vesicular glutamate uptake by alpha-keto acids accumulated in maple syrup urine disease

Maple syrup urine disease is a metabolic disorder caused by mutations of th e branched chain keto acid dehydrogenase complex. leading to accumulation o f alpha-keto acids and their amino acid precursors in the brain. We now rep ort that alpha-ketoisovaleric. alpha-keto-beta-methyl-n-valeric and alpha-k etoisocaproic acids accumulated in the disease inhibit glutamate uptake int o rat brain synaptic vesicles. The alpha-keto acids did not affect the elec trochemical proton gradient across the membrane, suggesting that they inter act directly with the vesicular glutamate carrier. Chloride anions have a b iphasic effect on glutamate uptake. Low concentrations activate the uptake (0.2 to 8 mM), while higher concentrations are inhibitory. The (alpha-keto acids inhibited glutamate uptake by a new mechanism, involving a change in the chloride dependence for the activation of glutamate uptake. The activat ion of glutamate uptake by low chloride concentrations was shifted toward h igher concentrations of the anion in the presence of alpha-keto acids. Inhi bition by alpha-keto acids was abolished at high chloride concentrations (2 0 to 80 mM), indicating that alpha-keto acids specifically change the stimu latory effect of low chloride concentrations. High glutamate concentrations also reduced the inhibition by alpha-keto acids, an effect observed both i n the absence and in the presence of low chloride concentrations. The resul ts suggest that in addition to their possible pathophysiological role in ma ple syrup urine disease. alpha-keto acids are valuable tools to study the m echanism of vesicular transport of glutamate. (C) 2000 Elsevier science B.V . All rights reserved.