Maple syrup urine disease is a metabolic disorder caused by mutations of the
branched chain keto acid dehydrogenase complex, leading to accumulation of alpha-
keto acids and their
amino acid precursors in the brain. We now report that alpha-ketoisovaleric, alpha-keto-beta-methyl-n-valeric and alpha-ketoisocaproic
acids accumulated in the disease inhibit
glutamate uptake into rat brain synaptic vesicles. The alpha-
keto acids did not affect the electrochemical
proton gradient across the membrane, suggesting that they interact directly with the vesicular
glutamate carrier.
Chloride anions have a biphasic 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 activation of
glutamate uptake by low
chloride concentrations was shifted toward higher concentrations of the
anion in the presence of alpha-
keto acids. Inhibition by alpha-
keto acids was abolished at high
chloride concentrations (20 to 80 mM), indicating that alpha-
keto acids specifically change the stimulatory effect of low
chloride concentrations. High
glutamate concentrations also reduced the inhibition by alpha-
keto acids, an effect observed both in the absence and in the presence of low
chloride concentrations. The results suggest that in addition to their possible pathophysiological role in
maple syrup urine disease, alpha-
keto acids are valuable tools to study the mechanism of vesicular transport of
glutamate.