Alpha-methyl-L-glutamic acid uptake by high affinity dicarboxylic amino acid transport system in Streptococcus...

The transport of alpha-methyl-L-glutamic acid was studied in Streptococcus faecalis. Energey-dependent uptake against substantial concentration gradients was observed. Kinetic experiments indicated that, in contrast to L-glutamic acid, only a single catalytic component (high affinity) and a diffusion controlled process participated in alpha-methyl-L-glutamic acid uptake. At concentrations up to 10 mM, alpha-methyl-glutamate transport was almost completely abolished in a mutant strain lacking a high affinity dicarboxylic amino acid transport system. In competition experiments, alpha-methylglutamic acid antagonized glutamate uptake via the high affinity system, and only slightly via the low affinity system. Column chromatography of cell extracts showed that very little (approx. 5%) of the accumulated amino acid was converted to metabolites during short term incubations. These studies indicate that, at concentrations up to 3-5 mM, alpha-methyl-L-glutamic acid can be used as a specific, relatively metabolically inert substrate of the high affinity dicarboxylic amino acid transport system in S. faecalis.