An enzymatic synthesis of 4-methyl-L-glutamic acid diastereoisomers.

(2S,4R)-"erythro"- and (2S,4S)-"threo"-4-methylglutamic acids have been prepared by glutamate dehydrogenase-catalyzed reductive amination of 2-keto-(R,S)-4-methylglutaric acid, in the presence of NH4+ ions, NAD+, and a NADH recycling system (ethanol-alcohol dehydrogenase), followed by separation by anion-exchange chromatography. Nuclear magnetic resonance, electrophoretic, chromatographic, and optical properties of both isomers are reported, together with the conditions leading to the four-carbon epimerization.