Amino Acid Isomerization in the Production of l -Phenylalanine from d -Phenylalanine by Bacteria

To establish an advantageous method for the production of l -amino acids, microbial isomerization of d - and dl -amino acids to l -amino acids was studied. Screening experiments on a number of microorganisms showed that cell suspensions of Pseudomonas fluorescens and P. miyamizu were capable of isomerizing d - and dl -phenylalanines to l -phenylalanine. Various conditions suitable for isomerization by these organisms were investigated. Cells grown in a medium containing d -phenylalanine showed highest isomerization activity, and almost completely converted d - or dl -phenylalanine into l -phenylalanine within 24 to 48 hr of incubation. Enzymatic studies on this isomerizing system suggested that the isomerization of d - or dl -phenylalanine is not catalyzed by a single enzyme, “amino acid isomerase,” but the conversion proceeds by a two step system as follows: d -pheylalanine is oxidized to phenylpyruvic acid by d -amino acid oxidase, and the acid is converted to l -phenylalanine by transamination or reductive amination.