Biochemistry of Amino Acids - A. Master 1961

General Biochemistry and Physiology of Amino Acid Metabolism
Perreamination
Transamination of D-Amino Acids

Until recently, investigating Transamination reactions of D-Amino Acids was hampered by the lack of sufficiently pure D-isomer preparations. Early studies reported observations suggesting the involvement of D-amino acids in transamination; however, these findings were met with skepticism because the D-amino acid preparations used may have contained L-amino acid impurities [256, 257, 310]. Today, it is difficult to evaluate those results since the studies failed to provide Evidence of the optical purity of The amino acid isomers used.

Relatively recent studies by Thorne and co-workers [311–314] conclusively demonstrated the involvement of D-amino acids in enzymatic transamination reactions in certain Bacteria that synthesize extracellular polyglutamic acids with a predominance of the D-configuration. Thorne found that Cell-free preparations from Bacillus subtilis catalyze The formation of D-glutamic acid, D-aspartic acid, and several other D-amino acids (e.g., D-Methionine, D-Serine) from D-Alanine and the corresponding a-keto acids. Freshly prepared enzyme extracts also mediated the transamination reaction between L-aspartic and a-ketoglutaric acids; in this case, the resulting glutamic acid had the L-configuration. If the extracts were stored for some time and then subjected to dialysis, The activity of D-transaminase (upon addition of Pyridoxal phosphate as a coenzyme) significantly exceeded that of L-transaminase.

Fractionation of the extracts with ammonium sulfate yielded preparations containing exclusively D-transaminase. The formation of D-alanine via transamination between pyruvic acid and the D-isomers of phenylalanine, Tryptophan, methionine, Histidine, and leucine was observed in experiments using enzyme preparations from Bacillus anthracis. In this enzyme system, pyruvic acid cannot be replaced by a-ketoglutaric acid. Among the D-amino acids tested in the experiment, only D-alanine reacted with a-ketoglutarate. Of the L-amino acids, only L-glutamic acid interacted with Pyruvate.

Thorne's studies provided conclusive proof of the involvement of D-amino acids in enzymatic transamination reactions. Evidently, the Cells of B. subtilis and B. anthracis contain L- and D-transaminases characterized by high stereospecificity. To date, only a single amino acid racemase has been found in these microorganisms, namely alanine racemase (p. 241). It is quite likely that D-transaminases also occur in other microorganisms, especially those whose cells contain D-amino acids. Definitive data confirming the involvement of D-amino acids in transamination reactions in animal Tissues have not been obtained, although reports describing such reactions do appear in the literature. The possibility that such reactions occur in animal organisms cannot be ruled out entirely.

When conducting non-enzymatic model transamination reactions using D-amino acids, the resulting amino acid may, under certain conditions, contain a slight excess of the D-isomer; analogous results were obtained in model reactions with L-amino acid isomers. For instance, non-enzymatic transamination between D-alanine and a-ketoglutaric acid (in the presence of Cu⁺⁺ ions and pyridoxal) yielded glutamic acid containing a minor excess of the D-isomer [705].