Occurrence of d-amino acids in a few archaea and dehydrogenase activities in hyperthermophile Pyrobaculum islandicum

Abstract

The contents of d-enantiomers of serine, alanine, proline, glutamate (glutamine) and aspartate (asparagine) were examined in the membrane fractions, soluble proteins and free amino acids from some species of archaea, Pyrobaculum islandicum, Methanosarcina barkeri and Halobacterium salinarium. Around 2% ( d/ d+ l) of d-aspartate was found in the membrane fractions. In the soluble proteins, the d-amino acid content was higher in P. islandicum than that in the other archaeal cells: the concentrations in P. islandicum were 3 and 4% for d-serine and d-aspartate, respectively. High concentrations of free d-amino acids were found in P. islandicum and H. salinarium; the concentrations of d-serine (12–13%), d-aspartate (4–7%) and d-proline (3–4%) were higher than those of d-alanine and d-glutamate. This result showed a resemblance between these archaea and not bacterial, but eukaryotic cells. The presence of d-amino acids was confirmed by their digestion with d-amino acid oxidase and d-aspartate oxidase. The occurrence of d-amino acids was also confirmed by the presence of activities catalyzing catabolism of d-amino acids in the P. islandicum homogenate, as measured by 2-oxo acid formation. The catalytic activities oxidizing d-alanine, d-aspartate and d-serine at 90°C were considerably high. Under anaerobic conditions, dehydrogenase activities of the homogenate were 69, 84 and 30% of the above oxidase activities toward d-alanine, d-aspartate and d-serine, respectively. Comparable or higher dehydrogenase activities were also detected with these d-amino acids as substrate by the reduction of 2,6-dichlorophenolindophenol. No d-amino acid oxidase activity was detected in the homogenates of M. barkeri and H. salinarium.