d-Amino acid contents of mitochondria and some purple bacteria

The endosymbiont most likely to have given rise to mitochondria is an aerobic bacterium belonging to the α subdivision of the so-called purple bacteria such as Rickettsia, Bradythizobium and Agrobacterium [1,2]. Contents of the d-enantiomers of serine, alanine, proline, glutamate and aspartate in rat liver whole mitochondria, mitochondrial outer membranes, inner membranes and matrix, soluble proteins and free amino acids were detected. These values for d-amino acid content were compared with those in soluble proteins and free amino acids from the purple bacteria Paracoccus denitrificans, Pseudomonas aeruginosa and Escherichia coli, members, respectively of the α, β, and γ subdivisions, to find any similarity between mitochondria and these purple bacteria. A similarity was observed in protein d-amino acid contents which were low (<1.5%, D-type/D-type+L-type) both in the membrane and soluble protein fractions from mitochondria and in soluble protein from bacteria. Oddly, substantial amounts of free d-serine and free d-aspartate (around 2%) were found for the first time in mitochondria. The contents of d-serine and d-aspartate were higher than those of d-alanine, d-proline and d-glutamate. In purple bacteria, the concentration of d-serine (<2%) was the lowest of the five amino acids examined, and those of d-alanine (27–32%) and d-glutamate (7–26%) were high. Therefore, no similarity was shown in the free d-amino acid content between mitochondria and any of the three purple bacteria.