Coming now to mammalians and human beings, we note that the identification of free D-amino acids has been reported for various mammalian tissues. Free impressively high activity. The d-enantiomer in the peptide structure is thus essential for the biological activity of dermorphin.
3 Achatin I was isolated in tissues of the African giant snail Achatina fulica and is described to enhance cardiac activity. Achatin II is the epimer peptide using l-phenylalanine. It was also isolated from the snail's tissue but shows, however, minor biological activity as compared to achatin I (Fujii 2002).
D-aspartic acid enantiomers were found in the brain of rats, chickens, and humans. Here, concentrations of D-aspartic acid seem to be related to developmental stages since concentrations increase and then rapidly decrease to trace amounts again in adult tissues. For example, in the prefrontal cortex of the human brain, as much as 60% of the total aspartic acid is present in its D-configuration at week 14 of gestation, but rapidly decreases to trace levels by the time of birth (Hashimoto et al. 1993). Nagata et al. (1994) identified a similar timely distribution pattern for the concentration of D-serine in the forebrain of mammals such as mouse, rat, and bull.4
D-serine, D-a-alanine, and D-proline were reported to be present in human plasma in D/L-ratios up to 0.24 (Nagata et al. 1992a) and also in mouse kidney in D/L-ratios up to 0.035 (Nagata et al. 1992b). In these experiments, particular attention had to be paid to the enantioselective analysis of serine. Unexpectedly, under the chosen hydrolysis conditions using 6 molar hydrochloric acid, the D-enantiomer decomposed faster than the L-enantiomer (Nagata and Tagashira 1998).5 Experimental data on D/L-ratios of serine are therefore to be handled with care.
In contrast to the importance of frog dermorphin and snail achatin, the physiological role of free D-amino acids found in mammals is not yet known. Further investigations are strongly required to elucidate the source of D-amino acids as well as their physiological, pharmacological, and pathological significance.
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