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D-Amino Acids in Soils. I. Uptake and Metabolism by Seedling Maize and Ryegrass¹

Although normally not constituents of proteins, Dconfiguration amino acids are important components of certain bacteria, insects, earthworms, and animals. while the contribution or fate of these D-amino acids within the ever-revolving nitrogen fraction of soils remains unknown, there is little question that they are present in soils. Their possible uptake by and effect on plants is thus of agronomic interest. This study has shown:

1. That both ryegrass and maize seedlings readily absorbed any of the D-amino acids applied. To date, those applied include D-valine, D-leucine, D-alanine, Dmethionine, and D-lysine as representatives of the different major groups of metabolically related amino acids.

2. That these plants possess essential enzymatic systems to metabolize the so-called "unnatural", "non-protein" D-configuration amino acids. The amount metabolized during a 34-hour exposure period ranged as a minimum between 53 and 87%.

3. That at least some of the initial metabolic conversions appear the same as those for the corresponding L--amino acids. The -keto acid analog of the supplied D-amino acid was a significant product and possibly a major intermediate in every case. Extensive decarboxylation occurred as evidenced by loss of radioactivity when the ¹⁴C label was in the carboxyl position. Utilization of the metabolites was evident from examples such as ¹⁴C-D-alanine ¹⁴C-pyruvic acid ¹⁴C-valine.

4. That other metabolic reactions are involved which have not been commonly reported for corresponding "natural" L-amino acid isomers. For example, there was apparent conversion of carboxyl¹⁴C-D-alanine to labeled ¹⁴C-alpha-methylserine, indicating the presence of alphamethylserine hydroxymethyl-transferase. From ¹⁴C-D-alanine, labeled valine was detected in grass roots but labeled isoleudne appeared in grass shoots.

Key Words: Alpha-keto acids • ¹⁴C-D-amino acids • Plants nutrition

² Research Associate, Department of Soils, Oregon State University and Research Chemist, SWC-ARS-USDA, Corvallis, Oregoa. Present address of senior author: Institut fur Bodenkunde, 34 Gottingen, von Sieboldstrasse 4, West Germany.

Received for publication August 2, 1969.

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