Aspects of D-leucine and D-lysine metabolism in maize and ryegrass seedlings

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Abstract
Maize and ryegrass seedlings (2.5 weeks old), the roots of which were dipped into 10-3M 14C-carboxyl-labeled D-leucine and 14C-ε-labeled D-lysine, readyly absorbed and converted or conjugated within 34 hr some 75–90% of the labeled compound supplied. The metabolic intermediates and products were generally similar for both maize and ryegrass. Radioactive intermediates from the carboxyllabeled D-leucine were L-leucine, N-malonyl-D-leucine (provisionally identified), and α-ketoisocaproic acid. Intermediates from ε-labeled D-lysine were numerous, with greater amounts and numbers detected in roots than in tops. Pipecolic acid was a major intermediate particularly in shoot tissue. Pathways of conversion appeared analogous to those for the L-isomer, and conversion may be by the usual L-configuration machinery, since the labeled L-isomer of the originally supplied 14C-D-amino acid was always found. How the 14C-D-amino acid gets to 14C-L-isomer is not known, but finding significant proportions of unlabeled D-alanine in plants treated with both the labeled L-leucine and D-lysine suggested that formation of the α-keto-acid analog and subsequent reamination was possibly an important route.