The Metabolism of Labelled Lysine and Pipecolic Acid byAcaciaPhyllodes

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Abstract
C14-lysine and C14- and H3-pipecolic acids have been used to study the metabolism of the lysine family of amino-acids in Acacia, which contains 4-hydroxypipecolic acid as a characteristic component of the soluble-nitrogen fraction. Degradation of C14-lysine was rapid and was far more extensive than that observed earlier in higher plants. Pipecolic acid was the major radioactive product in short-term experiments. After longer metabolic periods, radioactivity was distributed over a wide range of amino-acids, organic acids, and sugars. A tentative metabolic scheme is produced to explain these observations concerning lysine degradation. The distribution of radioactivity in the amino-acids of the protein present in the phyllodes was determined 24 hours after supplying C14-lysine. Specific activities of free and bound amino-acids are compared at this time. Hydroxyproline forms a notable component of the phyllode protein. Pipecolic acid degradation has been demonstrated for the first time in a biological system. The breakdown pathway was studied in Acacia phyllodes using H2-pipecolic acid. Substances tentatively identified as Δ′-piperdine-2-carboxylic acid and ε-amino-α-hydroxycaproic acid were amongst the early degradation products. Ultimately, lysine and α-aminoadipic acid became labelled. In contrast to the experiments with C organic acids and sugars did not become radioactive. The explanation of this finding is probably to be found in the ease with which H3 atoms present in certain chemical groupings may undergo exchange with normal hydrogen atoms of water molecules. The biosynthesis of 4-hydroxypipecolic acid probably involves a direct hydroxylation of the parent imino-acid.

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