Conversion of D-Lysine to L-Lysine via L-Pipecolic Acid in Neurospora crassa

Pipecolic acid (4) (Fig. 6 ), an imino acid oc­ curring in plants 1, animals 2> 3, and microorgan­ isms 4, is known to be a product of lysine cata­ bolism. Tracer studies have shown that lysine labelled in different positions is incorporated into pipecolic acid in such diverse organisms as Phaseolus sp. 5) 6, Triticum sp . 7, Zea m a y s8, Acacia hom alophylla9, Leucaena glauca10, Mimosa pudic a n , Rattus sp . 12_15, Neurospora crassa16,17, Rhizoctonia legum inicola18, Pseudomonas putida 19_21, and Achromobacter sp. 22. The reactions leading to pipecolic acid are out­ lined in Fig. 6. The pathway involves a-deamination of lysine and formation of an intramolecular Schiff’s base (3 ), zl1-piperideine-2-carboxylic acid (3), whose stereospecific reduction at C2 yields L-pipecolic acid (4) 16. Whereas the origin of pipecolic acid from lysine is well documented, there are con­ flicting reports whether the D or L isomer of lysine gives rise to pipecolic acid. Before 1967 it was ac­ cepted that pipecolic acid was derived from Llysine 2) 6> 9> 13,17, 22. Since 1968 evidence has ac­ cumulated which leads to the conclusion — with one exception18 — that pipecolic acid is derived from D-lysine 8’ 141519~21’ 23. In many cases inferences were based on a com­ parison of incorporation efficiences of radioactively labelled samples of D or L-lysine in separate ex­ periments. Shortcomings of such an approach have been recently pointed o u t23.