Identification of L-amino acid/L-lysine alpha-amino oxidase in mouse brain

Lysine, an essential amino acid is catabolized in brain through only the pipecolic acid pathway. During the formation of pipecolic acid, alpha-deamination of lysine, and the formation of the alpha-keto acid as well as its cyclized product are pre-requisites. The enzyme mediated alpha-deamination of L-lysine and the formation of the alpha-keto acid and the cyclized product are not demonstrated so far. Both lysine and pipecolic acid are known to increase in brain under the conditions of fasting, studies were therefore undertaken to identify the enzyme responsible for the alpha-deamination of L-lysine in the brain tissue of mice which were fasted. The detection of the alpha-keto acid of L-lysine -alpha-keto-epsilon-amino caproic acid and its cyclized product-delta-piperidine-2-carboxylate was facilitated by the use of L-[U-14C]-lysine as the substrate. The quantitation of the radioactivity in reaction products was done after separation by ion exchange chromatographic methods. The formation of the alpha-keto acid was enzyme mediated, the alpha-keto acid formed was established by reaction with N-methyl benzothiazolinone hydrazone hydrochloride. The cyclized product was accounted in a fraction which matched the resolution of authentic pipecolic acid on the Dowex column, and the cyclized product was confirmed by spectrophotometry. The hitherto undemonstrated alpha-amino deaminating enzyme of L-lysine in brain tissue, the alpha-keto acid of L-lysine and its cyclized product in a mammalian system could thus be demonstrated in the present study. These findings confirm the involvement of L-lysine oxidase/L-amino acid oxidase in the formation of pipecolic acid from L-lysine.