Yung-Feng Chang

Pipecolic acid pathway: The major lysine metabolic route in the rat brain

Summary Lysine metabolism was studied in the rat brain by intraventricular injection of 14C-labeled L- and D-lysine. The major metabolic intermediate in the brain labeled from either lysine isomer was found to be pipecolic acid which was characterized to be in the L-configuration. Small amounts of labeled α-aminoadipic acid could also be detected in the brain samples from rats 24 hours after injection with either lysine isomer. L- or D-lysine, therefore, appears to be metabolized in the rat brain via pipecolic acid to α-aminoadipic acid which differs from the major saccharopine pathway known to be operative in the liver or other mammalian tissues.

L-α-Aminoadipate Inhibits Kynurenate Synthesis in Rat Brain Hippocampus and Tissue Culture

Intracerebral administration of L-α-aminoadipic acid (L-AAA) at 500 mg/kg body weight to rats caused a complex behavioral change with sporadic wet-dog shakes. Animals developed severe limbic seizures between 1 and 6 h after L-AAA injection, characterized by generalized convulsions. Twenty days after L-AAA injection kynurenine aminotransferase (KAT) activity measured in hippocampal brain tissue slices prepared with a McIlwain chopper at 30 μm showed a significant 43% decrease. Subcutaneous injection of kynurenine at 500 mg/kg showed a 63% increase in KAT activity twenty days later. This increase was offset by a concomitant administration of 500 mg/kg L-AAA stereotaxically on day one. In astrocyte culture kynurenic acid synthesis is inhibited by L-AAA and L-pipecolic acid. The possible involvement of kynurenic acid in the modulation of neuronal degeneration is discussed.

L-lysine is a barbiturate-like anticonvulsant and modulator of the benzodiazepine receptor

Our earlier observations showed thatl-lysine enhanced the activity of diazepam against seizures induced by pentylenetetrazol (PTZ), and increased the affinity of benzodiazepine receptor binding in a manner additive to that caused by γ-aminobutyric acid (GABA). The present paper provides additional evidence to show thatl-lysine has central nervous system depressant-like characteristics.l-lysine enhanced [3H]flunitrazepam (FTZ) binding in brain membranes was dose-dependent and stimulated by chloride, bromide and iodide, but not fluoride. Enhancement of [3H]FTZ binding byl-lysine at a fixed concentration was increased by GABA but inhibited by pentobarbital between 10−7 to 10−3M. While GABA enhancement of [3H]FTZ binding was inhibited by the GABA mimetics imidazole acetic acid and tetrahydroisoxazol pyridinol, the enhancement by pentobarbital andl-lysine of [3H]FTZ binding was dose-dependently increased by these two GABA mimetics. The above results suggest thatl-lysine and pentobarbital acted at the same site of the GABA/benzodiazepine receptor complex which was different from the GABA binding site. The benzodiazepine receptor antagonist imidazodiazepine Ro15-1788 blocked the antiseizure activity of diazepam against PTZ. Similar to pentobarbital, the anti-PTZ effect ofl-lysine was not blocked by Ro15-1788. Picrotoxinin and the GABA, receptor antagonist bicuculline partially inhibitedl-lysines enhancement of [3H]FTZ binding with the IC50s of 2 μM and 0.1 μM, respectively. The convulsant benzodiazepine Ro5-3663 dose-dependently inhibited the enhancement of [3H]FTZ binding byl-lysine. This article shows the basic amino acidl-lysine to have a central nervous system depressant characteristics with an anti-PTZ seizure activity and an enhancement of [3H]FTZ binding similar to that of barbiturates but different from GABA.

Induction of separate catabolic pathways for L- and D-lysine in Pseudomonas putida

Abstract A strain of Pseudomonas putida was found to oxidize L-lysine by an inducible pathway through δ-aminovalerate, and D-lysine via Δ 1 -piperideine-2-carboxylate and pipecolic acid. Each pathway is selectively induced by L- or D-lysine and the appropriate intermediates. Lysine racemase is inadequate to permit growth on L-lysine after a block in the L-pathway, but is sufficient to permit cross induction of D-lysine-related enzymes.

Invitro synthesis of L-pipecolate from L-lysine: Inconsistent with ϵ-N-acetyl-L-lysine as an obligatory intermediate

In vitro synthesis of 14C-L-pipecolic acid from L-[U-14C]lysine was demonstrated for the first time in the rat tissues. Acetyl donor was not required for this synthesis. No labeling of ϵ-N-acetyl-L-lysine was detected during 14C-pipecolate synthesis. When ϵ-N-acetyl-L-[U-14C]lysine was substrate, large quantities of 14C-lysine but only very small quantities of 14C-pipecolate were detected, indicating the synthesis of L-pipecolate from ϵ-N-acetyl-L-lysine to be by way of L-lysine. Although the present data are inconsistent with the hypothesis that removal of α-NH2 group from L-lysine in preparation for L-pipecolate formation requires ϵ-N-substitution, the involvement of an enzyme-bound ϵ-N-substitutedL-lysine in this pathway cannot be ruled out.

Modulation of benzodiazepine by lysine and pipecolic acid on pentylenetetrazol-induced seizures

L-lysine, an essential amino acid for man and animals, and its metabolite pipecolic acid (PA) have been studied for their effects on pentylenetetrazol (PTZ)-induced seizures in mice. L-Lysine or L-PA i.p. significantly increased clonic and tonic latencies in a dose-dependent manner against 90 mg/kg PTZ-induced seizures. L-Lysine but not L-PA enhanced the anticonvulsant effect of diazepam (DZ) (0.2 mg/kg). L-PA (0.1 mmol/kg) i.c.v. showed a slight decrease in clonic latency; it did not enhance the antiseizure activity of DZ; it caused seizures at 0.6 mmol/kg. D-PA (0.1 mmol/kg) i.c.v. displayed an opposite effect compared to its L-isomer. The anticonvulsant effect of L-lysine in terms of increase in seizure latency and survival was even more amplified when tested with a submaximal PTZ concentration (65 mg/kg). L-Lysine showed an enhancement of specific 3H-flunitrazepam (FZ) binding to mouse brain membranes both in vitro and in vivo. The possibility of L-lysine acting as a modulator for the GABA/benzodiazepine receptors was demonstrated. Since L-PA showed enhancement of 3H-FZ binding only in vitro but not in vivo, the anticonvulsant effect of L-PA may not be linked to the GABA/benzodiazepine receptor.

Enhancement of hexobarbital-induced sleep by lysine and its metabolites

Abstract Lysine has been shown to be metabolized in the rat brain to pipecolic acid which is a precursor of piperidine. Lysine and its proposed metabolites in this pathway were studied for the first time for their effect on the sleeping time induced by hexobarbital in the rat. Only L -lysine and D -lysine were found to prolong sleeping time significantly without toxic effect. A 3-day pretreatment with L -lysine produced an even more profound sleep prolongation. In most cases sleep enhancement was accompanied by a significant shortening of the time of sleep onset. Quantification of brain hexobarbital levels in the control and treated rats indicates that prolongation of sleeping time was not produced by inhibition of hexobarbital metabolism. The sleep prolonging effect of lysine, therefore, may be a direct action of lysine, or the metabolite(s) derived in vivo from lysine, on the central nervous system.

Effects of l-lysine and its metabolites on pentylenetetrazol-induced seizures

Lysine and its metabolic intermediates were studied for their effect on pentylenetetrazol (PTZ)-induced seizures in mice. L-Lysine at dosages above 2 mmol/kg given i.p. significantly increased seizure protection and seizure latency (the time required to develop seizures after PTZ injection) with a peak effect dose at 10 mmol/kg. A pretreatment time of 15 min was required to significantly prolong seizure latency with a peak effect time of 45 min. D-Lysine at 10 mmol/kg i.p. afforded some seizure protection and significantly prolonged seizure latency but has a peak effect time of 15 min. When administered intracerebroventricularly, both L-lysine and piperidine at 0.1 mmol/kg prolonged seizure latency significantly, and increased seizure protection slightly. L-Pipecolic acid at the same dose given through the same route, however, shortened seizure latency significantly. L-alpha-Aminoadipic acid, on the other hand, had no significant effect. Lysine metabolites that prolonged seizure latency also increased seizure protection and decreased seizure death, and one that shortened seizure latency had the opposite effect. The anticonvulsant activity of lysine and its metabolites was explained on the basis of their connection with the GABAergic transmission.

Blood-brain barrier transport ofL-pipecolic acid in various rat brain regions

Blood-brain barrier transport ofL-[l-14C]pipecolic acid was studied in the rat by single intracarotid injection using3H2O as a diffusible internal standard. Brain uptake index (BUI) forL-[14C]pipecolic acid (0.036 mM) was found to be 18.1, 10.5, and 12.6 for the cerebral cortex, brain stem, and cerebellum, respectively which was substantially higher than that reported for its analogL-proline in the whole brain. Influx ofL-pipecolic acid into the brain was concentration dependent and differed significantly between the cerebral cortex and the brain stem, and between the cerebral cortex and the cerebellum, but not between the brain stem and the cerebellum. Kinetic study ofL-pipecolic acid influx revealed a low- and a high-capacity uptake mechanisms. The low-capacity saturable component hasKm values ranging from 38 to 73 μM, andVmax values ranging from 10 to 13 nmol/g/min for the three brain regions. The nonsaturable component has aKm of 4 mM, aVmax of 200 nmol/g/min and similar diffusion constant (Kd) (0.03 to 0.06 mlg−1 min−1) for all three brain regions. A possible role of the two-component brain uptake mechanism in the regulation of the neuronal function ofL-pipecolic acid was suggested.

Uptake and metabolism of Δ1-piperidine-2-carboxylic acid by synaptosomes from rat cerebral cortex

Abstract Δ1-Piperidine-2-carboxylic acid (P2C), an intermediate of the l -lysine metabolic pathway in the brain, was studied for its uptake metabolism in the synaptosome of the rat cerebral cortex. The results of this study showed that the uptake of P2C into the synaptosome was NA+-and temperature-dependent with a two-tier transport kinetic (Km = 2.6 and 0.7 μM; Vmax = 1.6 and 0.73 pmol/min/mg). P2C uptake was only moderately inhibited (≈20%) by l -lysine and its metabolites, l -α-aminoadipic acid at up to 100 μM, and the putative amino acid neurotransmitters, γ-aminobutyric acid, l -glutamic acid and l -spartic acid (25–31%) at 5–500 μM. The synaptosomal preparation only has a very low activity for metabolizing P2C to its product l -pipecolic acid. The metabolic activity for P2C was mainly contained in the 27 000 × g supernatant S2 fraction. Since P2C is the precursor of the putative neuromodulator l -pipecolic acid, the understanding of its uptake and metabolic characteristics in the brain should be of significance.