Kinetics and mechanism of thermal decomposition of kynurenines and biomolecular conjugates: ramifications for the modification of mammalian eye lens proteins

Formation of fluorophores from the kynurenine pathway metabolite N-formylkynurenine and cyclic amines involves transamidation and carbon-carbon bond formation at the 2-position of the amine

BACKGROUND

Tryptophan catabolism along the kynurenine pathway is associated with a number of pathologies including cataract formation and cancer. Whilst the chemical reactions of kynurenine are well studied, less is known about the reactivity of its precursor N-formylkynurenine (NFK). We previously reported the generation of a strong fluorophore in an aqueous reaction of NFK with piperidine, and herein we describe its structure and mechanism of formation.

METHODS

Compounds were identified using NMR, mass and UV spectroscopic techniques. The products from the reaction of amines with amino acids were quantified using HPLC-MS.

RESULTS

The novel fluorophore was identified as a tetrahydroquinolone adduct (PIP-THQ), where piperidine is N-formylated and attached at its 2-position to the quinolone. NFK is initially deaminated to generate an unsaturated enone, which forms an adduct with piperidine and is subsequently converted into the fluorophore. Testing of a variety of other secondary amines showed that only cyclic amines unsubstituted at both positions adjacent to nitrogen could form fluorophores efficiently. The amino acids tryptophan and kynurenine, which lack the formamide group do not form such fluorophores.

CONCLUSIONS

NFK forms fluorophores in a not previously published reaction with cyclic amines.

GENERAL SIGNIFICANCE

Our study is the first to provide evidence for concurrent transamidation and substitution at the 2-position of a cyclic amine occurring under moderately-heated aqueous conditions with no added catalysts. The high reactivity of NFK demonstrated here could result in formation of biologically relevant metabolites yet to be characterised.

The therapeutic effect of mitochondria-targeted antioxidant SkQ1 and Cistanche deserticola is associated with increased levels of tryptophan and kynurenine in the rat lens

Supplementation of senescence-accelerated OXYS rats with the mitochondria-targeted antioxidant SkQ1 and with the powder from Cistanche deserticola results in the deceleration of the cataract development and even in the improvement of lens transparency. The therapeutic effect of these preparations correlates with a significant elevation of tryptophan and kynurenine levels in the lens. This finding is attributed to a deceleration of the tryptophan and kynurenine oxidation due to antioxidant-assisted reduction of oxidative stress in the lens.