Oxygen radical induced fluorescence in proteins; identification of the fluorescent tryptophan metabolite, N-formyl kynurenine, as a biological index of radical damage

Summary

The effect of oxygen derived free radicals (OFR) on aromatic and sulphur containing amino acids has been investigated, both in their free form and within protein backbones. Aerated amino acids and proteins in solution were exposed to three discrete OFR generating systems; (1) gamma radiation in the presence or absence of formate (2) photolysis by UV light at 254 and 366 nm, and (3) site specific modification by H₂O₂ in the presence of CuII ions.

A sensitive reverse-phase HPLC technique with dual detection systems (UV absorbance and fluorescence monitoring) was developed to analyse the products of amino acid oxidation. OFR denatured amino acids were chromatographed by this procedure, and all radical species generated, with the exception of the superoxide anion, resulted in the formation of identifiable fluorescent metabolites of tryptophan, kynurenines. The identity of peaks was confimed by spiking with authentic material and scanning absorption spectroscopy. After complete proteolytic hydrolysis, OFR treated proteins were also analysed by this technique; again the dose dependent production of kynurenines was detected in IgG,γ lens crystallins and albumin. Bityrosine was not detected in any of the proteins studied using this procedure, however, several novel unidentified fluorophores were detected in proteolytic hydrolysates, possibly the product of two different amino acid radicals.

Immunoglobulin G isolated from the sera of normals and rheumatoid arthritis (RA) patients was examined for the presence of one specific tryptophan metabolite, N-formyl kynurenine. Significantly elevated levels of this metabolite were detected in rheumatoid sera, suggesting increased OFR activity in RA.

These results have demonstrated firstly, that specific oxidised products of amino acids are retained in the protein backbone after exposure to OFR generating systems. Secondly, in aerated solution, oxidised tryptophan residues confer the major new visible fluorescence in non-haem proteins, not tyrosine products. In addition, this work has demonstrated that the measurement of a specific product of an oxidised amino acid can be applied to biological macromolecules, and may be important in implicating free radical reactions in certain disease processes.