COPPER CHELATION BY TRYPTOPHAN INHIBITS THE COPPER-ASCORBATE OXIDATION OF TRYPTOPHAN

The in vitro oxidation of tryptophan (Trp) by pro-oxidant systems such as iron-ascorbate indicates that Trp is a target for oxygen radicals in vivo. The Trp in albumin and lipoproteins has been reported to be a ctively oxidized by hydroxyl radical (HO.) generating systems such as copper-ascorbate or PUFA (polyunsaturated fatty acids) respectively. T he super-physiological concentrations of the oxidants used in these st udies prompted us to examine the effect of low copper and ascorbate co ncentrations on Trp oxidation. Trp (10-5000 mu mol/L) was incubated wi th 1.5 mu mol/L copper plus ascorbate (0.113 and 1.13 mmol/L) at 37 de grees C and its oxidation followed by fluorescence and high-performanc e liquid chromatography. The percentage of Trp oxidized by the ascorba te-copper system was inversely related to its concentration and positi vely related to the ascorbate concentration. High concentrations of Tr p (above 50 mu mol/L for 0.113 mmol/L and 500 mu mol/L for 1.13 mmol/L ascorbate) are not significantly oxidized in the presence of ascorbat e. The large drop in the percentage Trp oxidation at higher concentrat ions may be due to the chelation of copper by Trp. High concentrations of Trp (over 50 mu mol/L) strongly prevented ascorbate oxidation by c opper, and therefore inhibited the production of HO. needed for Trp ox idation. Protein Trp is less readily oxidized by the ascorbate-copper system than free Trp. Proteins chelate copper much better than Trp, an d so inhibit its oxidative activity, at least against ascorbic acid.