Copper chelation by tryptophan inhibits the copper-ascorbate oxidation of tryptophan

Abstract

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 actively oxidized by hydroxyl radical (HO(•)) generating systems such as copper-ascorbate or PUFA (polyunsaturated fatty acids) respectively. The super-physiological concentrations of the oxidants used in these studies prompted us to examine the effect of low copper and ascorbate concentrations on Trp oxidation. Trp (10-5000 μmol/L) was incubated with 1.5 μmol/L copper plus ascorbate (0.113 and 1.13 mmol/L) at 37°C and its oxidation followed by fluorescence and high-performance liquid chromatography. The percentage of Trp oxidized by the ascorbate-copper system was inversely related to its concentration and positively related to the ascorbate concentration. High concentrations of Trp (above 50 μmol/L for 0.113 mmol/L and 500 μmol/L for 1.13 mmol/L ascorbate) are not significantly oxidized in the presence of ascorbate. The large drop in the percentage Trp oxidation at higher concentrations may be due to the chelation of copper by Trp. High concentrations of Trp (over 50 μmol/L) strongly prevented ascorbate oxidation by copper, and therefore inhibited the production of HO(•) needed for Trp oxidation. Protein Trp is less readily oxidized by the ascorbate-copper system than free Trp. Proteins chelate copper much better than Trp, and so inhibit its oxidative activity, at least against ascorbic acid.

Copper-ligand interactions and physiological free radical processes. Part 3. Influence of histidine, salicylic acid and anthranilic acid on copper-driven Fenton chemistryIn Vitro

“…Likewise, the effective competition of ML2H at the expense of ML2 at pH 5.5 (Table II) as compared to pH 7.4 (Table I) should normally entail a lesser decrease of the redox potential of the Cu(II)/Cu(I) couple at pH 5.5 than at pH 7.4 (the latter being unfortunately impossible to determine), hence the lesser inhibitory effect actually observed. The strong decrease in the copper-mediated oxidation of tryptophan recently reported to occur as a function of tryptophan concentration [35] is also in line with this interpretation. …”

Oxidative and Nitrosative Stress Induced in Myofibrillar Proteins by a Hydroxyl-Radical-Generating System: Impact of Nitrite and Ascorbate

“…While the food science literature on this topic is scarce, studies on medical research often varied and, at times, provided contradictory results on the effect of ascorbate on TRP. Some authors highlight the pro-oxidant action of ascorbate under physiological conditions, 29,30 while others described the ability of ascorbate to inhibit and even revert oxidation and nitrosation of TRP residues. 31 The present results show that ascorbate offers antioxidant protection to TRP in MP as a likely consequence of scavenging hydroxyl radicals.…”

Chemistry of copper in white wine: a review

“…Central to the role of Cu as a catalyst, it was argued, is the reduction of Cu(II) to Cu(I) and the consequent production of free radicals (Moreaux et al. ). Similar to the oxidation mechanism in the absence of ascorbic acid (see section on Accelerating the rate of wine oxidation), Cu could also perform this catalytic role in concert with Fe.…”